diff --git a/compiler/rustc_abi/src/callconv.rs b/compiler/rustc_abi/src/callconv.rs
index 9fb70b80c9efb..4529ab8058e9a 100644
--- a/compiler/rustc_abi/src/callconv.rs
+++ b/compiler/rustc_abi/src/callconv.rs
@@ -65,8 +65,6 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
         Ty: TyAbiInterface<'a, C> + Copy,
     {
         match self.backend_repr {
-            BackendRepr::Uninhabited => Err(Heterogeneous),
-
             // The primitive for this algorithm.
             BackendRepr::Scalar(scalar) => {
                 let kind = match scalar.primitive() {
diff --git a/compiler/rustc_abi/src/layout.rs b/compiler/rustc_abi/src/layout.rs
index 45cd0b517f6c7..53243266f992b 100644
--- a/compiler/rustc_abi/src/layout.rs
+++ b/compiler/rustc_abi/src/layout.rs
@@ -130,6 +130,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             },
             backend_repr: BackendRepr::ScalarPair(a, b),
             largest_niche,
+            uninhabited: false,
             align,
             size,
             max_repr_align: None,
@@ -221,8 +222,9 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         LayoutData {
             variants: Variants::Empty,
             fields: FieldsShape::Primitive,
-            backend_repr: BackendRepr::Uninhabited,
+            backend_repr: BackendRepr::Memory { sized: true },
             largest_niche: None,
+            uninhabited: true,
             align: dl.i8_align,
             size: Size::ZERO,
             max_repr_align: None,
@@ -400,6 +402,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             fields: FieldsShape::Union(union_field_count),
             backend_repr: abi,
             largest_niche: None,
+            uninhabited: false,
             align,
             size: size.align_to(align.abi),
             max_repr_align,
@@ -447,7 +450,6 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 Scalar::Union { .. } => {}
             };
             match &mut st.backend_repr {
-                BackendRepr::Uninhabited => {}
                 BackendRepr::Scalar(scalar) => hide_niches(scalar),
                 BackendRepr::ScalarPair(a, b) => {
                     hide_niches(a);
@@ -639,9 +641,8 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             let same_size = size == variant_layouts[largest_variant_index].size;
             let same_align = align == variant_layouts[largest_variant_index].align;
 
-            let abi = if variant_layouts.iter().all(|v| v.is_uninhabited()) {
-                BackendRepr::Uninhabited
-            } else if same_size && same_align && others_zst {
+            let uninhabited = variant_layouts.iter().all(|v| v.is_uninhabited());
+            let abi = if same_size && same_align && others_zst {
                 match variant_layouts[largest_variant_index].backend_repr {
                     // When the total alignment and size match, we can use the
                     // same ABI as the scalar variant with the reserved niche.
@@ -683,6 +684,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 },
                 backend_repr: abi,
                 largest_niche,
+                uninhabited,
                 size,
                 align,
                 max_repr_align,
@@ -853,9 +855,8 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
         };
         let mut abi = BackendRepr::Memory { sized: true };
 
-        if layout_variants.iter().all(|v| v.is_uninhabited()) {
-            abi = BackendRepr::Uninhabited;
-        } else if tag.size(dl) == size {
+        let uninhabited = layout_variants.iter().all(|v| v.is_uninhabited());
+        if tag.size(dl) == size {
             // Make sure we only use scalar layout when the enum is entirely its
             // own tag (i.e. it has no padding nor any non-ZST variant fields).
             abi = BackendRepr::Scalar(tag);
@@ -995,6 +996,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 memory_index: [0].into(),
             },
             largest_niche,
+            uninhabited,
             backend_repr: abi,
             align,
             size,
@@ -1355,9 +1357,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                 _ => {}
             }
         }
-        if fields.iter().any(|f| f.is_uninhabited()) {
-            abi = BackendRepr::Uninhabited;
-        }
+        let uninhabited = fields.iter().any(|f| f.is_uninhabited());
 
         let unadjusted_abi_align = if repr.transparent() {
             match layout_of_single_non_zst_field {
@@ -1378,6 +1378,7 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
             fields: FieldsShape::Arbitrary { offsets, memory_index },
             backend_repr: abi,
             largest_niche,
+            uninhabited,
             align,
             size,
             max_repr_align,
diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
index dbb4bed5cdd99..098638b6bcfa7 100644
--- a/compiler/rustc_abi/src/lib.rs
+++ b/compiler/rustc_abi/src/lib.rs
@@ -1404,7 +1404,6 @@ impl AddressSpace {
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
 #[cfg_attr(feature = "nightly", derive(HashStable_Generic))]
 pub enum BackendRepr {
-    Uninhabited,
     Scalar(Scalar),
     ScalarPair(Scalar, Scalar),
     Vector {
@@ -1423,10 +1422,9 @@ impl BackendRepr {
     #[inline]
     pub fn is_unsized(&self) -> bool {
         match *self {
-            BackendRepr::Uninhabited
-            | BackendRepr::Scalar(_)
-            | BackendRepr::ScalarPair(..)
-            | BackendRepr::Vector { .. } => false,
+            BackendRepr::Scalar(_) | BackendRepr::ScalarPair(..) | BackendRepr::Vector { .. } => {
+                false
+            }
             BackendRepr::Memory { sized } => !sized,
         }
     }
@@ -1445,12 +1443,6 @@ impl BackendRepr {
         }
     }
 
-    /// Returns `true` if this is an uninhabited type
-    #[inline]
-    pub fn is_uninhabited(&self) -> bool {
-        matches!(*self, BackendRepr::Uninhabited)
-    }
-
     /// Returns `true` if this is a scalar type
     #[inline]
     pub fn is_scalar(&self) -> bool {
@@ -1471,7 +1463,7 @@ impl BackendRepr {
             BackendRepr::Vector { element, count } => {
                 cx.data_layout().vector_align(element.size(cx) * count)
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => return None,
+            BackendRepr::Memory { .. } => return None,
         })
     }
 
@@ -1492,7 +1484,7 @@ impl BackendRepr {
                 // to make the size a multiple of align (e.g. for vectors of size 3).
                 (element.size(cx) * count).align_to(self.inherent_align(cx)?.abi)
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => return None,
+            BackendRepr::Memory { .. } => return None,
         })
     }
 
@@ -1506,9 +1498,7 @@ impl BackendRepr {
             BackendRepr::Vector { element, count } => {
                 BackendRepr::Vector { element: element.to_union(), count }
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {
-                BackendRepr::Memory { sized: true }
-            }
+            BackendRepr::Memory { .. } => BackendRepr::Memory { sized: true },
         }
     }
 
@@ -1704,6 +1694,11 @@ pub struct LayoutData<FieldIdx: Idx, VariantIdx: Idx> {
     /// The leaf scalar with the largest number of invalid values
     /// (i.e. outside of its `valid_range`), if it exists.
     pub largest_niche: Option<Niche>,
+    /// Is this type known to be uninhabted?
+    ///
+    /// This is separate from BackendRepr because uninhabited return types can affect ABI,
+    /// especially in the case of by-pointer struct returns, which allocate stack even when unused.
+    pub uninhabited: bool,
 
     pub align: AbiAndPrefAlign,
     pub size: Size,
@@ -1735,14 +1730,14 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
     /// Returns `true` if this is an aggregate type (including a ScalarPair!)
     pub fn is_aggregate(&self) -> bool {
         match self.backend_repr {
-            BackendRepr::Uninhabited | BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => false,
+            BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => false,
             BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => true,
         }
     }
 
     /// Returns `true` if this is an uninhabited type
     pub fn is_uninhabited(&self) -> bool {
-        self.backend_repr.is_uninhabited()
+        self.uninhabited
     }
 
     pub fn scalar<C: HasDataLayout>(cx: &C, scalar: Scalar) -> Self {
@@ -1778,6 +1773,7 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
             fields: FieldsShape::Primitive,
             backend_repr: BackendRepr::Scalar(scalar),
             largest_niche,
+            uninhabited: false,
             size,
             align,
             max_repr_align: None,
@@ -1802,6 +1798,7 @@ where
             backend_repr,
             fields,
             largest_niche,
+            uninhabited,
             variants,
             max_repr_align,
             unadjusted_abi_align,
@@ -1813,6 +1810,7 @@ where
             .field("abi", backend_repr)
             .field("fields", fields)
             .field("largest_niche", largest_niche)
+            .field("uninhabited", uninhabited)
             .field("variants", variants)
             .field("max_repr_align", max_repr_align)
             .field("unadjusted_abi_align", unadjusted_abi_align)
@@ -1877,7 +1875,6 @@ impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {
             BackendRepr::Scalar(_) | BackendRepr::ScalarPair(..) | BackendRepr::Vector { .. } => {
                 false
             }
-            BackendRepr::Uninhabited => self.size.bytes() == 0,
             BackendRepr::Memory { sized } => sized && self.size.bytes() == 0,
         }
     }
diff --git a/compiler/rustc_codegen_cranelift/src/value_and_place.rs b/compiler/rustc_codegen_cranelift/src/value_and_place.rs
index a9b8e1bd39353..1b3f86c8405de 100644
--- a/compiler/rustc_codegen_cranelift/src/value_and_place.rs
+++ b/compiler/rustc_codegen_cranelift/src/value_and_place.rs
@@ -638,9 +638,7 @@ impl<'tcx> CPlace<'tcx> {
             }
             CPlaceInner::Addr(_, Some(_)) => bug!("Can't write value to unsized place {:?}", self),
             CPlaceInner::Addr(to_ptr, None) => {
-                if dst_layout.size == Size::ZERO
-                    || dst_layout.backend_repr == BackendRepr::Uninhabited
-                {
+                if dst_layout.size == Size::ZERO {
                     return;
                 }
 
diff --git a/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs b/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
index 5322b731d8bb5..433868e238a4c 100644
--- a/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
+++ b/compiler/rustc_codegen_gcc/src/intrinsic/mod.rs
@@ -315,7 +315,7 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tc
                 let layout = self.layout_of(tp_ty).layout;
                 let _use_integer_compare = match layout.backend_repr() {
                     Scalar(_) | ScalarPair(_, _) => true,
-                    Uninhabited | Vector { .. } => false,
+                    Vector { .. } => false,
                     Memory { .. } => {
                         // For rusty ABIs, small aggregates are actually passed
                         // as `RegKind::Integer` (see `FnAbi::adjust_for_abi`),
diff --git a/compiler/rustc_codegen_gcc/src/type_of.rs b/compiler/rustc_codegen_gcc/src/type_of.rs
index 8b8b54753e7fd..bac4fc51300a2 100644
--- a/compiler/rustc_codegen_gcc/src/type_of.rs
+++ b/compiler/rustc_codegen_gcc/src/type_of.rs
@@ -84,7 +84,7 @@ fn uncached_gcc_type<'gcc, 'tcx>(
                 false,
             );
         }
-        BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {}
+        BackendRepr::Memory { .. } => {}
     }
 
     let name = match *layout.ty.kind() {
@@ -179,19 +179,16 @@ impl<'tcx> LayoutGccExt<'tcx> for TyAndLayout<'tcx> {
     fn is_gcc_immediate(&self) -> bool {
         match self.backend_repr {
             BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => true,
-            BackendRepr::ScalarPair(..) | BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {
-                false
-            }
+            BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => false,
         }
     }
 
     fn is_gcc_scalar_pair(&self) -> bool {
         match self.backend_repr {
             BackendRepr::ScalarPair(..) => true,
-            BackendRepr::Uninhabited
-            | BackendRepr::Scalar(_)
-            | BackendRepr::Vector { .. }
-            | BackendRepr::Memory { .. } => false,
+            BackendRepr::Scalar(_) | BackendRepr::Vector { .. } | BackendRepr::Memory { .. } => {
+                false
+            }
         }
     }
 
diff --git a/compiler/rustc_codegen_llvm/src/back/lto.rs b/compiler/rustc_codegen_llvm/src/back/lto.rs
index 7262fce491191..3e25b94961b00 100644
--- a/compiler/rustc_codegen_llvm/src/back/lto.rs
+++ b/compiler/rustc_codegen_llvm/src/back/lto.rs
@@ -362,8 +362,8 @@ fn fat_lto(
                 ptr as *const *const libc::c_char,
                 symbols_below_threshold.len() as libc::size_t,
             );
-            save_temp_bitcode(cgcx, &module, "lto.after-restriction");
         }
+        save_temp_bitcode(cgcx, &module, "lto.after-restriction");
     }
 
     Ok(LtoModuleCodegen::Fat(module))
diff --git a/compiler/rustc_codegen_llvm/src/callee.rs b/compiler/rustc_codegen_llvm/src/callee.rs
index aa9a0f34f55e9..ea9ab5c02bd6c 100644
--- a/compiler/rustc_codegen_llvm/src/callee.rs
+++ b/compiler/rustc_codegen_llvm/src/callee.rs
@@ -66,9 +66,7 @@ pub(crate) fn get_fn<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'t
             // LLVM will prefix the name with `__imp_`. Ideally, we'd like the
             // existing logic below to set the Storage Class, but it has an
             // exemption for MinGW for backwards compatibility.
-            unsafe {
-                llvm::LLVMSetDLLStorageClass(llfn, llvm::DLLStorageClass::DllImport);
-            }
+            llvm::set_dllimport_storage_class(llfn);
             llfn
         } else {
             cx.declare_fn(sym, fn_abi, Some(instance))
@@ -99,65 +97,61 @@ pub(crate) fn get_fn<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, instance: Instance<'t
         // has been applied to the definition (wherever that definition may be).
 
         llvm::set_linkage(llfn, llvm::Linkage::ExternalLinkage);
-        unsafe {
-            let is_generic = instance.args.non_erasable_generics().next().is_some();
-
-            let is_hidden = if is_generic {
-                // This is a monomorphization of a generic function.
-                if !(cx.tcx.sess.opts.share_generics()
-                    || tcx.codegen_fn_attrs(instance_def_id).inline
-                        == rustc_attr_parsing::InlineAttr::Never)
-                {
-                    // When not sharing generics, all instances are in the same
-                    // crate and have hidden visibility.
-                    true
-                } else {
-                    if let Some(instance_def_id) = instance_def_id.as_local() {
-                        // This is a monomorphization of a generic function
-                        // defined in the current crate. It is hidden if:
-                        // - the definition is unreachable for downstream
-                        //   crates, or
-                        // - the current crate does not re-export generics
-                        //   (because the crate is a C library or executable)
-                        cx.tcx.is_unreachable_local_definition(instance_def_id)
-                            || !cx.tcx.local_crate_exports_generics()
-                    } else {
-                        // This is a monomorphization of a generic function
-                        // defined in an upstream crate. It is hidden if:
-                        // - it is instantiated in this crate, and
-                        // - the current crate does not re-export generics
-                        instance.upstream_monomorphization(tcx).is_none()
-                            && !cx.tcx.local_crate_exports_generics()
-                    }
-                }
-            } else {
-                // This is a non-generic function. It is hidden if:
-                // - it is instantiated in the local crate, and
-                //   - it is defined an upstream crate (non-local), or
-                //   - it is not reachable
-                cx.tcx.is_codegened_item(instance_def_id)
-                    && (!instance_def_id.is_local()
-                        || !cx.tcx.is_reachable_non_generic(instance_def_id))
-            };
-            if is_hidden {
-                llvm::set_visibility(llfn, llvm::Visibility::Hidden);
-            }
+        let is_generic = instance.args.non_erasable_generics().next().is_some();
 
-            // MinGW: For backward compatibility we rely on the linker to decide whether it
-            // should use dllimport for functions.
-            if cx.use_dll_storage_attrs
-                && let Some(library) = tcx.native_library(instance_def_id)
-                && library.kind.is_dllimport()
-                && !matches!(tcx.sess.target.env.as_ref(), "gnu" | "uclibc")
+        let is_hidden = if is_generic {
+            // This is a monomorphization of a generic function.
+            if !(cx.tcx.sess.opts.share_generics()
+                || tcx.codegen_fn_attrs(instance_def_id).inline
+                    == rustc_attr_parsing::InlineAttr::Never)
             {
-                llvm::LLVMSetDLLStorageClass(llfn, llvm::DLLStorageClass::DllImport);
+                // When not sharing generics, all instances are in the same
+                // crate and have hidden visibility.
+                true
+            } else {
+                if let Some(instance_def_id) = instance_def_id.as_local() {
+                    // This is a monomorphization of a generic function
+                    // defined in the current crate. It is hidden if:
+                    // - the definition is unreachable for downstream
+                    //   crates, or
+                    // - the current crate does not re-export generics
+                    //   (because the crate is a C library or executable)
+                    cx.tcx.is_unreachable_local_definition(instance_def_id)
+                        || !cx.tcx.local_crate_exports_generics()
+                } else {
+                    // This is a monomorphization of a generic function
+                    // defined in an upstream crate. It is hidden if:
+                    // - it is instantiated in this crate, and
+                    // - the current crate does not re-export generics
+                    instance.upstream_monomorphization(tcx).is_none()
+                        && !cx.tcx.local_crate_exports_generics()
+                }
             }
+        } else {
+            // This is a non-generic function. It is hidden if:
+            // - it is instantiated in the local crate, and
+            //   - it is defined an upstream crate (non-local), or
+            //   - it is not reachable
+            cx.tcx.is_codegened_item(instance_def_id)
+                && (!instance_def_id.is_local()
+                    || !cx.tcx.is_reachable_non_generic(instance_def_id))
+        };
+        if is_hidden {
+            llvm::set_visibility(llfn, llvm::Visibility::Hidden);
+        }
 
-            if cx.should_assume_dso_local(llfn, true) {
-                llvm::LLVMRustSetDSOLocal(llfn, true);
-            }
+        // MinGW: For backward compatibility we rely on the linker to decide whether it
+        // should use dllimport for functions.
+        if cx.use_dll_storage_attrs
+            && let Some(library) = tcx.native_library(instance_def_id)
+            && library.kind.is_dllimport()
+            && !matches!(tcx.sess.target.env.as_ref(), "gnu" | "uclibc")
+        {
+            llvm::set_dllimport_storage_class(llfn);
         }
 
+        cx.assume_dso_local(llfn, true);
+
         llfn
     };
 
diff --git a/compiler/rustc_codegen_llvm/src/consts.rs b/compiler/rustc_codegen_llvm/src/consts.rs
index 4a5491ec7a18c..330e8a8f4069d 100644
--- a/compiler/rustc_codegen_llvm/src/consts.rs
+++ b/compiler/rustc_codegen_llvm/src/consts.rs
@@ -336,12 +336,7 @@ impl<'ll> CodegenCx<'ll, '_> {
             llvm::set_thread_local_mode(g, self.tls_model);
         }
 
-        let dso_local = self.should_assume_dso_local(g, true);
-        if dso_local {
-            unsafe {
-                llvm::LLVMRustSetDSOLocal(g, true);
-            }
-        }
+        let dso_local = self.assume_dso_local(g, true);
 
         if !def_id.is_local() {
             let needs_dll_storage_attr = self.use_dll_storage_attrs
@@ -375,9 +370,7 @@ impl<'ll> CodegenCx<'ll, '_> {
                 // is actually present in the current crate. We can find out via the
                 // is_codegened_item query.
                 if !self.tcx.is_codegened_item(def_id) {
-                    unsafe {
-                        llvm::LLVMSetDLLStorageClass(g, llvm::DLLStorageClass::DllImport);
-                    }
+                    llvm::set_dllimport_storage_class(g);
                 }
             }
         }
@@ -387,9 +380,7 @@ impl<'ll> CodegenCx<'ll, '_> {
             && library.kind.is_dllimport()
         {
             // For foreign (native) libs we know the exact storage type to use.
-            unsafe {
-                llvm::LLVMSetDLLStorageClass(g, llvm::DLLStorageClass::DllImport);
-            }
+            llvm::set_dllimport_storage_class(g);
         }
 
         self.instances.borrow_mut().insert(instance, g);
@@ -460,9 +451,7 @@ impl<'ll> CodegenCx<'ll, '_> {
             set_global_alignment(self, g, alloc.align);
             llvm::set_initializer(g, v);
 
-            if self.should_assume_dso_local(g, true) {
-                llvm::LLVMRustSetDSOLocal(g, true);
-            }
+            self.assume_dso_local(g, true);
 
             // Forward the allocation's mutability (picked by the const interner) to LLVM.
             if alloc.mutability.is_not() {
diff --git a/compiler/rustc_codegen_llvm/src/intrinsic.rs b/compiler/rustc_codegen_llvm/src/intrinsic.rs
index 7e1a9d361e6fb..f68365f6c69bc 100644
--- a/compiler/rustc_codegen_llvm/src/intrinsic.rs
+++ b/compiler/rustc_codegen_llvm/src/intrinsic.rs
@@ -476,7 +476,7 @@ impl<'ll, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 let layout = self.layout_of(tp_ty).layout;
                 let use_integer_compare = match layout.backend_repr() {
                     Scalar(_) | ScalarPair(_, _) => true,
-                    Uninhabited | Vector { .. } => false,
+                    Vector { .. } => false,
                     Memory { .. } => {
                         // For rusty ABIs, small aggregates are actually passed
                         // as `RegKind::Integer` (see `FnAbi::adjust_for_abi`),
diff --git a/compiler/rustc_codegen_llvm/src/llvm/mod.rs b/compiler/rustc_codegen_llvm/src/llvm/mod.rs
index efc9cf2ef691b..5ec934241314c 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/mod.rs
@@ -403,3 +403,15 @@ pub(crate) fn add_module_flag_str(
         );
     }
 }
+
+pub(crate) fn set_dllimport_storage_class<'ll>(v: &'ll Value) {
+    unsafe {
+        LLVMSetDLLStorageClass(v, DLLStorageClass::DllImport);
+    }
+}
+
+pub(crate) fn set_dso_local<'ll>(v: &'ll Value) {
+    unsafe {
+        LLVMRustSetDSOLocal(v, true);
+    }
+}
diff --git a/compiler/rustc_codegen_llvm/src/mono_item.rs b/compiler/rustc_codegen_llvm/src/mono_item.rs
index 70edee21bd63a..a64627eaf5986 100644
--- a/compiler/rustc_codegen_llvm/src/mono_item.rs
+++ b/compiler/rustc_codegen_llvm/src/mono_item.rs
@@ -38,11 +38,7 @@ impl<'tcx> PreDefineCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
 
         llvm::set_linkage(g, base::linkage_to_llvm(linkage));
         llvm::set_visibility(g, base::visibility_to_llvm(visibility));
-        unsafe {
-            if self.should_assume_dso_local(g, false) {
-                llvm::LLVMRustSetDSOLocal(g, true);
-            }
-        }
+        self.assume_dso_local(g, false);
 
         self.instances.borrow_mut().insert(instance, g);
     }
@@ -79,9 +75,7 @@ impl<'tcx> PreDefineCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
 
         debug!("predefine_fn: instance = {:?}", instance);
 
-        if self.should_assume_dso_local(lldecl, false) {
-            unsafe { llvm::LLVMRustSetDSOLocal(lldecl, true) };
-        }
+        self.assume_dso_local(lldecl, false);
 
         self.instances.borrow_mut().insert(instance, lldecl);
     }
@@ -90,11 +84,16 @@ impl<'tcx> PreDefineCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
 impl CodegenCx<'_, '_> {
     /// Whether a definition or declaration can be assumed to be local to a group of
     /// libraries that form a single DSO or executable.
-    pub(crate) fn should_assume_dso_local(
-        &self,
-        llval: &llvm::Value,
-        is_declaration: bool,
-    ) -> bool {
+    /// Marks the local as DSO if so.
+    pub(crate) fn assume_dso_local(&self, llval: &llvm::Value, is_declaration: bool) -> bool {
+        let assume = self.should_assume_dso_local(llval, is_declaration);
+        if assume {
+            llvm::set_dso_local(llval);
+        }
+        assume
+    }
+
+    fn should_assume_dso_local(&self, llval: &llvm::Value, is_declaration: bool) -> bool {
         let linkage = llvm::get_linkage(llval);
         let visibility = llvm::get_visibility(llval);
 
diff --git a/compiler/rustc_codegen_llvm/src/type_of.rs b/compiler/rustc_codegen_llvm/src/type_of.rs
index b0b6da869da68..ba01fbff38539 100644
--- a/compiler/rustc_codegen_llvm/src/type_of.rs
+++ b/compiler/rustc_codegen_llvm/src/type_of.rs
@@ -23,7 +23,7 @@ fn uncached_llvm_type<'a, 'tcx>(
             let element = layout.scalar_llvm_type_at(cx, element);
             return cx.type_vector(element, count);
         }
-        BackendRepr::Uninhabited | BackendRepr::Memory { .. } | BackendRepr::ScalarPair(..) => {}
+        BackendRepr::Memory { .. } | BackendRepr::ScalarPair(..) => {}
     }
 
     let name = match layout.ty.kind() {
@@ -172,19 +172,16 @@ impl<'tcx> LayoutLlvmExt<'tcx> for TyAndLayout<'tcx> {
     fn is_llvm_immediate(&self) -> bool {
         match self.backend_repr {
             BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => true,
-            BackendRepr::ScalarPair(..) | BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {
-                false
-            }
+            BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => false,
         }
     }
 
     fn is_llvm_scalar_pair(&self) -> bool {
         match self.backend_repr {
             BackendRepr::ScalarPair(..) => true,
-            BackendRepr::Uninhabited
-            | BackendRepr::Scalar(_)
-            | BackendRepr::Vector { .. }
-            | BackendRepr::Memory { .. } => false,
+            BackendRepr::Scalar(_) | BackendRepr::Vector { .. } | BackendRepr::Memory { .. } => {
+                false
+            }
         }
     }
 
diff --git a/compiler/rustc_codegen_ssa/src/back/link.rs b/compiler/rustc_codegen_ssa/src/back/link.rs
index 105a4cb81f0d1..a8d917f0fdb58 100644
--- a/compiler/rustc_codegen_ssa/src/back/link.rs
+++ b/compiler/rustc_codegen_ssa/src/back/link.rs
@@ -1990,6 +1990,7 @@ fn add_pre_link_args(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavor)
     if let Some(args) = sess.target.pre_link_args.get(&flavor) {
         cmd.verbatim_args(args.iter().map(Deref::deref));
     }
+
     cmd.verbatim_args(&sess.opts.unstable_opts.pre_link_args);
 }
 
@@ -2518,6 +2519,12 @@ fn add_order_independent_options(
             "--target-cpu",
             &codegen_results.crate_info.target_cpu,
         ]);
+        if codegen_results.crate_info.target_features.len() > 0 {
+            cmd.link_arg(&format!(
+                "--target-feature={}",
+                &codegen_results.crate_info.target_features.join(",")
+            ));
+        }
     } else if flavor == LinkerFlavor::Ptx {
         cmd.link_args(&["--fallback-arch", &codegen_results.crate_info.target_cpu]);
     } else if flavor == LinkerFlavor::Bpf {
diff --git a/compiler/rustc_codegen_ssa/src/back/linker.rs b/compiler/rustc_codegen_ssa/src/back/linker.rs
index e3ace01c1eb54..8900405c1b8f8 100644
--- a/compiler/rustc_codegen_ssa/src/back/linker.rs
+++ b/compiler/rustc_codegen_ssa/src/back/linker.rs
@@ -153,6 +153,7 @@ pub(crate) fn get_linker<'a>(
             hinted_static: None,
             is_ld: cc == Cc::No,
             is_gnu: flavor.is_gnu(),
+            uses_lld: flavor.uses_lld(),
         }) as Box<dyn Linker>,
         LinkerFlavor::Msvc(..) => Box::new(MsvcLinker { cmd, sess }) as Box<dyn Linker>,
         LinkerFlavor::EmCc => Box::new(EmLinker { cmd, sess }) as Box<dyn Linker>,
@@ -361,6 +362,7 @@ struct GccLinker<'a> {
     // Link as ld
     is_ld: bool,
     is_gnu: bool,
+    uses_lld: bool,
 }
 
 impl<'a> GccLinker<'a> {
@@ -552,6 +554,7 @@ impl<'a> Linker for GccLinker<'a> {
                 self.link_args(&["--entry", "_initialize"]);
             }
         }
+
         // VxWorks compiler driver introduced `--static-crt` flag specifically for rustc,
         // it switches linking for libc and similar system libraries to static without using
         // any `#[link]` attributes in the `libc` crate, see #72782 for details.
@@ -567,6 +570,15 @@ impl<'a> Linker for GccLinker<'a> {
         {
             self.cc_arg("--static-crt");
         }
+
+        // avr-none doesn't have default ISA, users must specify which specific
+        // CPU (well, microcontroller) they are targetting using `-Ctarget-cpu`.
+        //
+        // Currently this makes sense only when using avr-gcc as a linker, since
+        // it brings a couple of hand-written important intrinsics from libgcc.
+        if self.sess.target.arch == "avr" && !self.uses_lld {
+            self.verbatim_arg(format!("-mmcu={}", self.target_cpu));
+        }
     }
 
     fn link_dylib_by_name(&mut self, name: &str, verbatim: bool, as_needed: bool) {
diff --git a/compiler/rustc_codegen_ssa/src/base.rs b/compiler/rustc_codegen_ssa/src/base.rs
index a989e9c64c0f9..40238f4b4915a 100644
--- a/compiler/rustc_codegen_ssa/src/base.rs
+++ b/compiler/rustc_codegen_ssa/src/base.rs
@@ -915,6 +915,7 @@ impl CrateInfo {
         let n_crates = crates.len();
         let mut info = CrateInfo {
             target_cpu,
+            target_features: tcx.global_backend_features(()).clone(),
             crate_types,
             exported_symbols,
             linked_symbols,
diff --git a/compiler/rustc_codegen_ssa/src/lib.rs b/compiler/rustc_codegen_ssa/src/lib.rs
index 428a45975f1ec..9d2ac219d592c 100644
--- a/compiler/rustc_codegen_ssa/src/lib.rs
+++ b/compiler/rustc_codegen_ssa/src/lib.rs
@@ -190,6 +190,7 @@ impl From<&cstore::NativeLib> for NativeLib {
 #[derive(Debug, Encodable, Decodable)]
 pub struct CrateInfo {
     pub target_cpu: String,
+    pub target_features: Vec<String>,
     pub crate_types: Vec<CrateType>,
     pub exported_symbols: UnordMap<CrateType, Vec<String>>,
     pub linked_symbols: FxIndexMap<CrateType, Vec<(String, SymbolExportKind)>>,
@@ -230,6 +231,7 @@ pub fn provide(providers: &mut Providers) {
     crate::base::provide(providers);
     crate::target_features::provide(providers);
     crate::codegen_attrs::provide(providers);
+    providers.queries.global_backend_features = |_tcx: TyCtxt<'_>, ()| vec![];
 }
 
 /// Checks if the given filename ends with the `.rcgu.o` extension that `rustc`
diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index 0620f08fc731b..49074996174a7 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -4,9 +4,7 @@ use rustc_abi::{BackendRepr, ExternAbi, HasDataLayout, Reg, WrappingRange};
 use rustc_ast as ast;
 use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
 use rustc_hir::lang_items::LangItem;
-use rustc_middle::mir::{
-    self, AssertKind, BasicBlock, InlineAsmMacro, SwitchTargets, UnwindTerminateReason,
-};
+use rustc_middle::mir::{self, AssertKind, InlineAsmMacro, SwitchTargets, UnwindTerminateReason};
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, ValidityRequirement};
 use rustc_middle::ty::print::{with_no_trimmed_paths, with_no_visible_paths};
 use rustc_middle::ty::{self, Instance, Ty};
@@ -942,7 +940,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     &fn_abi.ret,
                     &mut llargs,
                     Some(intrinsic),
-                    target,
                 );
                 let dest = match ret_dest {
                     _ if fn_abi.ret.is_indirect() => llargs[0],
@@ -998,19 +995,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         };
 
         let mut llargs = Vec::with_capacity(arg_count);
-        let destination = target.as_ref().map(|&target| {
-            (
-                self.make_return_dest(
-                    bx,
-                    destination,
-                    &fn_abi.ret,
-                    &mut llargs,
-                    None,
-                    Some(target),
-                ),
-                target,
-            )
-        });
+
+        // We still need to call `make_return_dest` even if there's no `target`, since
+        // `fn_abi.ret` could be `PassMode::Indirect`, even if it is uninhabited,
+        // and `make_return_dest` adds the return-place indirect pointer to `llargs`.
+        let return_dest = self.make_return_dest(bx, destination, &fn_abi.ret, &mut llargs, None);
+        let destination = target.map(|target| (return_dest, target));
 
         // Split the rust-call tupled arguments off.
         let (first_args, untuple) = if abi == ExternAbi::RustCall && !args.is_empty() {
@@ -1813,11 +1803,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         fn_ret: &ArgAbi<'tcx, Ty<'tcx>>,
         llargs: &mut Vec<Bx::Value>,
         intrinsic: Option<ty::IntrinsicDef>,
-        target: Option<BasicBlock>,
     ) -> ReturnDest<'tcx, Bx::Value> {
-        if target.is_none() {
-            return ReturnDest::Nothing;
-        }
         // If the return is ignored, we can just return a do-nothing `ReturnDest`.
         if fn_ret.is_ignore() {
             return ReturnDest::Nothing;
diff --git a/compiler/rustc_codegen_ssa/src/mir/operand.rs b/compiler/rustc_codegen_ssa/src/mir/operand.rs
index 958a52a2cb1dc..faf1300e4731e 100644
--- a/compiler/rustc_codegen_ssa/src/mir/operand.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/operand.rs
@@ -422,9 +422,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
                          See <https://github.com/rust-lang/rust/issues/137108>",
                     );
                 }
-                BackendRepr::Uninhabited
-                | BackendRepr::ScalarPair(_, _)
-                | BackendRepr::Memory { sized: false } => bug!(),
+                BackendRepr::ScalarPair(_, _) | BackendRepr::Memory { sized: false } => bug!(),
             })
         };
 
diff --git a/compiler/rustc_const_eval/src/interpret/operand.rs b/compiler/rustc_const_eval/src/interpret/operand.rs
index 5d905cff1f216..36da9037e43d9 100644
--- a/compiler/rustc_const_eval/src/interpret/operand.rs
+++ b/compiler/rustc_const_eval/src/interpret/operand.rs
@@ -385,7 +385,7 @@ impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
             (Immediate::Uninit, _) => Immediate::Uninit,
             // If the field is uninhabited, we can forget the data (can happen in ConstProp).
             // `enum S { A(!), B, C }` is an example of an enum with Scalar layout that
-            // has an `Uninhabited` variant, which means this case is possible.
+            // has an uninhabited variant, which means this case is possible.
             _ if layout.is_uninhabited() => Immediate::Uninit,
             // the field contains no information, can be left uninit
             // (Scalar/ScalarPair can contain even aligned ZST, not just 1-ZST)
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index 0ac34f4633b57..40dec0cb39e7b 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -1264,21 +1264,20 @@ impl<'rt, 'tcx, M: Machine<'tcx>> ValueVisitor<'tcx, M> for ValidityVisitor<'rt,
             }
         }
 
-        // *After* all of this, check the ABI. We need to check the ABI to handle
-        // types like `NonNull` where the `Scalar` info is more restrictive than what
-        // the fields say (`rustc_layout_scalar_valid_range_start`).
-        // But in most cases, this will just propagate what the fields say,
-        // and then we want the error to point at the field -- so, first recurse,
-        // then check ABI.
+        // *After* all of this, check further information stored in the layout. We need to check
+        // this to handle types like `NonNull` where the `Scalar` info is more restrictive than what
+        // the fields say (`rustc_layout_scalar_valid_range_start`). But in most cases, this will
+        // just propagate what the fields say, and then we want the error to point at the field --
+        // so, we first recurse, then we do this check.
         //
         // FIXME: We could avoid some redundant checks here. For newtypes wrapping
         // scalars, we do the same check on every "level" (e.g., first we check
         // MyNewtype and then the scalar in there).
+        if val.layout.is_uninhabited() {
+            let ty = val.layout.ty;
+            throw_validation_failure!(self.path, UninhabitedVal { ty });
+        }
         match val.layout.backend_repr {
-            BackendRepr::Uninhabited => {
-                let ty = val.layout.ty;
-                throw_validation_failure!(self.path, UninhabitedVal { ty });
-            }
             BackendRepr::Scalar(scalar_layout) => {
                 if !scalar_layout.is_uninit_valid() {
                     // There is something to check here.
diff --git a/compiler/rustc_const_eval/src/util/check_validity_requirement.rs b/compiler/rustc_const_eval/src/util/check_validity_requirement.rs
index 79baf91c3ce64..6426bca2332d4 100644
--- a/compiler/rustc_const_eval/src/util/check_validity_requirement.rs
+++ b/compiler/rustc_const_eval/src/util/check_validity_requirement.rs
@@ -111,13 +111,15 @@ fn check_validity_requirement_lax<'tcx>(
     };
 
     // Check the ABI.
-    let valid = match this.backend_repr {
-        BackendRepr::Uninhabited => false, // definitely UB
-        BackendRepr::Scalar(s) => scalar_allows_raw_init(s),
-        BackendRepr::ScalarPair(s1, s2) => scalar_allows_raw_init(s1) && scalar_allows_raw_init(s2),
-        BackendRepr::Vector { element: s, count } => count == 0 || scalar_allows_raw_init(s),
-        BackendRepr::Memory { .. } => true, // Fields are checked below.
-    };
+    let valid = !this.is_uninhabited() // definitely UB if uninhabited
+        && match this.backend_repr {
+            BackendRepr::Scalar(s) => scalar_allows_raw_init(s),
+            BackendRepr::ScalarPair(s1, s2) => {
+                scalar_allows_raw_init(s1) && scalar_allows_raw_init(s2)
+            }
+            BackendRepr::Vector { element: s, count } => count == 0 || scalar_allows_raw_init(s),
+            BackendRepr::Memory { .. } => true, // Fields are checked below.
+        };
     if !valid {
         // This is definitely not okay.
         return Ok(false);
diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index f98504e19decb..ec0d1dfa351c0 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -201,7 +201,7 @@ fn check_well_formed(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(), ErrorGua
         hir::Node::ImplItem(item) => check_impl_item(tcx, item),
         hir::Node::ForeignItem(item) => check_foreign_item(tcx, item),
         hir::Node::OpaqueTy(_) => Ok(crate::check::check::check_item_type(tcx, def_id)),
-        _ => unreachable!(),
+        _ => unreachable!("{node:?}"),
     };
 
     if let Some(generics) = node.generics() {
@@ -1108,7 +1108,13 @@ fn check_associated_item(
                 let ty = tcx.type_of(item.def_id).instantiate_identity();
                 let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
                 wfcx.register_wf_obligation(span, loc, ty.into());
-                check_sized_if_body(wfcx, item.def_id.expect_local(), ty, Some(span));
+                check_sized_if_body(
+                    wfcx,
+                    item.def_id.expect_local(),
+                    ty,
+                    Some(span),
+                    ObligationCauseCode::SizedConstOrStatic,
+                );
                 Ok(())
             }
             ty::AssocKind::Fn => {
@@ -1354,7 +1360,7 @@ fn check_item_type(
                 traits::ObligationCause::new(
                     ty_span,
                     wfcx.body_def_id,
-                    ObligationCauseCode::WellFormed(None),
+                    ObligationCauseCode::SizedConstOrStatic,
                 ),
                 wfcx.param_env,
                 item_ty,
@@ -1698,6 +1704,7 @@ fn check_fn_or_method<'tcx>(
             hir::FnRetTy::Return(ty) => Some(ty.span),
             hir::FnRetTy::DefaultReturn(_) => None,
         },
+        ObligationCauseCode::SizedReturnType,
     );
 }
 
@@ -1706,13 +1713,14 @@ fn check_sized_if_body<'tcx>(
     def_id: LocalDefId,
     ty: Ty<'tcx>,
     maybe_span: Option<Span>,
+    code: ObligationCauseCode<'tcx>,
 ) {
     let tcx = wfcx.tcx();
     if let Some(body) = tcx.hir_maybe_body_owned_by(def_id) {
         let span = maybe_span.unwrap_or(body.value.span);
 
         wfcx.register_bound(
-            ObligationCause::new(span, def_id, traits::ObligationCauseCode::SizedReturnType),
+            ObligationCause::new(span, def_id, code),
             wfcx.param_env,
             ty,
             tcx.require_lang_item(LangItem::Sized, Some(span)),
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index 1c63b8b365594..c39ed29d0d315 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -1810,7 +1810,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         crate::GatherLocalsVisitor::new(&fcx).visit_body(body);
 
         let ty = fcx.check_expr_with_expectation(body.value, expected);
-        fcx.require_type_is_sized(ty, body.value.span, ObligationCauseCode::ConstSized);
+        fcx.require_type_is_sized(ty, body.value.span, ObligationCauseCode::SizedConstOrStatic);
         fcx.write_ty(block.hir_id, ty);
         ty
     }
diff --git a/compiler/rustc_middle/src/mir/basic_blocks.rs b/compiler/rustc_middle/src/mir/basic_blocks.rs
index 107c3198525a1..171542d1279c7 100644
--- a/compiler/rustc_middle/src/mir/basic_blocks.rs
+++ b/compiler/rustc_middle/src/mir/basic_blocks.rs
@@ -79,7 +79,7 @@ impl<'tcx> BasicBlocks<'tcx> {
     #[inline]
     pub fn reverse_postorder(&self) -> &[BasicBlock] {
         self.cache.reverse_postorder.get_or_init(|| {
-            let mut rpo: Vec<_> = Postorder::new(&self.basic_blocks, START_BLOCK, ()).collect();
+            let mut rpo: Vec<_> = Postorder::new(&self.basic_blocks, START_BLOCK, None).collect();
             rpo.reverse();
             rpo
         })
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 71fed4b3e1e9c..0f1d9f0af7fc9 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -98,20 +98,13 @@ impl<'tcx> HasLocalDecls<'tcx> for Body<'tcx> {
 }
 
 impl MirPhase {
-    /// Gets the index of the current MirPhase within the set of all `MirPhase`s.
-    ///
-    /// FIXME(JakobDegen): Return a `(usize, usize)` instead.
-    pub fn phase_index(&self) -> usize {
-        const BUILT_PHASE_COUNT: usize = 1;
-        const ANALYSIS_PHASE_COUNT: usize = 2;
-        match self {
-            MirPhase::Built => 1,
-            MirPhase::Analysis(analysis_phase) => {
-                1 + BUILT_PHASE_COUNT + (*analysis_phase as usize)
-            }
-            MirPhase::Runtime(runtime_phase) => {
-                1 + BUILT_PHASE_COUNT + ANALYSIS_PHASE_COUNT + (*runtime_phase as usize)
-            }
+    /// Gets the (dialect, phase) index of the current `MirPhase`. Both numbers
+    /// are 1-indexed.
+    pub fn index(&self) -> (usize, usize) {
+        match *self {
+            MirPhase::Built => (1, 1),
+            MirPhase::Analysis(analysis_phase) => (2, 1 + analysis_phase as usize),
+            MirPhase::Runtime(runtime_phase) => (3, 1 + runtime_phase as usize),
         }
     }
 
diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index 1e3b8d029e1b3..dd3de48fc14fd 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -231,7 +231,8 @@ fn dump_path<'tcx>(
     let pass_num = if tcx.sess.opts.unstable_opts.dump_mir_exclude_pass_number {
         String::new()
     } else if pass_num {
-        format!(".{:03}-{:03}", body.phase.phase_index(), body.pass_count)
+        let (dialect_index, phase_index) = body.phase.index();
+        format!(".{}-{}-{:03}", dialect_index, phase_index, body.pass_count)
     } else {
         ".-------".to_string()
     };
diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index 389792a04f8a0..af6f0e4c55183 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -23,68 +23,77 @@ use crate::ty::{self, GenericArgsRef, List, Region, Ty, UserTypeAnnotationIndex}
 
 /// Represents the "flavors" of MIR.
 ///
-/// All flavors of MIR use the same data structure, but there are some important differences. These
-/// differences come in two forms: Dialects and phases.
+/// The MIR pipeline is structured into a few major dialects, with one or more phases within each
+/// dialect. A MIR flavor is identified by a dialect-phase pair. A single `MirPhase` value
+/// specifies such a pair. All flavors of MIR use the same data structure to represent the program.
 ///
-/// Dialects represent a stronger distinction than phases. This is because the transitions between
-/// dialects are semantic changes, and therefore technically *lowerings* between distinct IRs. In
-/// other words, the same [`Body`](crate::mir::Body) might be well-formed for multiple dialects, but
-/// have different semantic meaning and different behavior at runtime.
+/// Different MIR dialects have different semantics. (The differences between dialects are small,
+/// but they do exist.) The progression from one MIR dialect to the next is technically a lowering
+/// from one IR to another. In other words, a single well-formed [`Body`](crate::mir::Body) might
+/// have different semantic meaning and different behavior at runtime in the different dialects.
+/// The specific differences between dialects are described on the variants below.
 ///
-/// Each dialect additionally has a number of phases. However, phase changes never involve semantic
-/// changes. If some MIR is well-formed both before and after a phase change, it is also guaranteed
-/// that it has the same semantic meaning. In this sense, phase changes can only add additional
-/// restrictions on what MIR is well-formed.
+/// Phases exist only to place restrictions on what language constructs are permitted in
+/// well-formed MIR, and subsequent phases mostly increase those restrictions. I.e. to convert MIR
+/// from one phase to the next might require removing/replacing certain MIR constructs.
 ///
-/// When adding phases, remember to update [`MirPhase::phase_index`].
+/// When adding dialects or phases, remember to update [`MirPhase::index`].
 #[derive(Copy, Clone, TyEncodable, TyDecodable, Debug, PartialEq, Eq, PartialOrd, Ord)]
 #[derive(HashStable)]
 pub enum MirPhase {
-    /// The MIR that is generated by MIR building.
+    /// The "built MIR" dialect, as generated by MIR building.
     ///
     /// The only things that operate on this dialect are unsafeck, the various MIR lints, and const
     /// qualifs.
     ///
-    /// This has no distinct phases.
+    /// This dialect has just the one (implicit) phase, which places few restrictions on what MIR
+    /// constructs are allowed.
     Built,
-    /// The MIR used for most analysis.
+
+    /// The "analysis MIR" dialect, used for borrowck and friends.
     ///
-    /// The only semantic change between analysis and built MIR is constant promotion. In built MIR,
-    /// sequences of statements that would generally be subject to constant promotion are
-    /// semantically constants, while in analysis MIR all constants are explicit.
+    /// The only semantic difference between built MIR and analysis MIR relates to constant
+    /// promotion. In built MIR, sequences of statements that would generally be subject to
+    /// constant promotion are semantically constants, while in analysis MIR all constants are
+    /// explicit.
     ///
-    /// The result of const promotion is available from the `mir_promoted` and `promoted_mir` queries.
+    /// The result of const promotion is available from the `mir_promoted` and `promoted_mir`
+    /// queries.
     ///
-    /// This is the version of MIR used by borrowck and friends.
+    /// The phases of this dialect are described in `AnalysisPhase`.
     Analysis(AnalysisPhase),
-    /// The MIR used for CTFE, optimizations, and codegen.
-    ///
-    /// The semantic changes that occur in the lowering from analysis to runtime MIR are as follows:
-    ///
-    ///  - Drops: In analysis MIR, `Drop` terminators represent *conditional* drops; roughly speaking,
-    ///    if dataflow analysis determines that the place being dropped is uninitialized, the drop will
-    ///    not be executed. The exact semantics of this aren't written down anywhere, which means they
-    ///    are essentially "what drop elaboration does." In runtime MIR, the drops are unconditional;
-    ///    when a `Drop` terminator is reached, if the type has drop glue that drop glue is always
-    ///    executed. This may be UB if the underlying place is not initialized.
-    ///  - Packed drops: Places might in general be misaligned - in most cases this is UB, the exception
-    ///    is fields of packed structs. In analysis MIR, `Drop(P)` for a `P` that might be misaligned
-    ///    for this reason implicitly moves `P` to a temporary before dropping. Runtime MIR has no such
-    ///    rules, and dropping a misaligned place is simply UB.
-    ///  - Unwinding: in analysis MIR, unwinding from a function which may not unwind aborts. In runtime
-    ///    MIR, this is UB.
-    ///  - Retags: If `-Zmir-emit-retag` is enabled, analysis MIR has "implicit" retags in the same way
-    ///    that Rust itself has them. Where exactly these are is generally subject to change, and so we
-    ///    don't document this here. Runtime MIR has most retags explicit (though implicit retags
-    ///    can still occur at `Rvalue::{Ref,AddrOf}`).
-    ///  - Coroutine bodies: In analysis MIR, locals may actually be behind a pointer that user code has
-    ///    access to. This occurs in coroutine bodies. Such locals do not behave like other locals,
-    ///    because they eg may be aliased in surprising ways. Runtime MIR has no such special locals -
-    ///    all coroutine bodies are lowered and so all places that look like locals really are locals.
+
+    /// The "runtime MIR" dialect, used for CTFE, optimizations, and codegen.
+    ///
+    /// The semantic differences between analysis MIR and runtime MIR are as follows.
+    ///
+    /// - Drops: In analysis MIR, `Drop` terminators represent *conditional* drops; roughly
+    ///   speaking, if dataflow analysis determines that the place being dropped is uninitialized,
+    ///   the drop will not be executed. The exact semantics of this aren't written down anywhere,
+    ///   which means they are essentially "what drop elaboration does." In runtime MIR, the drops
+    ///   are unconditional; when a `Drop` terminator is reached, if the type has drop glue that
+    ///   drop glue is always executed. This may be UB if the underlying place is not initialized.
+    /// - Packed drops: Places might in general be misaligned - in most cases this is UB, the
+    ///   exception is fields of packed structs. In analysis MIR, `Drop(P)` for a `P` that might be
+    ///   misaligned for this reason implicitly moves `P` to a temporary before dropping. Runtime
+    ///   MIR has no such rules, and dropping a misaligned place is simply UB.
+    /// - Unwinding: in analysis MIR, unwinding from a function which may not unwind aborts. In
+    ///   runtime MIR, this is UB.
+    /// - Retags: If `-Zmir-emit-retag` is enabled, analysis MIR has "implicit" retags in the same
+    ///   way that Rust itself has them. Where exactly these are is generally subject to change,
+    ///   and so we don't document this here. Runtime MIR has most retags explicit (though implicit
+    ///   retags can still occur at `Rvalue::{Ref,AddrOf}`).
+    /// - Coroutine bodies: In analysis MIR, locals may actually be behind a pointer that user code
+    ///   has access to. This occurs in coroutine bodies. Such locals do not behave like other
+    ///   locals, because they e.g. may be aliased in surprising ways. Runtime MIR has no such
+    ///   special locals. All coroutine bodies are lowered and so all places that look like locals
+    ///   really are locals.
     ///
     /// Also note that the lint pass which reports eg `200_u8 + 200_u8` as an error is run as a part
     /// of analysis to runtime MIR lowering. To ensure lints are reported reliably, this means that
-    /// transformations which may suppress such errors should not run on analysis MIR.
+    /// transformations that can suppress such errors should not run on analysis MIR.
+    ///
+    /// The phases of this dialect are described in `RuntimePhase`.
     Runtime(RuntimePhase),
 }
 
@@ -111,7 +120,8 @@ pub enum AnalysisPhase {
     /// * [`TerminatorKind::FalseEdge`]
     /// * [`StatementKind::FakeRead`]
     /// * [`StatementKind::AscribeUserType`]
-    /// * [`StatementKind::Coverage`] with [`CoverageKind::BlockMarker`] or [`CoverageKind::SpanMarker`]
+    /// * [`StatementKind::Coverage`] with [`CoverageKind::BlockMarker`] or
+    ///   [`CoverageKind::SpanMarker`]
     /// * [`Rvalue::Ref`] with `BorrowKind::Fake`
     /// * [`CastKind::PointerCoercion`] with any of the following:
     ///   * [`PointerCoercion::ArrayToPointer`]
diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs
index 0e7dcc24dafa6..5950ac295af19 100644
--- a/compiler/rustc_middle/src/mir/traversal.rs
+++ b/compiler/rustc_middle/src/mir/traversal.rs
@@ -104,23 +104,21 @@ impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
 /// ```
 ///
 /// A Postorder traversal of this graph is `D B C A` or `D C B A`
-pub struct Postorder<'a, 'tcx, C> {
+pub struct Postorder<'a, 'tcx> {
     basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
     visited: DenseBitSet<BasicBlock>,
     visit_stack: Vec<(BasicBlock, Successors<'a>)>,
     root_is_start_block: bool,
-    extra: C,
+    /// A non-empty `extra` allows for a precise calculation of the successors.
+    extra: Option<(TyCtxt<'tcx>, Instance<'tcx>)>,
 }
 
-impl<'a, 'tcx, C> Postorder<'a, 'tcx, C>
-where
-    C: Customization<'tcx>,
-{
+impl<'a, 'tcx> Postorder<'a, 'tcx> {
     pub fn new(
         basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
         root: BasicBlock,
-        extra: C,
-    ) -> Postorder<'a, 'tcx, C> {
+        extra: Option<(TyCtxt<'tcx>, Instance<'tcx>)>,
+    ) -> Postorder<'a, 'tcx> {
         let mut po = Postorder {
             basic_blocks,
             visited: DenseBitSet::new_empty(basic_blocks.len()),
@@ -140,7 +138,11 @@ where
             return;
         }
         let data = &self.basic_blocks[bb];
-        let successors = C::successors(data, self.extra);
+        let successors = if let Some(extra) = self.extra {
+            data.mono_successors(extra.0, extra.1)
+        } else {
+            data.terminator().successors()
+        };
         self.visit_stack.push((bb, successors));
     }
 
@@ -198,10 +200,7 @@ where
     }
 }
 
-impl<'tcx, C> Iterator for Postorder<'_, 'tcx, C>
-where
-    C: Customization<'tcx>,
-{
+impl<'tcx> Iterator for Postorder<'_, 'tcx> {
     type Item = BasicBlock;
 
     fn next(&mut self) -> Option<BasicBlock> {
@@ -241,32 +240,12 @@ pub fn postorder<'a, 'tcx>(
     reverse_postorder(body).rev()
 }
 
-/// Lets us plug in some additional logic and data into a Postorder traversal. Or not.
-pub trait Customization<'tcx>: Copy {
-    fn successors<'a>(_: &'a BasicBlockData<'tcx>, _: Self) -> Successors<'a>;
-}
-
-impl<'tcx> Customization<'tcx> for () {
-    fn successors<'a>(data: &'a BasicBlockData<'tcx>, _: ()) -> Successors<'a> {
-        data.terminator().successors()
-    }
-}
-
-impl<'tcx> Customization<'tcx> for (TyCtxt<'tcx>, Instance<'tcx>) {
-    fn successors<'a>(
-        data: &'a BasicBlockData<'tcx>,
-        (tcx, instance): (TyCtxt<'tcx>, Instance<'tcx>),
-    ) -> Successors<'a> {
-        data.mono_successors(tcx, instance)
-    }
-}
-
 pub fn mono_reachable_reverse_postorder<'a, 'tcx>(
     body: &'a Body<'tcx>,
     tcx: TyCtxt<'tcx>,
     instance: Instance<'tcx>,
 ) -> Vec<BasicBlock> {
-    let mut iter = Postorder::new(&body.basic_blocks, START_BLOCK, (tcx, instance));
+    let mut iter = Postorder::new(&body.basic_blocks, START_BLOCK, Some((tcx, instance)));
     let mut items = Vec::with_capacity(body.basic_blocks.len());
     while let Some(block) = iter.next() {
         items.push(block);
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index f039da772fd4d..673751b7e100d 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -273,7 +273,7 @@ pub enum ObligationCauseCode<'tcx> {
     },
 
     /// Constant expressions must be sized.
-    ConstSized,
+    SizedConstOrStatic,
 
     /// `static` items must have `Sync` type.
     SharedStatic,
diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index 19fa832357499..eb14ed20fbace 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/compiler/rustc_middle/src/ty/layout.rs
@@ -794,8 +794,9 @@ where
                         Some(fields) => FieldsShape::Union(fields),
                         None => FieldsShape::Arbitrary { offsets: IndexVec::new(), memory_index: IndexVec::new() },
                     },
-                    backend_repr: BackendRepr::Uninhabited,
+                    backend_repr: BackendRepr::Memory { sized: true },
                     largest_niche: None,
+                    uninhabited: true,
                     align: tcx.data_layout.i8_align,
                     size: Size::ZERO,
                     max_repr_align: None,
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index 77a3854ebdef3..2f8a305019999 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -1558,8 +1558,11 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             return true;
         };
 
+        if layout.uninhabited {
+            return true;
+        }
+
         match layout.backend_repr {
-            BackendRepr::Uninhabited => true,
             BackendRepr::Scalar(a) => !a.is_always_valid(&self.ecx),
             BackendRepr::ScalarPair(a, b) => {
                 !a.is_always_valid(&self.ecx) || !b.is_always_valid(&self.ecx)
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 04c9375b83176..5df12ac4d8bc2 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -513,6 +513,24 @@ fn mir_drops_elaborated_and_const_checked(tcx: TyCtxt<'_>, def: LocalDefId) -> &
         body.tainted_by_errors = Some(error_reported);
     }
 
+    // Also taint the body if it's within a top-level item that is not well formed.
+    //
+    // We do this check here and not during `mir_promoted` because that may result
+    // in borrowck cycles if WF requires looking into an opaque hidden type.
+    let root = tcx.typeck_root_def_id(def.to_def_id());
+    match tcx.def_kind(root) {
+        DefKind::Fn
+        | DefKind::AssocFn
+        | DefKind::Static { .. }
+        | DefKind::Const
+        | DefKind::AssocConst => {
+            if let Err(guar) = tcx.check_well_formed(root.expect_local()) {
+                body.tainted_by_errors = Some(guar);
+            }
+        }
+        _ => {}
+    }
+
     run_analysis_to_runtime_passes(tcx, &mut body);
 
     tcx.alloc_steal_mir(body)
diff --git a/compiler/rustc_smir/src/rustc_smir/convert/abi.rs b/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
index fb2e838cdc998..a627e0e69b62f 100644
--- a/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
+++ b/compiler/rustc_smir/src/rustc_smir/convert/abi.rs
@@ -202,7 +202,6 @@ impl<'tcx> Stable<'tcx> for rustc_abi::BackendRepr {
 
     fn stable(&self, tables: &mut Tables<'_>) -> Self::T {
         match *self {
-            rustc_abi::BackendRepr::Uninhabited => ValueAbi::Uninhabited,
             rustc_abi::BackendRepr::Scalar(scalar) => ValueAbi::Scalar(scalar.stable(tables)),
             rustc_abi::BackendRepr::ScalarPair(first, second) => {
                 ValueAbi::ScalarPair(first.stable(tables), second.stable(tables))
diff --git a/compiler/rustc_target/src/callconv/loongarch.rs b/compiler/rustc_target/src/callconv/loongarch.rs
index 47566bde6b4a4..3fa67c624a7be 100644
--- a/compiler/rustc_target/src/callconv/loongarch.rs
+++ b/compiler/rustc_target/src/callconv/loongarch.rs
@@ -80,7 +80,7 @@ where
                 }
             }
         },
-        BackendRepr::Vector { .. } | BackendRepr::Uninhabited => return Err(CannotUseFpConv),
+        BackendRepr::Vector { .. } => return Err(CannotUseFpConv),
         BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => match arg_layout.fields {
             FieldsShape::Primitive => {
                 unreachable!("aggregates can't have `FieldsShape::Primitive`")
diff --git a/compiler/rustc_target/src/callconv/mod.rs b/compiler/rustc_target/src/callconv/mod.rs
index c2a176facdfa1..1c044fe98b3ef 100644
--- a/compiler/rustc_target/src/callconv/mod.rs
+++ b/compiler/rustc_target/src/callconv/mod.rs
@@ -38,7 +38,7 @@ mod xtensa;
 pub enum PassMode {
     /// Ignore the argument.
     ///
-    /// The argument is either uninhabited or a ZST.
+    /// The argument is a ZST.
     Ignore,
     /// Pass the argument directly.
     ///
@@ -350,7 +350,6 @@ impl<'a, Ty> ArgAbi<'a, Ty> {
         scalar_attrs: impl Fn(&TyAndLayout<'a, Ty>, Scalar, Size) -> ArgAttributes,
     ) -> Self {
         let mode = match layout.backend_repr {
-            BackendRepr::Uninhabited => PassMode::Ignore,
             BackendRepr::Scalar(scalar) => {
                 PassMode::Direct(scalar_attrs(&layout, scalar, Size::ZERO))
             }
@@ -737,7 +736,9 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             // to 128-bit-sized vectors.
             "x86" if spec.rustc_abi == Some(RustcAbi::X86Sse2) => arg.layout.size.bits() <= 128,
             "x86_64" if spec.rustc_abi != Some(RustcAbi::X86Softfloat) => {
-                arg.layout.size.bits() <= 128
+                // FIXME once https://github.com/bytecodealliance/wasmtime/issues/10254 is fixed
+                // accept vectors up to 128bit rather than vectors of exactly 128bit.
+                arg.layout.size.bits() == 128
             }
             // So far, we haven't implemented this logic for any other target.
             _ => false,
diff --git a/compiler/rustc_target/src/callconv/riscv.rs b/compiler/rustc_target/src/callconv/riscv.rs
index 265ae20f9919d..785175229b031 100644
--- a/compiler/rustc_target/src/callconv/riscv.rs
+++ b/compiler/rustc_target/src/callconv/riscv.rs
@@ -86,7 +86,7 @@ where
                 }
             }
         },
-        BackendRepr::Vector { .. } | BackendRepr::Uninhabited => return Err(CannotUseFpConv),
+        BackendRepr::Vector { .. } => return Err(CannotUseFpConv),
         BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => match arg_layout.fields {
             FieldsShape::Primitive => {
                 unreachable!("aggregates can't have `FieldsShape::Primitive`")
diff --git a/compiler/rustc_target/src/callconv/x86.rs b/compiler/rustc_target/src/callconv/x86.rs
index 5f4f4cd57f654..7e5aab0201b6d 100644
--- a/compiler/rustc_target/src/callconv/x86.rs
+++ b/compiler/rustc_target/src/callconv/x86.rs
@@ -108,9 +108,7 @@ where
                 Ty: TyAbiInterface<'a, C> + Copy,
             {
                 match layout.backend_repr {
-                    BackendRepr::Uninhabited
-                    | BackendRepr::Scalar(_)
-                    | BackendRepr::ScalarPair(..) => false,
+                    BackendRepr::Scalar(_) | BackendRepr::ScalarPair(..) => false,
                     BackendRepr::Vector { .. } => true,
                     BackendRepr::Memory { .. } => {
                         for i in 0..layout.fields.count() {
diff --git a/compiler/rustc_target/src/callconv/x86_64.rs b/compiler/rustc_target/src/callconv/x86_64.rs
index b15d82c26dac2..ab306e202396b 100644
--- a/compiler/rustc_target/src/callconv/x86_64.rs
+++ b/compiler/rustc_target/src/callconv/x86_64.rs
@@ -51,8 +51,6 @@ where
         }
 
         let mut c = match layout.backend_repr {
-            BackendRepr::Uninhabited => return Ok(()),
-
             BackendRepr::Scalar(scalar) => match scalar.primitive() {
                 Primitive::Int(..) | Primitive::Pointer(_) => Class::Int,
                 Primitive::Float(_) => Class::Sse,
diff --git a/compiler/rustc_target/src/callconv/x86_win64.rs b/compiler/rustc_target/src/callconv/x86_win64.rs
index 2ef5127de04ae..4d99a9f9ba064 100644
--- a/compiler/rustc_target/src/callconv/x86_win64.rs
+++ b/compiler/rustc_target/src/callconv/x86_win64.rs
@@ -8,7 +8,7 @@ use crate::spec::{HasTargetSpec, RustcAbi};
 pub(crate) fn compute_abi_info<Ty>(cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<'_, Ty>) {
     let fixup = |a: &mut ArgAbi<'_, Ty>, is_ret: bool| {
         match a.layout.backend_repr {
-            BackendRepr::Uninhabited | BackendRepr::Memory { sized: false } => {}
+            BackendRepr::Memory { sized: false } => {}
             BackendRepr::ScalarPair(..) | BackendRepr::Memory { sized: true } => {
                 match a.layout.size.bits() {
                     8 => a.cast_to(Reg::i8()),
diff --git a/compiler/rustc_target/src/spec/base/avr_gnu.rs b/compiler/rustc_target/src/spec/base/avr.rs
similarity index 91%
rename from compiler/rustc_target/src/spec/base/avr_gnu.rs
rename to compiler/rustc_target/src/spec/base/avr.rs
index 3554dcfcb4a13..85b73e61e52cd 100644
--- a/compiler/rustc_target/src/spec/base/avr_gnu.rs
+++ b/compiler/rustc_target/src/spec/base/avr.rs
@@ -1,45 +1,5 @@
 use object::elf;
 
-use crate::spec::{Cc, LinkerFlavor, Lld, RelocModel, Target, TargetOptions};
-
-/// A base target for AVR devices using the GNU toolchain.
-///
-/// Requires GNU avr-gcc and avr-binutils on the host system.
-/// FIXME: Remove the second parameter when const string concatenation is possible.
-pub(crate) fn target(target_cpu: &'static str, mmcu: &'static str) -> Target {
-    Target {
-        arch: "avr".into(),
-        metadata: crate::spec::TargetMetadata {
-            description: None,
-            tier: None,
-            host_tools: None,
-            std: None,
-        },
-        data_layout: "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8".into(),
-        llvm_target: "avr-unknown-unknown".into(),
-        pointer_width: 16,
-        options: TargetOptions {
-            env: "gnu".into(),
-
-            c_int_width: "16".into(),
-            cpu: target_cpu.into(),
-            exe_suffix: ".elf".into(),
-
-            linker: Some("avr-gcc".into()),
-            eh_frame_header: false,
-            pre_link_args: TargetOptions::link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &[mmcu]),
-            late_link_args: TargetOptions::link_args(
-                LinkerFlavor::Gnu(Cc::Yes, Lld::No),
-                &["-lgcc"],
-            ),
-            max_atomic_width: Some(16),
-            atomic_cas: false,
-            relocation_model: RelocModel::Static,
-            ..TargetOptions::default()
-        },
-    }
-}
-
 /// Resolve the value of the EF_AVR_ARCH field for AVR ELF files, given the
 /// name of the target CPU / MCU.
 ///
diff --git a/compiler/rustc_target/src/spec/base/mod.rs b/compiler/rustc_target/src/spec/base/mod.rs
index b9139c8452c5f..6f88be5d37f16 100644
--- a/compiler/rustc_target/src/spec/base/mod.rs
+++ b/compiler/rustc_target/src/spec/base/mod.rs
@@ -1,7 +1,7 @@
 pub(crate) mod aix;
 pub(crate) mod android;
 pub(crate) mod apple;
-pub(crate) mod avr_gnu;
+pub(crate) mod avr;
 pub(crate) mod bpf;
 pub(crate) mod cygwin;
 pub(crate) mod dragonfly;
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index f7e467b0c1155..65736770efb8c 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -60,7 +60,7 @@ pub mod crt_objects;
 mod base;
 mod json;
 
-pub use base::avr_gnu::ef_avr_arch;
+pub use base::avr::ef_avr_arch;
 
 /// Linker is called through a C/C++ compiler.
 #[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
@@ -303,15 +303,16 @@ impl LinkerFlavor {
         }
     }
 
-    fn infer_linker_hints(linker_stem: &str) -> (Option<Cc>, Option<Lld>) {
+    fn infer_linker_hints(linker_stem: &str) -> Result<Self, (Option<Cc>, Option<Lld>)> {
         // Remove any version postfix.
         let stem = linker_stem
             .rsplit_once('-')
             .and_then(|(lhs, rhs)| rhs.chars().all(char::is_numeric).then_some(lhs))
             .unwrap_or(linker_stem);
 
-        // GCC/Clang can have an optional target prefix.
-        if stem == "emcc"
+        if stem == "llvm-bitcode-linker" {
+            Ok(Self::Llbc)
+        } else if stem == "emcc" // GCC/Clang can have an optional target prefix.
             || stem == "gcc"
             || stem.ends_with("-gcc")
             || stem == "g++"
@@ -321,7 +322,7 @@ impl LinkerFlavor {
             || stem == "clang++"
             || stem.ends_with("-clang++")
         {
-            (Some(Cc::Yes), Some(Lld::No))
+            Err((Some(Cc::Yes), Some(Lld::No)))
         } else if stem == "wasm-ld"
             || stem.ends_with("-wasm-ld")
             || stem == "ld.lld"
@@ -329,11 +330,11 @@ impl LinkerFlavor {
             || stem == "rust-lld"
             || stem == "lld-link"
         {
-            (Some(Cc::No), Some(Lld::Yes))
+            Err((Some(Cc::No), Some(Lld::Yes)))
         } else if stem == "ld" || stem.ends_with("-ld") || stem == "link" {
-            (Some(Cc::No), Some(Lld::No))
+            Err((Some(Cc::No), Some(Lld::No)))
         } else {
-            (None, None)
+            Err((None, None))
         }
     }
 
@@ -357,7 +358,10 @@ impl LinkerFlavor {
     }
 
     pub fn with_linker_hints(self, linker_stem: &str) -> LinkerFlavor {
-        self.with_hints(LinkerFlavor::infer_linker_hints(linker_stem))
+        match LinkerFlavor::infer_linker_hints(linker_stem) {
+            Ok(linker_flavor) => linker_flavor,
+            Err(hints) => self.with_hints(hints),
+        }
     }
 
     pub fn check_compatibility(self, cli: LinkerFlavorCli) -> Option<String> {
@@ -1797,7 +1801,7 @@ supported_targets! {
     ("riscv64gc-unknown-fuchsia", riscv64gc_unknown_fuchsia),
     ("x86_64-unknown-fuchsia", x86_64_unknown_fuchsia),
 
-    ("avr-unknown-gnu-atmega328", avr_unknown_gnu_atmega328),
+    ("avr-none", avr_none),
 
     ("x86_64-unknown-l4re-uclibc", x86_64_unknown_l4re_uclibc),
 
@@ -2001,7 +2005,7 @@ supported_targets! {
     ("x86_64-pc-nto-qnx710", x86_64_pc_nto_qnx710),
     ("x86_64-pc-nto-qnx710_iosock", x86_64_pc_nto_qnx710_iosock),
     ("x86_64-pc-nto-qnx800", x86_64_pc_nto_qnx800),
-    ("i586-pc-nto-qnx700", i586_pc_nto_qnx700),
+    ("i686-pc-nto-qnx700", i686_pc_nto_qnx700),
 
     ("aarch64-unknown-linux-ohos", aarch64_unknown_linux_ohos),
     ("armv7-unknown-linux-ohos", armv7_unknown_linux_ohos),
@@ -3062,7 +3066,10 @@ impl Target {
             &self.post_link_args,
         ] {
             for (&flavor, flavor_args) in args {
-                check!(!flavor_args.is_empty(), "linker flavor args must not be empty");
+                check!(
+                    !flavor_args.is_empty() || self.arch == "avr",
+                    "linker flavor args must not be empty"
+                );
                 // Check that flavors mentioned in link args are compatible with the default flavor.
                 match self.linker_flavor {
                     LinkerFlavor::Gnu(..) => {
diff --git a/compiler/rustc_target/src/spec/targets/avr_none.rs b/compiler/rustc_target/src/spec/targets/avr_none.rs
new file mode 100644
index 0000000000000..91d3197d0998e
--- /dev/null
+++ b/compiler/rustc_target/src/spec/targets/avr_none.rs
@@ -0,0 +1,32 @@
+use crate::spec::{Cc, LinkerFlavor, Lld, RelocModel, Target, TargetOptions};
+
+pub(crate) fn target() -> Target {
+    Target {
+        arch: "avr".into(),
+        metadata: crate::spec::TargetMetadata {
+            description: None,
+            tier: None,
+            host_tools: None,
+            std: None,
+        },
+        data_layout: "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8".into(),
+        llvm_target: "avr-unknown-unknown".into(),
+        pointer_width: 16,
+        options: TargetOptions {
+            c_int_width: "16".into(),
+            exe_suffix: ".elf".into(),
+            linker: Some("avr-gcc".into()),
+            eh_frame_header: false,
+            pre_link_args: TargetOptions::link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &[]),
+            late_link_args: TargetOptions::link_args(
+                LinkerFlavor::Gnu(Cc::Yes, Lld::No),
+                &["-lgcc"],
+            ),
+            max_atomic_width: Some(16),
+            atomic_cas: false,
+            relocation_model: RelocModel::Static,
+            need_explicit_cpu: true,
+            ..TargetOptions::default()
+        },
+    }
+}
diff --git a/compiler/rustc_target/src/spec/targets/avr_unknown_gnu_atmega328.rs b/compiler/rustc_target/src/spec/targets/avr_unknown_gnu_atmega328.rs
deleted file mode 100644
index c3d6fb722733d..0000000000000
--- a/compiler/rustc_target/src/spec/targets/avr_unknown_gnu_atmega328.rs
+++ /dev/null
@@ -1,5 +0,0 @@
-use crate::spec::{Target, base};
-
-pub(crate) fn target() -> Target {
-    base::avr_gnu::target("atmega328", "-mmcu=atmega328")
-}
diff --git a/compiler/rustc_target/src/spec/targets/i586_pc_nto_qnx700.rs b/compiler/rustc_target/src/spec/targets/i686_pc_nto_qnx700.rs
similarity index 100%
rename from compiler/rustc_target/src/spec/targets/i586_pc_nto_qnx700.rs
rename to compiler/rustc_target/src/spec/targets/i686_pc_nto_qnx700.rs
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index ee0ab0dfbb82a..2de005ef0192d 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -3125,8 +3125,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     Applicability::MachineApplicable,
                 );
             }
-            ObligationCauseCode::ConstSized => {
-                err.note("constant expressions must have a statically known size");
+            ObligationCauseCode::SizedConstOrStatic => {
+                err.note("statics and constants must have a statically known size");
             }
             ObligationCauseCode::InlineAsmSized => {
                 err.note("all inline asm arguments must have a statically known size");
diff --git a/compiler/rustc_ty_utils/src/abi.rs b/compiler/rustc_ty_utils/src/abi.rs
index c5d2d0afbfad2..0ff82f0c256de 100644
--- a/compiler/rustc_ty_utils/src/abi.rs
+++ b/compiler/rustc_ty_utils/src/abi.rs
@@ -464,7 +464,7 @@ fn fn_abi_sanity_check<'tcx>(
 
         match &arg.mode {
             PassMode::Ignore => {
-                assert!(arg.layout.is_zst() || arg.layout.is_uninhabited());
+                assert!(arg.layout.is_zst());
             }
             PassMode::Direct(_) => {
                 // Here the Rust type is used to determine the actual ABI, so we have to be very
@@ -472,9 +472,7 @@ fn fn_abi_sanity_check<'tcx>(
                 // `layout.backend_repr` and ignore everything else. We should just reject
                 //`Aggregate` entirely here, but some targets need to be fixed first.
                 match arg.layout.backend_repr {
-                    BackendRepr::Uninhabited
-                    | BackendRepr::Scalar(_)
-                    | BackendRepr::Vector { .. } => {}
+                    BackendRepr::Scalar(_) | BackendRepr::Vector { .. } => {}
                     BackendRepr::ScalarPair(..) => {
                         panic!("`PassMode::Direct` used for ScalarPair type {}", arg.layout.ty)
                     }
diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
index 556512e0236e5..b6a14d147cad4 100644
--- a/compiler/rustc_ty_utils/src/layout.rs
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -348,19 +348,17 @@ fn layout_of_uncached<'tcx>(
                 .checked_mul(count, dl)
                 .ok_or_else(|| error(cx, LayoutError::SizeOverflow(ty)))?;
 
-            let abi = if count != 0 && ty.is_privately_uninhabited(tcx, cx.typing_env) {
-                BackendRepr::Uninhabited
-            } else {
-                BackendRepr::Memory { sized: true }
-            };
+            let abi = BackendRepr::Memory { sized: true };
 
             let largest_niche = if count != 0 { element.largest_niche } else { None };
+            let uninhabited = if count != 0 { element.uninhabited } else { false };
 
             tcx.mk_layout(LayoutData {
                 variants: Variants::Single { index: FIRST_VARIANT },
                 fields: FieldsShape::Array { stride: element.size, count },
                 backend_repr: abi,
                 largest_niche,
+                uninhabited,
                 align: element.align,
                 size,
                 max_repr_align: None,
@@ -375,6 +373,7 @@ fn layout_of_uncached<'tcx>(
                 fields: FieldsShape::Array { stride: element.size, count: 0 },
                 backend_repr: BackendRepr::Memory { sized: false },
                 largest_niche: None,
+                uninhabited: false,
                 align: element.align,
                 size: Size::ZERO,
                 max_repr_align: None,
@@ -390,6 +389,7 @@ fn layout_of_uncached<'tcx>(
             fields: FieldsShape::Array { stride: Size::from_bytes(1), count: 0 },
             backend_repr: BackendRepr::Memory { sized: false },
             largest_niche: None,
+            uninhabited: false,
             align: dl.i8_align,
             size: Size::ZERO,
             max_repr_align: None,
@@ -555,6 +555,7 @@ fn layout_of_uncached<'tcx>(
                 fields,
                 backend_repr: abi,
                 largest_niche: e_ly.largest_niche,
+                uninhabited: false,
                 size,
                 align,
                 max_repr_align: None,
@@ -1014,13 +1015,8 @@ fn coroutine_layout<'tcx>(
 
     size = size.align_to(align.abi);
 
-    let abi = if prefix.backend_repr.is_uninhabited()
-        || variants.iter().all(|v| v.backend_repr.is_uninhabited())
-    {
-        BackendRepr::Uninhabited
-    } else {
-        BackendRepr::Memory { sized: true }
-    };
+    let uninhabited = prefix.uninhabited || variants.iter().all(|v| v.is_uninhabited());
+    let abi = BackendRepr::Memory { sized: true };
 
     // this is similar to how ReprOptions populates its field_shuffle_seed
     let def_hash = tcx.def_path_hash(def_id).0.to_smaller_hash();
@@ -1041,6 +1037,7 @@ fn coroutine_layout<'tcx>(
         // See <https://github.com/rust-lang/rust/issues/63818>, <https://github.com/rust-lang/miri/issues/3780>.
         // FIXME: Remove when <https://github.com/rust-lang/rust/issues/125735> is implemented and aliased coroutine fields are wrapped in `UnsafePinned`.
         largest_niche: None,
+        uninhabited,
         size,
         align,
         max_repr_align: None,
diff --git a/compiler/rustc_ty_utils/src/layout/invariant.rs b/compiler/rustc_ty_utils/src/layout/invariant.rs
index 8d5403ed32468..5ea6716c5ca14 100644
--- a/compiler/rustc_ty_utils/src/layout/invariant.rs
+++ b/compiler/rustc_ty_utils/src/layout/invariant.rs
@@ -10,7 +10,11 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
 
     // Type-level uninhabitedness should always imply ABI uninhabitedness.
     if layout.ty.is_privately_uninhabited(tcx, cx.typing_env) {
-        assert!(layout.is_uninhabited());
+        assert!(
+            layout.is_uninhabited(),
+            "{:?} is type-level uninhabited but not ABI-uninhabited?",
+            layout.ty
+        );
     }
 
     if layout.size.bytes() % layout.align.abi.bytes() != 0 {
@@ -71,7 +75,7 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
         let Some((align, size)) = align.zip(size) else {
             assert_matches!(
                 layout.layout.backend_repr(),
-                BackendRepr::Uninhabited | BackendRepr::Memory { .. },
+                BackendRepr::Memory { .. },
                 "ABI unexpectedly missing alignment and/or size in {layout:#?}"
             );
             return;
@@ -235,7 +239,7 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
                 assert!(align >= element.align(cx).abi); // just sanity-checking `vector_align`.
                 // FIXME: Do some kind of check of the inner type, like for Scalar and ScalarPair.
             }
-            BackendRepr::Uninhabited | BackendRepr::Memory { .. } => {} // Nothing to check.
+            BackendRepr::Memory { .. } => {} // Nothing to check.
         }
     }
 
@@ -291,8 +295,8 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
                     || variant.is_uninhabited()
                 {
                     // These are never actually accessed anyway, so we can skip the coherence check
-                    // for them. They also fail that check, since they have
-                    // `Aggregate`/`Uninhabited` ABI even when the main type is
+                    // for them. They also fail that check, since they may have
+                    // a different ABI even when the main type is
                     // `Scalar`/`ScalarPair`. (Note that sometimes, variants with fields have size
                     // 0, and sometimes, variants without fields have non-0 size.)
                     continue;
@@ -306,7 +310,6 @@ pub(super) fn layout_sanity_check<'tcx>(cx: &LayoutCx<'tcx>, layout: &TyAndLayou
                     (BackendRepr::ScalarPair(a1, b1), BackendRepr::ScalarPair(a2, b2)) => {
                         scalar_coherent(a1, a2) && scalar_coherent(b1, b2)
                     }
-                    (BackendRepr::Uninhabited, _) => true,
                     (BackendRepr::Memory { .. }, _) => true,
                     _ => false,
                 };
diff --git a/compiler/stable_mir/src/abi.rs b/compiler/stable_mir/src/abi.rs
index 861b6692b5367..091f3e1a95e95 100644
--- a/compiler/stable_mir/src/abi.rs
+++ b/compiler/stable_mir/src/abi.rs
@@ -227,7 +227,6 @@ pub enum TagEncoding {
 /// in terms of categories of C types there are ABI rules for.
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Serialize)]
 pub enum ValueAbi {
-    Uninhabited,
     Scalar(Scalar),
     ScalarPair(Scalar, Scalar),
     Vector {
@@ -244,10 +243,7 @@ impl ValueAbi {
     /// Returns `true` if the layout corresponds to an unsized type.
     pub fn is_unsized(&self) -> bool {
         match *self {
-            ValueAbi::Uninhabited
-            | ValueAbi::Scalar(_)
-            | ValueAbi::ScalarPair(..)
-            | ValueAbi::Vector { .. } => false,
+            ValueAbi::Scalar(_) | ValueAbi::ScalarPair(..) | ValueAbi::Vector { .. } => false,
             ValueAbi::Aggregate { sized } => !sized,
         }
     }
diff --git a/config.example.toml b/config.example.toml
index fd27b24e46191..a17d3ec9f8841 100644
--- a/config.example.toml
+++ b/config.example.toml
@@ -793,12 +793,12 @@
 # C compiler to be used to compile C code. Note that the
 # default value is platform specific, and if not specified it may also depend on
 # what platform is crossing to what platform.
-# See `src/bootstrap/cc_detect.rs` for details.
+# See `src/bootstrap/src/utils/cc_detect.rs` for details.
 #cc = "cc" (path)
 
 # C++ compiler to be used to compile C++ code (e.g. LLVM and our LLVM shims).
 # This is only used for host targets.
-# See `src/bootstrap/cc_detect.rs` for details.
+# See `src/bootstrap/src/utils/cc_detect.rs` for details.
 #cxx = "c++" (path)
 
 # Archiver to be used to assemble static libraries compiled from C/C++ code.
diff --git a/library/std/src/sys/pal/windows/c.rs b/library/std/src/sys/pal/windows/c.rs
index 8dc61edb603f4..4fbdc839939c9 100644
--- a/library/std/src/sys/pal/windows/c.rs
+++ b/library/std/src/sys/pal/windows/c.rs
@@ -204,7 +204,7 @@ compat_fn_with_fallback! {
     pub fn NtReleaseKeyedEvent(
         EventHandle: HANDLE,
         Key: *const c_void,
-        Alertable: BOOLEAN,
+        Alertable: bool,
         Timeout: *mut i64
     ) -> NTSTATUS {
         panic!("keyed events not available")
@@ -213,7 +213,7 @@ compat_fn_with_fallback! {
     pub fn NtWaitForKeyedEvent(
         EventHandle: HANDLE,
         Key: *const c_void,
-        Alertable: BOOLEAN,
+        Alertable: bool,
         Timeout: *mut i64
     ) -> NTSTATUS {
         panic!("keyed events not available")
diff --git a/library/std/src/sys/random/windows.rs b/library/std/src/sys/random/windows.rs
index 7566000f9e6ff..f5da637f56ca9 100644
--- a/library/std/src/sys/random/windows.rs
+++ b/library/std/src/sys/random/windows.rs
@@ -14,7 +14,7 @@ pub fn fill_bytes(mut bytes: &mut [u8]) {
     while !bytes.is_empty() {
         let len = bytes.len().try_into().unwrap_or(u32::MAX);
         let ret = unsafe { c::RtlGenRandom(bytes.as_mut_ptr().cast(), len) };
-        assert_ne!(ret, 0, "failed to generate random data");
+        assert!(ret, "failed to generate random data");
         bytes = &mut bytes[len as usize..];
     }
 }
diff --git a/library/std/src/sys/sync/mutex/windows7.rs b/library/std/src/sys/sync/mutex/windows7.rs
index 689dba10f01ed..0b57de78ba6dd 100644
--- a/library/std/src/sys/sync/mutex/windows7.rs
+++ b/library/std/src/sys/sync/mutex/windows7.rs
@@ -44,7 +44,7 @@ impl Mutex {
 
     #[inline]
     pub fn try_lock(&self) -> bool {
-        unsafe { c::TryAcquireSRWLockExclusive(raw(self)) != 0 }
+        unsafe { c::TryAcquireSRWLockExclusive(raw(self)) }
     }
 
     #[inline]
diff --git a/library/std/src/sys/sync/thread_parking/windows7.rs b/library/std/src/sys/sync/thread_parking/windows7.rs
index f7585e882f055..a1a0f8427cd83 100644
--- a/library/std/src/sys/sync/thread_parking/windows7.rs
+++ b/library/std/src/sys/sync/thread_parking/windows7.rs
@@ -195,7 +195,7 @@ mod keyed_events {
 
     pub unsafe fn park(parker: Pin<&Parker>) {
         // Wait for unpark() to produce this event.
-        c::NtWaitForKeyedEvent(keyed_event_handle(), parker.ptr(), 0, ptr::null_mut());
+        c::NtWaitForKeyedEvent(keyed_event_handle(), parker.ptr(), false, ptr::null_mut());
         // Set the state back to EMPTY (from either PARKED or NOTIFIED).
         // Note that we don't just write EMPTY, but use swap() to also
         // include an acquire-ordered read to synchronize with unpark()'s
@@ -218,7 +218,7 @@ mod keyed_events {
 
         // Wait for unpark() to produce this event.
         let unparked =
-            c::NtWaitForKeyedEvent(handle, parker.ptr(), 0, &mut timeout) == c::STATUS_SUCCESS;
+            c::NtWaitForKeyedEvent(handle, parker.ptr(), false, &mut timeout) == c::STATUS_SUCCESS;
 
         // Set the state back to EMPTY (from either PARKED or NOTIFIED).
         let prev_state = parker.state.swap(EMPTY, Acquire);
@@ -228,7 +228,7 @@ mod keyed_events {
             // was set to NOTIFIED, which means we *just* missed an
             // unpark(), which is now blocked on us to wait for it.
             // Wait for it to consume the event and unblock that thread.
-            c::NtWaitForKeyedEvent(handle, parker.ptr(), 0, ptr::null_mut());
+            c::NtWaitForKeyedEvent(handle, parker.ptr(), false, ptr::null_mut());
         }
     }
     pub unsafe fn unpark(parker: Pin<&Parker>) {
@@ -239,7 +239,7 @@ mod keyed_events {
         // To prevent this thread from blocking indefinitely in that case,
         // park_impl() will, after seeing the state set to NOTIFIED after
         // waking up, call NtWaitForKeyedEvent again to unblock us.
-        c::NtReleaseKeyedEvent(keyed_event_handle(), parker.ptr(), 0, ptr::null_mut());
+        c::NtReleaseKeyedEvent(keyed_event_handle(), parker.ptr(), false, ptr::null_mut());
     }
 
     fn keyed_event_handle() -> c::HANDLE {
diff --git a/library/std/src/sys/thread_local/os.rs b/library/std/src/sys/thread_local/os.rs
index 00d2e30bd6036..fe6af27db3a17 100644
--- a/library/std/src/sys/thread_local/os.rs
+++ b/library/std/src/sys/thread_local/os.rs
@@ -28,9 +28,7 @@ pub macro thread_local_inner {
         // user provided type or type alias with a matching name. Please update the shadowing test
         // in `tests/thread.rs` if these types are renamed.
         unsafe {
-            // Inlining does not work on windows-gnu due to linking errors around
-            // dllimports. See https://github.com/rust-lang/rust/issues/109797.
-            $crate::thread::LocalKey::new(#[cfg_attr(windows, inline(never))] |init| {
+            $crate::thread::LocalKey::new(|init| {
                 static VAL: $crate::thread::local_impl::Storage<$t>
                     = $crate::thread::local_impl::Storage::new();
                 VAL.get(init, __init)
diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index b5cd3864620d3..c4e5c1aac2f5d 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -301,7 +301,7 @@ target | std | host | notes
 [`armv8r-none-eabihf`](platform-support/armv8r-none-eabihf.md) | * |  | Bare Armv8-R, hardfloat
 [`armv7a-nuttx-eabi`](platform-support/nuttx.md) | ✓ |  | ARMv7-A with NuttX
 [`armv7a-nuttx-eabihf`](platform-support/nuttx.md) | ✓ |  | ARMv7-A with NuttX, hardfloat
-`avr-unknown-gnu-atmega328` | * |  | AVR. Requires `-Z build-std=core`
+`avr-none` | * |  | AVR; requires `-Zbuild-std=core` and `-Ctarget-cpu=...`
 `bpfeb-unknown-none` | * |  | BPF (big endian)
 `bpfel-unknown-none` | * |  | BPF (little endian)
 `csky-unknown-linux-gnuabiv2` | ✓ |  | C-SKY abiv2 Linux (little endian)
@@ -309,10 +309,10 @@ target | std | host | notes
 [`hexagon-unknown-linux-musl`](platform-support/hexagon-unknown-linux-musl.md) | ✓ | | Hexagon Linux with musl 1.2.3
 [`hexagon-unknown-none-elf`](platform-support/hexagon-unknown-none-elf.md)| * | | Bare Hexagon (v60+, HVX)
 [`i386-apple-ios`](platform-support/apple-ios.md) | ✓ |  | 32-bit x86 iOS (Penryn) [^x86_32-floats-return-ABI]
-[`i586-pc-nto-qnx700`](platform-support/nto-qnx.md) | * |  | 32-bit x86 QNX Neutrino 7.0 RTOS (Pentium 4) [^x86_32-floats-return-ABI]
 [`i586-unknown-netbsd`](platform-support/netbsd.md) | ✓ |  | 32-bit x86 (original Pentium) [^x86_32-floats-x87]
 [`i586-unknown-redox`](platform-support/redox.md) | ✓ |  | 32-bit x86 Redox OS (PentiumPro) [^x86_32-floats-x87]
 [`i686-apple-darwin`](platform-support/apple-darwin.md) | ✓ | ✓ | 32-bit macOS (10.12+, Sierra+, Penryn) [^x86_32-floats-return-ABI]
+[`i686-pc-nto-qnx700`](platform-support/nto-qnx.md) | * |  | 32-bit x86 QNX Neutrino 7.0 RTOS (Pentium 4) [^x86_32-floats-return-ABI]
 `i686-unknown-haiku` | ✓ | ✓ | 32-bit Haiku (Pentium 4) [^x86_32-floats-return-ABI]
 [`i686-unknown-hurd-gnu`](platform-support/hurd.md) | ✓ | ✓ | 32-bit GNU/Hurd (Pentium 4) [^x86_32-floats-return-ABI]
 [`i686-unknown-netbsd`](platform-support/netbsd.md) | ✓ | ✓ | NetBSD/i386 (Pentium 4) [^x86_32-floats-return-ABI]
diff --git a/src/doc/rustc/src/platform-support/avr-none.md b/src/doc/rustc/src/platform-support/avr-none.md
new file mode 100644
index 0000000000000..9c1836222c103
--- /dev/null
+++ b/src/doc/rustc/src/platform-support/avr-none.md
@@ -0,0 +1,81 @@
+# `avr-none`
+
+Series of microcontrollers from Atmel: ATmega8, ATmega328p etc.
+
+**Tier: 3**
+
+## Target maintainers
+
+- [Patryk Wychowaniec](https://github.com/Patryk27) <pwychowaniec@pm.me>
+
+## Requirements
+
+This target is only cross-compiled; x86-64 Linux, x86-64 macOS and aarch64 macOS
+hosts are confirmed to work, but in principle any machine able to run rustc and
+avr-gcc should be good.
+
+Compiling for this target requires `avr-gcc` installed, because a couple of
+intrinsics (like 32-bit multiplication) rely on [`libgcc`](https://github.com/gcc-mirror/gcc/blob/3269a722b7a03613e9c4e2862bc5088c4a17cc11/libgcc/config/avr/lib1funcs.S)
+and can't be provided through `compiler-builtins` yet. This is a limitation that
+[we hope to lift in the future](https://github.com/rust-lang/compiler-builtins/issues/711).
+
+You'll also need to setup the `.cargo/config` file - see below for details.
+
+## Building the target
+
+Rust comes with AVR support enabled, you don't have to rebuild the compiler.
+
+## Building Rust programs
+
+Install `avr-gcc`:
+
+```console
+# Ubuntu:
+$ sudo apt-get install gcc-avr
+
+# Mac:
+$ brew tap osx-cross/avr && brew install avr-gcc
+
+# NixOS (takes a couple of minutes, since it's not provided through Hydra):
+$ nix shell nixpkgs#pkgsCross.avr.buildPackages.gcc11
+```
+
+... setup `.cargo/config` for your project:
+
+```toml
+[build]
+target = "avr-none"
+rustflags = ["-C", "target-cpu=atmega328p"]
+
+[unstable]
+build-std = ["core"]
+```
+
+... and then simply run:
+
+```console
+$ cargo build --release
+```
+
+The final binary will be placed into
+`./target/avr-none/release/your-project.elf`.
+
+Note that since AVRs have rather small amounts of registers, ROM and RAM, it's
+recommended to always use `--release` to avoid running out of space.
+
+Also, please note that specifying `-C target-cpu` is required - here's a list of
+the possible variants:
+
+https://github.com/llvm/llvm-project/blob/093d4db2f3c874d4683fb01194b00dbb20e5c713/clang/lib/Basic/Targets/AVR.cpp#L32
+
+## Testing
+
+You can use [`simavr`](https://github.com/buserror/simavr) to emulate the
+resulting firmware on your machine:
+
+```console
+$ simavr -m atmega328p ./target/avr-none/release/your-project.elf
+```
+
+Alternatively, if you want to write a couple of actual `#[test]`s, you can use
+[`avr-tester`](https://github.com/Patryk27/avr-tester).
diff --git a/src/doc/rustc/src/platform-support/nto-qnx.md b/src/doc/rustc/src/platform-support/nto-qnx.md
index 339741f14727d..77e8caaee4cdc 100644
--- a/src/doc/rustc/src/platform-support/nto-qnx.md
+++ b/src/doc/rustc/src/platform-support/nto-qnx.md
@@ -31,7 +31,7 @@ Currently, the following QNX versions and compilation targets are supported:
 | `aarch64-unknown-nto-qnx710_iosock` | QNX Neutrino 7.1 with io-sock | AArch64             |      ?       |        ✓         |
 | `x86_64-pc-nto-qnx710_iosock`       | QNX Neutrino 7.1 with io-sock | x86_64              |      ?       |        ✓         |
 | `aarch64-unknown-nto-qnx700`        | QNX Neutrino 7.0              | AArch64             |      ?       |        ✓         |
-| `i586-pc-nto-qnx700`                | QNX Neutrino 7.0              | x86                 |              |        ✓         |
+| `i686-pc-nto-qnx700`                | QNX Neutrino 7.0              | x86                 |              |        ✓         |
 
 On QNX Neutrino 7.0 and 7.1, `io-pkt` is used as network stack by default.
 QNX Neutrino 7.1 includes the optional network stack `io-sock`.
diff --git a/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md b/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md
index 73264aba85809..ba95ab7af6d1a 100644
--- a/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md
+++ b/src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md
@@ -129,6 +129,8 @@ As of the time of this writing the proposals that are enabled by default (the
 * `mutable-globals`
 * `reference-types`
 * `sign-ext`
+* `nontrapping-fptoint` (Rust 1.87.0+, LLVM 20+)
+* `bulk-memory` (Rust 1.87.0+, LLVM 20+)
 
 If you're compiling WebAssembly code for an engine that does not support a
 feature in LLVM's default feature set then the feature must be disabled at
diff --git a/src/tools/clippy/clippy_utils/src/mir/mod.rs b/src/tools/clippy/clippy_utils/src/mir/mod.rs
index 637c0bafd964a..ffcfcd240ea5a 100644
--- a/src/tools/clippy/clippy_utils/src/mir/mod.rs
+++ b/src/tools/clippy/clippy_utils/src/mir/mod.rs
@@ -30,7 +30,7 @@ pub fn visit_local_usage(locals: &[Local], mir: &Body<'_>, location: Location) -
         locals.len()
     ];
 
-    traversal::Postorder::new(&mir.basic_blocks, location.block, ())
+    traversal::Postorder::new(&mir.basic_blocks, location.block, None)
         .collect::<Vec<_>>()
         .into_iter()
         .rev()
diff --git a/src/tools/compiletest/src/header/tests.rs b/src/tools/compiletest/src/header/tests.rs
index 55292c46bba9c..c65f10a52bd5b 100644
--- a/src/tools/compiletest/src/header/tests.rs
+++ b/src/tools/compiletest/src/header/tests.rs
@@ -465,7 +465,10 @@ fn profiler_runtime() {
 #[test]
 fn asm_support() {
     let asms = [
+        #[cfg(bootstrap)]
         ("avr-unknown-gnu-atmega328", false),
+        #[cfg(not(bootstrap))]
+        ("avr-none", false),
         ("i686-unknown-netbsd", true),
         ("riscv32gc-unknown-linux-gnu", true),
         ("riscv64imac-unknown-none-elf", true),
diff --git a/src/tools/llvm-bitcode-linker/src/bin/llvm-bitcode-linker.rs b/src/tools/llvm-bitcode-linker/src/bin/llvm-bitcode-linker.rs
index fdbf6171c53f2..608c6605304f4 100644
--- a/src/tools/llvm-bitcode-linker/src/bin/llvm-bitcode-linker.rs
+++ b/src/tools/llvm-bitcode-linker/src/bin/llvm-bitcode-linker.rs
@@ -27,6 +27,10 @@ pub struct Args {
     #[arg(long)]
     target_cpu: Option<String>,
 
+    /// The target features
+    #[arg(long)]
+    target_feature: Option<String>,
+
     /// Write output to the filename
     #[arg(short, long)]
     output: PathBuf,
@@ -49,7 +53,7 @@ fn main() -> anyhow::Result<()> {
 
     let args = Args::parse();
 
-    let mut linker = Session::new(args.target, args.target_cpu, args.output);
+    let mut linker = Session::new(args.target, args.target_cpu, args.target_feature, args.output);
 
     linker.add_exported_symbols(args.export_symbol);
 
diff --git a/src/tools/llvm-bitcode-linker/src/linker.rs b/src/tools/llvm-bitcode-linker/src/linker.rs
index 9f579d1009408..dafd847e76808 100644
--- a/src/tools/llvm-bitcode-linker/src/linker.rs
+++ b/src/tools/llvm-bitcode-linker/src/linker.rs
@@ -8,6 +8,7 @@ use crate::{Optimization, Target};
 pub struct Session {
     target: Target,
     cpu: Option<String>,
+    feature: Option<String>,
     symbols: Vec<String>,
 
     /// A file that `llvm-link` supports, like a bitcode file or an archive.
@@ -21,7 +22,12 @@ pub struct Session {
 }
 
 impl Session {
-    pub fn new(target: crate::Target, cpu: Option<String>, out_path: PathBuf) -> Self {
+    pub fn new(
+        target: crate::Target,
+        cpu: Option<String>,
+        feature: Option<String>,
+        out_path: PathBuf,
+    ) -> Self {
         let link_path = out_path.with_extension("o");
         let opt_path = out_path.with_extension("optimized.o");
         let sym_path = out_path.with_extension("symbols.txt");
@@ -29,6 +35,7 @@ impl Session {
         Session {
             target,
             cpu,
+            feature,
             symbols: Vec::new(),
             files: Vec::new(),
             link_path,
@@ -134,6 +141,10 @@ impl Session {
             lcc_command.arg("--mcpu").arg(mcpu);
         }
 
+        if let Some(mattr) = &self.feature {
+            lcc_command.arg(&format!("--mattr={}", mattr));
+        }
+
         let lcc_output = lcc_command
             .arg(&self.opt_path)
             .arg("-o").arg(&self.out_path)
diff --git a/src/tools/run-make-support/src/external_deps/rustc.rs b/src/tools/run-make-support/src/external_deps/rustc.rs
index fd4a20278addd..710ba025830a8 100644
--- a/src/tools/run-make-support/src/external_deps/rustc.rs
+++ b/src/tools/run-make-support/src/external_deps/rustc.rs
@@ -253,6 +253,13 @@ impl Rustc {
         self
     }
 
+    /// Specify the target CPU.
+    pub fn target_cpu<S: AsRef<str>>(&mut self, target_cpu: S) -> &mut Self {
+        let target_cpu = target_cpu.as_ref();
+        self.cmd.arg(format!("-Ctarget-cpu={target_cpu}"));
+        self
+    }
+
     /// Specify the crate type.
     pub fn crate_type(&mut self, crate_type: &str) -> &mut Self {
         self.cmd.arg("--crate-type");
diff --git a/src/tools/rust-analyzer/crates/hir-ty/src/layout.rs b/src/tools/rust-analyzer/crates/hir-ty/src/layout.rs
index b6f7c44c2aeee..f5a7b6581230f 100644
--- a/src/tools/rust-analyzer/crates/hir-ty/src/layout.rs
+++ b/src/tools/rust-analyzer/crates/hir-ty/src/layout.rs
@@ -194,6 +194,7 @@ fn layout_of_simd_ty(
         fields,
         backend_repr: BackendRepr::Vector { element: e_abi, count: e_len },
         largest_niche: e_ly.largest_niche,
+        uninhabited: false,
         size,
         align,
         max_repr_align: None,
@@ -297,20 +298,17 @@ pub fn layout_of_ty_query(
                 .checked_mul(count, dl)
                 .ok_or(LayoutError::BadCalc(LayoutCalculatorError::SizeOverflow))?;
 
-            let backend_repr =
-                if count != 0 && matches!(element.backend_repr, BackendRepr::Uninhabited) {
-                    BackendRepr::Uninhabited
-                } else {
-                    BackendRepr::Memory { sized: true }
-                };
+            let backend_repr = BackendRepr::Memory { sized: true };
 
             let largest_niche = if count != 0 { element.largest_niche } else { None };
+            let uninhabited = if count != 0 { element.uninhabited } else { false };
 
             Layout {
                 variants: Variants::Single { index: struct_variant_idx() },
                 fields: FieldsShape::Array { stride: element.size, count },
                 backend_repr,
                 largest_niche,
+                uninhabited,
                 align: element.align,
                 size,
                 max_repr_align: None,
@@ -325,6 +323,7 @@ pub fn layout_of_ty_query(
                 fields: FieldsShape::Array { stride: element.size, count: 0 },
                 backend_repr: BackendRepr::Memory { sized: false },
                 largest_niche: None,
+                uninhabited: false,
                 align: element.align,
                 size: Size::ZERO,
                 max_repr_align: None,
@@ -337,6 +336,7 @@ pub fn layout_of_ty_query(
             fields: FieldsShape::Array { stride: Size::from_bytes(1), count: 0 },
             backend_repr: BackendRepr::Memory { sized: false },
             largest_niche: None,
+            uninhabited: false,
             align: dl.i8_align,
             size: Size::ZERO,
             max_repr_align: None,
diff --git a/src/tools/tidy/src/target_policy.rs b/src/tools/tidy/src/target_policy.rs
index 776221d306233..b2681934417cb 100644
--- a/src/tools/tidy/src/target_policy.rs
+++ b/src/tools/tidy/src/target_policy.rs
@@ -21,6 +21,7 @@ const EXCEPTIONS: &[&str] = &[
     "xtensa_esp32s2_espidf",
     "xtensa_esp32s3_none_elf",
     "xtensa_esp32s3_espidf",
+    "i586_pc_nto_qnx700", // Renamed to i686-pc-nto-qnx700, see https://github.com/rust-lang/rust/issues/136495
 ];
 
 pub fn check(root_path: &Path, bad: &mut bool) {
diff --git a/tests/assembly/asm/avr-modifiers.rs b/tests/assembly/asm/avr-modifiers.rs
index 585fdd7b72534..124cad9bef6a8 100644
--- a/tests/assembly/asm/avr-modifiers.rs
+++ b/tests/assembly/asm/avr-modifiers.rs
@@ -1,6 +1,6 @@
 //@ add-core-stubs
 //@ assembly-output: emit-asm
-//@ compile-flags: --target avr-unknown-gnu-atmega328
+//@ compile-flags: --target avr-none -C target-cpu=atmega328p
 //@ needs-llvm-components: avr
 
 #![feature(no_core, asm_experimental_arch)]
diff --git a/tests/assembly/asm/avr-types.rs b/tests/assembly/asm/avr-types.rs
index 25cf3ec3b4bde..309405f4d51e7 100644
--- a/tests/assembly/asm/avr-types.rs
+++ b/tests/assembly/asm/avr-types.rs
@@ -1,6 +1,6 @@
 //@ add-core-stubs
 //@ assembly-output: emit-asm
-//@ compile-flags: --target avr-unknown-gnu-atmega328
+//@ compile-flags: --target avr-none -C target-cpu=atmega328p
 //@ needs-llvm-components: avr
 
 #![feature(no_core, asm_experimental_arch)]
diff --git a/tests/assembly/targets/targets-elf.rs b/tests/assembly/targets/targets-elf.rs
index b5c0ee5a10741..3e73d2c6eb72d 100644
--- a/tests/assembly/targets/targets-elf.rs
+++ b/tests/assembly/targets/targets-elf.rs
@@ -216,9 +216,9 @@
 //@ revisions: hexagon_unknown_none_elf
 //@ [hexagon_unknown_none_elf] compile-flags: --target hexagon-unknown-none-elf
 //@ [hexagon_unknown_none_elf] needs-llvm-components: hexagon
-//@ revisions: i586_pc_nto_qnx700
-//@ [i586_pc_nto_qnx700] compile-flags: --target i586-pc-nto-qnx700
-//@ [i586_pc_nto_qnx700] needs-llvm-components: x86
+//@ revisions: i686_pc_nto_qnx700
+//@ [i686_pc_nto_qnx700] compile-flags: --target i686-pc-nto-qnx700
+//@ [i686_pc_nto_qnx700] needs-llvm-components: x86
 //@ revisions: i586_unknown_linux_gnu
 //@ [i586_unknown_linux_gnu] compile-flags: --target i586-unknown-linux-gnu
 //@ [i586_unknown_linux_gnu] needs-llvm-components: x86
diff --git a/tests/assembly/targets/targets-pe.rs b/tests/assembly/targets/targets-pe.rs
index 1fa4dc821dd37..b74d0181c4d67 100644
--- a/tests/assembly/targets/targets-pe.rs
+++ b/tests/assembly/targets/targets-pe.rs
@@ -15,9 +15,9 @@
 //@ revisions: arm64ec_pc_windows_msvc
 //@ [arm64ec_pc_windows_msvc] compile-flags: --target arm64ec-pc-windows-msvc
 //@ [arm64ec_pc_windows_msvc] needs-llvm-components: aarch64
-//@ revisions: avr_unknown_gnu_atmega328
-//@ [avr_unknown_gnu_atmega328] compile-flags: --target avr-unknown-gnu-atmega328
-//@ [avr_unknown_gnu_atmega328] needs-llvm-components: avr
+//@ revisions: avr_none
+//@ [avr_none] compile-flags: --target avr-none -C target-cpu=atmega328p
+//@ [avr_none] needs-llvm-components: avr
 //@ revisions: bpfeb_unknown_none
 //@ [bpfeb_unknown_none] compile-flags: --target bpfeb-unknown-none
 //@ [bpfeb_unknown_none] needs-llvm-components: bpf
diff --git a/tests/codegen/asm/avr-clobbers.rs b/tests/codegen/asm/avr-clobbers.rs
index 56218cd7bcffb..9451127bf04ab 100644
--- a/tests/codegen/asm/avr-clobbers.rs
+++ b/tests/codegen/asm/avr-clobbers.rs
@@ -1,6 +1,6 @@
 //@ add-core-stubs
 //@ assembly-output: emit-asm
-//@ compile-flags: --target avr-unknown-gnu-atmega328
+//@ compile-flags: --target avr-none -C target-cpu=atmega328p
 //@ needs-llvm-components: avr
 
 #![crate_type = "rlib"]
diff --git a/tests/codegen/avr/avr-func-addrspace.rs b/tests/codegen/avr/avr-func-addrspace.rs
index ed8acccb1ad85..2ec7c86b5f43c 100644
--- a/tests/codegen/avr/avr-func-addrspace.rs
+++ b/tests/codegen/avr/avr-func-addrspace.rs
@@ -1,4 +1,4 @@
-//@ compile-flags: -Copt-level=3 --target=avr-unknown-gnu-atmega328 --crate-type=rlib -C panic=abort
+//@ compile-flags: -Copt-level=3 --target=avr-none -C target-cpu=atmega328p --crate-type=rlib -C panic=abort
 //@ needs-llvm-components: avr
 
 // This test validates that function pointers can be stored in global variables
diff --git a/tests/codegen/uninhabited-transparent-return-abi.rs b/tests/codegen/uninhabited-transparent-return-abi.rs
new file mode 100644
index 0000000000000..6e8b1683163ef
--- /dev/null
+++ b/tests/codegen/uninhabited-transparent-return-abi.rs
@@ -0,0 +1,44 @@
+//@ compile-flags: -Copt-level=3
+
+// See https://github.com/rust-lang/rust/issues/135802
+
+#![crate_type = "lib"]
+
+enum Void {}
+
+// Should be ABI-compatible with T, but wasn't prior to the PR adding this test.
+#[repr(transparent)]
+struct NoReturn<T>(T, Void);
+
+// Returned by invisible reference (in most ABIs)
+#[allow(dead_code)]
+struct Large(u64, u64, u64);
+
+extern "Rust" {
+    fn opaque() -> NoReturn<Large>;
+    fn opaque_with_arg(rsi: u32) -> NoReturn<Large>;
+}
+
+// CHECK-LABEL: @test_uninhabited_ret_by_ref
+#[no_mangle]
+pub fn test_uninhabited_ret_by_ref() {
+    // CHECK: %_1 = alloca [24 x i8], align {{8|4}}
+    // CHECK-NEXT: call void @llvm.lifetime.start.p0(i64 24, ptr nonnull %_1)
+    // CHECK-NEXT: call void @opaque(ptr noalias nocapture noundef nonnull sret([24 x i8]) align {{8|4}} dereferenceable(24) %_1) #2
+    // CHECK-NEXT: unreachable
+    unsafe {
+        opaque();
+    }
+}
+
+// CHECK-LABEL: @test_uninhabited_ret_by_ref_with_arg
+#[no_mangle]
+pub fn test_uninhabited_ret_by_ref_with_arg(rsi: u32) {
+    // CHECK: %_2 = alloca [24 x i8], align {{8|4}}
+    // CHECK-NEXT: call void @llvm.lifetime.start.p0(i64 24, ptr nonnull %_2)
+    // CHECK-NEXT: call void @opaque_with_arg(ptr noalias nocapture noundef nonnull sret([24 x i8]) align {{8|4}} dereferenceable(24) %_2, i32 noundef %rsi) #2
+    // CHECK-NEXT: unreachable
+    unsafe {
+        opaque_with_arg(rsi);
+    }
+}
diff --git a/tests/crashes/129095.rs b/tests/crashes/129095.rs
index d82474e18e706..b1bb74708c2d7 100644
--- a/tests/crashes/129095.rs
+++ b/tests/crashes/129095.rs
@@ -1,10 +1,13 @@
 //@ known-bug: rust-lang/rust#129095
 //@ compile-flags: -Zmir-enable-passes=+GVN -Zmir-enable-passes=+Inline -Zvalidate-mir
 
+#![feature(adt_const_params, unsized_const_params)]
+#![allow(incomplete_features)]
+
 pub fn function_with_bytes<const BYTES: &'static [u8; 4]>() -> &'static [u8] {
     BYTES
 }
 
 pub fn main() {
-    assert_eq!(function_with_bytes::<b"AAAAb">(), &[0x41, 0x41, 0x41, 0x41]);
+    assert_eq!(function_with_bytes::<b"AAAAA">(), &[0x41, 0x41, 0x41, 0x41]);
 }
diff --git a/tests/crashes/132960.rs b/tests/crashes/132960.rs
index 87d0ee7dedebd..c23a339342914 100644
--- a/tests/crashes/132960.rs
+++ b/tests/crashes/132960.rs
@@ -1,6 +1,6 @@
 //@ known-bug: #132960
 
-#![feature(adt_const_params, const_ptr_read, generic_const_exprs)]
+#![feature(adt_const_params, const_ptr_read, generic_const_exprs, unsized_const_params)]
 
 const fn concat_strs<const A: &'static str, const B: &'static str>() -> &'static str
 where
diff --git a/tests/crashes/134654.rs b/tests/crashes/134654.rs
index 8a8d18359e96f..f2323fe4ecdcd 100644
--- a/tests/crashes/134654.rs
+++ b/tests/crashes/134654.rs
@@ -2,6 +2,9 @@
 //@ compile-flags: -Zmir-enable-passes=+GVN -Zmir-enable-passes=+Inline -Zvalidate-mir
 //@ only-x86_64
 
+#![feature(adt_const_params, unsized_const_params)]
+#![allow(incomplete_features)]
+
 fn function_with_bytes<const BYTES:
     &'static [u8; 0xa9008fb6c9d81e42_0e25730562a601c8_u128]>() -> &'static [u8] {
     BYTES
diff --git a/tests/crashes/135128.rs b/tests/crashes/135128.rs
index 2ce17df824a0c..a8fd1ae1ff56a 100644
--- a/tests/crashes/135128.rs
+++ b/tests/crashes/135128.rs
@@ -1,6 +1,8 @@
 //@ known-bug: #135128
 //@ compile-flags: -Copt-level=1 --edition=2021
 
+#![feature(trivial_bounds)]
+
 async fn return_str() -> str
 where
     str: Sized,
diff --git a/tests/crashes/135570.rs b/tests/crashes/135570.rs
index a9eda97ef9de7..7919ceb26d50d 100644
--- a/tests/crashes/135570.rs
+++ b/tests/crashes/135570.rs
@@ -2,6 +2,9 @@
 //@compile-flags: -Zvalidate-mir -Zmir-enable-passes=+Inline -Copt-level=0 -Zmir-enable-passes=+GVN
 //@ only-x86_64
 
+#![feature(adt_const_params, unsized_const_params)]
+#![allow(incomplete_features)]
+
 fn function_with_bytes<const BYTES: &'static [u8; 0xc7b889180b67b07d_bc1a3c88783d35b5_u128]>(
 ) -> &'static [u8] {
     BYTES
diff --git a/tests/run-make/avr-rjmp-offset/rmake.rs b/tests/run-make/avr-rjmp-offset/rmake.rs
index de64b724eed2d..da314f26ca782 100644
--- a/tests/run-make/avr-rjmp-offset/rmake.rs
+++ b/tests/run-make/avr-rjmp-offset/rmake.rs
@@ -22,7 +22,12 @@ fn main() {
         .input("avr-rjmp-offsets.rs")
         .opt_level("s")
         .panic("abort")
-        .target("avr-unknown-gnu-atmega328")
+        .target("avr-none")
+        // rust-lld has some troubles understanding the -mmcu flag, so for the
+        // time being let's tell rustc to emit binary that's compatible with the
+        // target CPU that lld defaults to, i.e. just `avr` (that's simply the
+        // minimal common instruction set across all AVRs)
+        .target_cpu("avr")
         // normally one links with `avr-gcc`, but this is not available in CI,
         // hence this test diverges from the default behavior to enable linking
         // at all, which is necessary for the test (to resolve the labels). To
@@ -49,6 +54,7 @@ fn main() {
     // of the Rust compiler did produce a label `rjmp .-4` (misses the first
     // instruction in the loop).
     assert!(disassembly.contains("<main>"), "no main function in output");
+
     disassembly
         .trim()
         .lines()
diff --git a/tests/ui/abi/c-zst.aarch64-darwin.stderr b/tests/ui/abi/c-zst.aarch64-darwin.stderr
index c8f7d0a517c4c..57cc48aa9cf47 100644
--- a/tests/ui/abi/c-zst.aarch64-darwin.stderr
+++ b/tests/ui/abi/c-zst.aarch64-darwin.stderr
@@ -17,6 +17,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                                memory_index: [],
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -45,6 +46,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/abi/c-zst.powerpc-linux.stderr b/tests/ui/abi/c-zst.powerpc-linux.stderr
index 1015f7d889880..6738017673003 100644
--- a/tests/ui/abi/c-zst.powerpc-linux.stderr
+++ b/tests/ui/abi/c-zst.powerpc-linux.stderr
@@ -17,6 +17,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                                memory_index: [],
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -56,6 +57,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/abi/c-zst.s390x-linux.stderr b/tests/ui/abi/c-zst.s390x-linux.stderr
index 1015f7d889880..6738017673003 100644
--- a/tests/ui/abi/c-zst.s390x-linux.stderr
+++ b/tests/ui/abi/c-zst.s390x-linux.stderr
@@ -17,6 +17,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                                memory_index: [],
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -56,6 +57,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/abi/c-zst.sparc64-linux.stderr b/tests/ui/abi/c-zst.sparc64-linux.stderr
index 1015f7d889880..6738017673003 100644
--- a/tests/ui/abi/c-zst.sparc64-linux.stderr
+++ b/tests/ui/abi/c-zst.sparc64-linux.stderr
@@ -17,6 +17,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                                memory_index: [],
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -56,6 +57,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/abi/c-zst.x86_64-linux.stderr b/tests/ui/abi/c-zst.x86_64-linux.stderr
index c8f7d0a517c4c..57cc48aa9cf47 100644
--- a/tests/ui/abi/c-zst.x86_64-linux.stderr
+++ b/tests/ui/abi/c-zst.x86_64-linux.stderr
@@ -17,6 +17,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                                memory_index: [],
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -45,6 +46,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/abi/c-zst.x86_64-pc-windows-gnu.stderr b/tests/ui/abi/c-zst.x86_64-pc-windows-gnu.stderr
index 1015f7d889880..6738017673003 100644
--- a/tests/ui/abi/c-zst.x86_64-pc-windows-gnu.stderr
+++ b/tests/ui/abi/c-zst.x86_64-pc-windows-gnu.stderr
@@ -17,6 +17,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                                memory_index: [],
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -56,6 +57,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/abi/debug.stderr b/tests/ui/abi/debug.stderr
index e550e5bfcf3c3..5f73ff7d6bd58 100644
--- a/tests/ui/abi/debug.stderr
+++ b/tests/ui/abi/debug.stderr
@@ -20,6 +20,7 @@ error: fn_abi_of(test) = FnAbi {
                            ),
                            fields: Primitive,
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -67,6 +68,7 @@ error: fn_abi_of(test) = FnAbi {
                                valid_range: 0..=1,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -125,6 +127,7 @@ error: fn_abi_of(TestFnPtr) = FnAbi {
                                    valid_range: 0..=1,
                                },
                            ),
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -163,6 +166,7 @@ error: fn_abi_of(TestFnPtr) = FnAbi {
                        ),
                        fields: Primitive,
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -213,6 +217,7 @@ error: fn_abi_of(test_generic) = FnAbi {
                            ),
                            fields: Primitive,
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -248,6 +253,7 @@ error: fn_abi_of(test_generic) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -297,6 +303,7 @@ error: ABIs are not compatible
                            ),
                            fields: Primitive,
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -332,6 +339,7 @@ error: ABIs are not compatible
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -369,6 +377,7 @@ error: ABIs are not compatible
                            ),
                            fields: Primitive,
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -404,6 +413,7 @@ error: ABIs are not compatible
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -444,6 +454,7 @@ error: ABIs are not compatible
                                count: 32,
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -483,6 +494,7 @@ error: ABIs are not compatible
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -517,6 +529,7 @@ error: ABIs are not compatible
                                count: 32,
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -556,6 +569,7 @@ error: ABIs are not compatible
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -598,6 +612,7 @@ error: ABIs are not compatible
                            ),
                            fields: Primitive,
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -633,6 +648,7 @@ error: ABIs are not compatible
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -670,6 +686,7 @@ error: ABIs are not compatible
                            ),
                            fields: Primitive,
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -705,6 +722,7 @@ error: ABIs are not compatible
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -748,6 +766,7 @@ error: ABIs are not compatible
                            ),
                            fields: Primitive,
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -783,6 +802,7 @@ error: ABIs are not compatible
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -820,6 +840,7 @@ error: ABIs are not compatible
                            ),
                            fields: Primitive,
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -855,6 +876,7 @@ error: ABIs are not compatible
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -923,6 +945,7 @@ error: fn_abi_of(assoc_test) = FnAbi {
                                    valid_range: $NON_NULL,
                                },
                            ),
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -960,6 +983,7 @@ error: fn_abi_of(assoc_test) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/abi/sysv64-zst.stderr b/tests/ui/abi/sysv64-zst.stderr
index 781e9b2f4c9d5..ec85030c10686 100644
--- a/tests/ui/abi/sysv64-zst.stderr
+++ b/tests/ui/abi/sysv64-zst.stderr
@@ -17,6 +17,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                                memory_index: [],
                            },
                            largest_niche: None,
+                           uninhabited: false,
                            variants: Single {
                                index: 0,
                            },
@@ -45,6 +46,7 @@ error: fn_abi_of(pass_zst) = FnAbi {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/consts/const-slice-array-deref.stderr b/tests/ui/consts/const-slice-array-deref.stderr
index 346685380cc6c..b1d069280885d 100644
--- a/tests/ui/consts/const-slice-array-deref.stderr
+++ b/tests/ui/consts/const-slice-array-deref.stderr
@@ -5,6 +5,7 @@ LL | const ONE: [u16] = [1];
    |            ^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[u16]`
+   = note: statics and constants must have a statically known size
 
 error[E0308]: mismatched types
   --> $DIR/const-slice-array-deref.rs:1:20
diff --git a/tests/ui/consts/const-unsized.stderr b/tests/ui/consts/const-unsized.stderr
index 7931d7adafdb1..8328e19aac22d 100644
--- a/tests/ui/consts/const-unsized.stderr
+++ b/tests/ui/consts/const-unsized.stderr
@@ -5,6 +5,7 @@ LL | const CONST_0: dyn Debug + Sync = *(&0 as &(dyn Debug + Sync));
    |                ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn Debug + Sync + 'static)`
+   = note: statics and constants must have a statically known size
 
 error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> $DIR/const-unsized.rs:7:18
@@ -13,6 +14,7 @@ LL | const CONST_FOO: str = *"foo";
    |                  ^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
+   = note: statics and constants must have a statically known size
 
 error[E0277]: the size for values of type `(dyn Debug + Sync + 'static)` cannot be known at compilation time
   --> $DIR/const-unsized.rs:11:18
@@ -21,6 +23,7 @@ LL | static STATIC_1: dyn Debug + Sync = *(&1 as &(dyn Debug + Sync));
    |                  ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn Debug + Sync + 'static)`
+   = note: statics and constants must have a statically known size
 
 error[E0277]: the size for values of type `str` cannot be known at compilation time
   --> $DIR/const-unsized.rs:15:20
@@ -29,6 +32,7 @@ LL | static STATIC_BAR: str = *"bar";
    |                    ^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `str`
+   = note: statics and constants must have a statically known size
 
 error[E0507]: cannot move out of a shared reference
   --> $DIR/const-unsized.rs:3:35
diff --git a/tests/ui/consts/const_refs_to_static-ice-121413.stderr b/tests/ui/consts/const_refs_to_static-ice-121413.stderr
index 8665d9b685223..3980a7e9b93ba 100644
--- a/tests/ui/consts/const_refs_to_static-ice-121413.stderr
+++ b/tests/ui/consts/const_refs_to_static-ice-121413.stderr
@@ -30,6 +30,7 @@ LL |     static FOO: Sync = AtomicUsize::new(0);
    |                 ^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn Sync + 'static)`
+   = note: statics and constants must have a statically known size
 
 error: aborting due to 2 previous errors; 1 warning emitted
 
diff --git a/tests/ui/consts/dont-ctfe-unsized-initializer.rs b/tests/ui/consts/dont-ctfe-unsized-initializer.rs
new file mode 100644
index 0000000000000..cca38b760dcc8
--- /dev/null
+++ b/tests/ui/consts/dont-ctfe-unsized-initializer.rs
@@ -0,0 +1,7 @@
+static S: str = todo!();
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+
+const C: str = todo!();
+//~^ ERROR the size for values of type `str` cannot be known at compilation time
+
+fn main() {}
diff --git a/tests/ui/consts/dont-ctfe-unsized-initializer.stderr b/tests/ui/consts/dont-ctfe-unsized-initializer.stderr
new file mode 100644
index 0000000000000..e69790fc1823c
--- /dev/null
+++ b/tests/ui/consts/dont-ctfe-unsized-initializer.stderr
@@ -0,0 +1,21 @@
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/dont-ctfe-unsized-initializer.rs:1:11
+   |
+LL | static S: str = todo!();
+   |           ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: statics and constants must have a statically known size
+
+error[E0277]: the size for values of type `str` cannot be known at compilation time
+  --> $DIR/dont-ctfe-unsized-initializer.rs:4:10
+   |
+LL | const C: str = todo!();
+   |          ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `str`
+   = note: statics and constants must have a statically known size
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/extern/issue-36122-accessing-externed-dst.stderr b/tests/ui/extern/issue-36122-accessing-externed-dst.stderr
index 64178e6f84390..6f805aec1df02 100644
--- a/tests/ui/extern/issue-36122-accessing-externed-dst.stderr
+++ b/tests/ui/extern/issue-36122-accessing-externed-dst.stderr
@@ -5,6 +5,7 @@ LL |         static symbol: [usize];
    |                        ^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[usize]`
+   = note: statics and constants must have a statically known size
 
 error[E0133]: use of extern static is unsafe and requires unsafe function or block
   --> $DIR/issue-36122-accessing-externed-dst.rs:5:20
diff --git a/tests/ui/feature-gates/feature-gate-abi-avr-interrupt.rs b/tests/ui/feature-gates/feature-gate-abi-avr-interrupt.rs
index 5386628a8e069..ce3d3fc645d76 100644
--- a/tests/ui/feature-gates/feature-gate-abi-avr-interrupt.rs
+++ b/tests/ui/feature-gates/feature-gate-abi-avr-interrupt.rs
@@ -1,5 +1,5 @@
 //@ needs-llvm-components: avr
-//@ compile-flags: --target=avr-unknown-gnu-atmega328 --crate-type=rlib
+//@ compile-flags: --target=avr-none -C target-cpu=atmega328p --crate-type=rlib
 #![no_core]
 #![feature(no_core, lang_items)]
 #[lang="sized"]
diff --git a/tests/ui/issues/issue-54410.stderr b/tests/ui/issues/issue-54410.stderr
index 97e5990750e3c..2cd5a2a49ef5d 100644
--- a/tests/ui/issues/issue-54410.stderr
+++ b/tests/ui/issues/issue-54410.stderr
@@ -5,6 +5,7 @@ LL |     pub static mut symbol: [i8];
    |                            ^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[i8]`
+   = note: statics and constants must have a statically known size
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/layout/debug.stderr b/tests/ui/layout/debug.stderr
index 0daf2d3b9e7f2..07cad7766920f 100644
--- a/tests/ui/layout/debug.stderr
+++ b/tests/ui/layout/debug.stderr
@@ -31,6 +31,7 @@ error: layout_of(E) = Layout {
                    valid_range: 0..=0,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -56,6 +57,7 @@ error: layout_of(E) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -69,7 +71,9 @@ error: layout_of(E) = Layout {
                            abi: Align(4 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Memory {
+                           sized: true,
+                       },
                        fields: Arbitrary {
                            offsets: [
                                Size(4 bytes),
@@ -83,6 +87,7 @@ error: layout_of(E) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 1,
                        },
@@ -136,6 +141,7 @@ error: layout_of(S) = Layout {
                ],
            },
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -161,6 +167,7 @@ error: layout_of(U) = Layout {
                2,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -213,6 +220,7 @@ error: layout_of(Result<i32, i32>) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -255,6 +263,7 @@ error: layout_of(Result<i32, i32>) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -293,6 +302,7 @@ error: layout_of(Result<i32, i32>) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -328,6 +338,7 @@ error: layout_of(i32) = Layout {
            ),
            fields: Primitive,
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -353,6 +364,7 @@ error: layout_of(V) = Layout {
                2,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -378,6 +390,7 @@ error: layout_of(W) = Layout {
                2,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -403,6 +416,7 @@ error: layout_of(Y) = Layout {
                2,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -428,6 +442,7 @@ error: layout_of(P1) = Layout {
                1,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -453,6 +468,7 @@ error: layout_of(P2) = Layout {
                1,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -478,6 +494,7 @@ error: layout_of(P3) = Layout {
                1,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -503,6 +520,7 @@ error: layout_of(P4) = Layout {
                1,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -533,6 +551,7 @@ error: layout_of(P5) = Layout {
                2,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -563,6 +582,7 @@ error: layout_of(MaybeUninit<u8>) = Layout {
                2,
            ),
            largest_niche: None,
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
diff --git a/tests/ui/layout/hexagon-enum.stderr b/tests/ui/layout/hexagon-enum.stderr
index 96f0a8c874086..90c06ba1f834e 100644
--- a/tests/ui/layout/hexagon-enum.stderr
+++ b/tests/ui/layout/hexagon-enum.stderr
@@ -31,6 +31,7 @@ error: layout_of(A) = Layout {
                    valid_range: 0..=0,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -56,6 +57,7 @@ error: layout_of(A) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -107,6 +109,7 @@ error: layout_of(B) = Layout {
                    valid_range: 255..=255,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -132,6 +135,7 @@ error: layout_of(B) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -183,6 +187,7 @@ error: layout_of(C) = Layout {
                    valid_range: 256..=256,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -208,6 +213,7 @@ error: layout_of(C) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -259,6 +265,7 @@ error: layout_of(P) = Layout {
                    valid_range: 268435456..=268435456,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -284,6 +291,7 @@ error: layout_of(P) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -335,6 +343,7 @@ error: layout_of(T) = Layout {
                    valid_range: 2164260864..=2164260864,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -360,6 +369,7 @@ error: layout_of(T) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/layout/issue-96158-scalarpair-payload-might-be-uninit.stderr b/tests/ui/layout/issue-96158-scalarpair-payload-might-be-uninit.stderr
index cd9e4c0278150..3bdb9c5c143e4 100644
--- a/tests/ui/layout/issue-96158-scalarpair-payload-might-be-uninit.stderr
+++ b/tests/ui/layout/issue-96158-scalarpair-payload-might-be-uninit.stderr
@@ -37,6 +37,7 @@ error: layout_of(MissingPayloadField) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -78,6 +79,7 @@ error: layout_of(MissingPayloadField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -99,6 +101,7 @@ error: layout_of(MissingPayloadField) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -157,6 +160,7 @@ error: layout_of(CommonPayloadField) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -199,6 +203,7 @@ error: layout_of(CommonPayloadField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -237,6 +242,7 @@ error: layout_of(CommonPayloadField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -294,6 +300,7 @@ error: layout_of(CommonPayloadFieldIsMaybeUninit) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -335,6 +342,7 @@ error: layout_of(CommonPayloadFieldIsMaybeUninit) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -372,6 +380,7 @@ error: layout_of(CommonPayloadFieldIsMaybeUninit) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -429,6 +438,7 @@ error: layout_of(NicheFirst) = Layout {
                    valid_range: 0..=4,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -486,6 +496,7 @@ error: layout_of(NicheFirst) = Layout {
                                valid_range: 0..=2,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -507,6 +518,7 @@ error: layout_of(NicheFirst) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -528,6 +540,7 @@ error: layout_of(NicheFirst) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 2,
                        },
@@ -585,6 +598,7 @@ error: layout_of(NicheSecond) = Layout {
                    valid_range: 0..=4,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -642,6 +656,7 @@ error: layout_of(NicheSecond) = Layout {
                                valid_range: 0..=2,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -663,6 +678,7 @@ error: layout_of(NicheSecond) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -684,6 +700,7 @@ error: layout_of(NicheSecond) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 2,
                        },
diff --git a/tests/ui/layout/issue-96185-overaligned-enum.stderr b/tests/ui/layout/issue-96185-overaligned-enum.stderr
index 15a3f6004f50f..1d4e443644826 100644
--- a/tests/ui/layout/issue-96185-overaligned-enum.stderr
+++ b/tests/ui/layout/issue-96185-overaligned-enum.stderr
@@ -25,6 +25,7 @@ error: layout_of(Aligned1) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -50,6 +51,7 @@ error: layout_of(Aligned1) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -73,6 +75,7 @@ error: layout_of(Aligned1) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -128,6 +131,7 @@ error: layout_of(Aligned2) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -153,6 +157,7 @@ error: layout_of(Aligned2) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -176,6 +181,7 @@ error: layout_of(Aligned2) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
diff --git a/tests/ui/layout/thumb-enum.stderr b/tests/ui/layout/thumb-enum.stderr
index 120081d193c69..0c34331856474 100644
--- a/tests/ui/layout/thumb-enum.stderr
+++ b/tests/ui/layout/thumb-enum.stderr
@@ -31,6 +31,7 @@ error: layout_of(A) = Layout {
                    valid_range: 0..=0,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -56,6 +57,7 @@ error: layout_of(A) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -107,6 +109,7 @@ error: layout_of(B) = Layout {
                    valid_range: 255..=255,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -132,6 +135,7 @@ error: layout_of(B) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -183,6 +187,7 @@ error: layout_of(C) = Layout {
                    valid_range: 256..=256,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -208,6 +213,7 @@ error: layout_of(C) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -259,6 +265,7 @@ error: layout_of(P) = Layout {
                    valid_range: 268435456..=268435456,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -284,6 +291,7 @@ error: layout_of(P) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -335,6 +343,7 @@ error: layout_of(T) = Layout {
                    valid_range: 2164260864..=2164260864,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -360,6 +369,7 @@ error: layout_of(T) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
diff --git a/tests/ui/layout/zero-sized-array-enum-niche.stderr b/tests/ui/layout/zero-sized-array-enum-niche.stderr
index b6fcc14c063c5..33d2eede22090 100644
--- a/tests/ui/layout/zero-sized-array-enum-niche.stderr
+++ b/tests/ui/layout/zero-sized-array-enum-niche.stderr
@@ -25,6 +25,7 @@ error: layout_of(Result<[u32; 0], bool>) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -54,6 +55,7 @@ error: layout_of(Result<[u32; 0], bool>) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -88,6 +90,7 @@ error: layout_of(Result<[u32; 0], bool>) = Layout {
                                valid_range: 0..=1,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -133,6 +136,7 @@ error: layout_of(MultipleAlignments) = Layout {
                    valid_range: 0..=2,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -162,6 +166,7 @@ error: layout_of(MultipleAlignments) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -187,6 +192,7 @@ error: layout_of(MultipleAlignments) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -221,6 +227,7 @@ error: layout_of(MultipleAlignments) = Layout {
                                valid_range: 0..=1,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 2,
                        },
@@ -266,6 +273,7 @@ error: layout_of(Result<[u32; 0], Packed<NonZero<u16>>>) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -295,6 +303,7 @@ error: layout_of(Result<[u32; 0], Packed<NonZero<u16>>>) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -329,6 +338,7 @@ error: layout_of(Result<[u32; 0], Packed<NonZero<u16>>>) = Layout {
                                valid_range: 1..=65535,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -374,6 +384,7 @@ error: layout_of(Result<[u32; 0], Packed<U16IsZero>>) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -407,6 +418,7 @@ error: layout_of(Result<[u32; 0], Packed<U16IsZero>>) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -441,6 +453,7 @@ error: layout_of(Result<[u32; 0], Packed<U16IsZero>>) = Layout {
                                valid_range: 0..=0,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
diff --git a/tests/ui/repr/16-bit-repr-c-enum.rs b/tests/ui/repr/16-bit-repr-c-enum.rs
index 2b6bbf1265083..011076882d24f 100644
--- a/tests/ui/repr/16-bit-repr-c-enum.rs
+++ b/tests/ui/repr/16-bit-repr-c-enum.rs
@@ -2,7 +2,7 @@
 //@ revisions: avr msp430
 //
 //@ [avr] needs-llvm-components: avr
-//@ [avr] compile-flags: --target=avr-unknown-gnu-atmega328 --crate-type=rlib
+//@ [avr] compile-flags: --target=avr-none -C target-cpu=atmega328p --crate-type=rlib
 //@ [msp430] needs-llvm-components: msp430
 //@ [msp430] compile-flags: --target=msp430-none-elf --crate-type=rlib
 #![feature(no_core, lang_items, intrinsics, staged_api, rustc_attrs)]
diff --git a/tests/ui/repr/repr-c-dead-variants.aarch64-unknown-linux-gnu.stderr b/tests/ui/repr/repr-c-dead-variants.aarch64-unknown-linux-gnu.stderr
index 8e8f1d159b740..204db30786ee3 100644
--- a/tests/ui/repr/repr-c-dead-variants.aarch64-unknown-linux-gnu.stderr
+++ b/tests/ui/repr/repr-c-dead-variants.aarch64-unknown-linux-gnu.stderr
@@ -4,7 +4,15 @@ error: layout_of(Univariant) = Layout {
                abi: Align(4 bytes),
                pref: $SOME_ALIGN,
            },
-           abi: Uninhabited,
+           abi: Scalar(
+               Initialized {
+                   value: Int(
+                       I32,
+                       false,
+                   ),
+                   valid_range: 0..=0,
+               },
+           ),
            fields: Arbitrary {
                offsets: [
                    Size(0 bytes),
@@ -23,6 +31,7 @@ error: layout_of(Univariant) = Layout {
                    valid_range: 0..=0,
                },
            ),
+           uninhabited: true,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -40,7 +49,15 @@ error: layout_of(Univariant) = Layout {
                            abi: Align(4 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Scalar(
+                           Initialized {
+                               value: Int(
+                                   I32,
+                                   false,
+                               ),
+                               valid_range: 0..=0,
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(4 bytes),
@@ -50,6 +67,7 @@ error: layout_of(Univariant) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -107,6 +125,7 @@ error: layout_of(TwoVariants) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -119,12 +138,26 @@ error: layout_of(TwoVariants) = Layout {
                tag_field: 0,
                variants: [
                    Layout {
-                       size: Size(4 bytes),
+                       size: Size(8 bytes),
                        align: AbiAndPrefAlign {
                            abi: Align(4 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: ScalarPair(
+                           Initialized {
+                               value: Int(
+                                   I32,
+                                   false,
+                               ),
+                               valid_range: 0..=1,
+                           },
+                           Union {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(4 bytes),
@@ -134,6 +167,7 @@ error: layout_of(TwoVariants) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -171,6 +205,7 @@ error: layout_of(TwoVariants) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -216,6 +251,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -233,7 +269,9 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            abi: Align(8 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Memory {
+                           sized: true,
+                       },
                        fields: Arbitrary {
                            offsets: [
                                Size(8 bytes),
@@ -245,6 +283,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -272,6 +311,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
diff --git a/tests/ui/repr/repr-c-dead-variants.armebv7r-none-eabi.stderr b/tests/ui/repr/repr-c-dead-variants.armebv7r-none-eabi.stderr
index 2cd0960ce3eec..1fab00bf50c49 100644
--- a/tests/ui/repr/repr-c-dead-variants.armebv7r-none-eabi.stderr
+++ b/tests/ui/repr/repr-c-dead-variants.armebv7r-none-eabi.stderr
@@ -4,7 +4,15 @@ error: layout_of(Univariant) = Layout {
                abi: Align(1 bytes),
                pref: $SOME_ALIGN,
            },
-           abi: Uninhabited,
+           abi: Scalar(
+               Initialized {
+                   value: Int(
+                       I8,
+                       false,
+                   ),
+                   valid_range: 0..=0,
+               },
+           ),
            fields: Arbitrary {
                offsets: [
                    Size(0 bytes),
@@ -23,6 +31,7 @@ error: layout_of(Univariant) = Layout {
                    valid_range: 0..=0,
                },
            ),
+           uninhabited: true,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -40,7 +49,15 @@ error: layout_of(Univariant) = Layout {
                            abi: Align(1 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Scalar(
+                           Initialized {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                               valid_range: 0..=0,
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(1 bytes),
@@ -50,6 +67,7 @@ error: layout_of(Univariant) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -107,6 +125,7 @@ error: layout_of(TwoVariants) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -119,12 +138,26 @@ error: layout_of(TwoVariants) = Layout {
                tag_field: 0,
                variants: [
                    Layout {
-                       size: Size(1 bytes),
+                       size: Size(2 bytes),
                        align: AbiAndPrefAlign {
                            abi: Align(1 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: ScalarPair(
+                           Initialized {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                               valid_range: 0..=1,
+                           },
+                           Union {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(1 bytes),
@@ -134,6 +167,7 @@ error: layout_of(TwoVariants) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -171,6 +205,7 @@ error: layout_of(TwoVariants) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -216,6 +251,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -233,7 +269,9 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            abi: Align(8 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Memory {
+                           sized: true,
+                       },
                        fields: Arbitrary {
                            offsets: [
                                Size(8 bytes),
@@ -245,6 +283,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -272,6 +311,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
diff --git a/tests/ui/repr/repr-c-dead-variants.i686-pc-windows-msvc.stderr b/tests/ui/repr/repr-c-dead-variants.i686-pc-windows-msvc.stderr
index 8e8f1d159b740..204db30786ee3 100644
--- a/tests/ui/repr/repr-c-dead-variants.i686-pc-windows-msvc.stderr
+++ b/tests/ui/repr/repr-c-dead-variants.i686-pc-windows-msvc.stderr
@@ -4,7 +4,15 @@ error: layout_of(Univariant) = Layout {
                abi: Align(4 bytes),
                pref: $SOME_ALIGN,
            },
-           abi: Uninhabited,
+           abi: Scalar(
+               Initialized {
+                   value: Int(
+                       I32,
+                       false,
+                   ),
+                   valid_range: 0..=0,
+               },
+           ),
            fields: Arbitrary {
                offsets: [
                    Size(0 bytes),
@@ -23,6 +31,7 @@ error: layout_of(Univariant) = Layout {
                    valid_range: 0..=0,
                },
            ),
+           uninhabited: true,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -40,7 +49,15 @@ error: layout_of(Univariant) = Layout {
                            abi: Align(4 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Scalar(
+                           Initialized {
+                               value: Int(
+                                   I32,
+                                   false,
+                               ),
+                               valid_range: 0..=0,
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(4 bytes),
@@ -50,6 +67,7 @@ error: layout_of(Univariant) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -107,6 +125,7 @@ error: layout_of(TwoVariants) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -119,12 +138,26 @@ error: layout_of(TwoVariants) = Layout {
                tag_field: 0,
                variants: [
                    Layout {
-                       size: Size(4 bytes),
+                       size: Size(8 bytes),
                        align: AbiAndPrefAlign {
                            abi: Align(4 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: ScalarPair(
+                           Initialized {
+                               value: Int(
+                                   I32,
+                                   false,
+                               ),
+                               valid_range: 0..=1,
+                           },
+                           Union {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(4 bytes),
@@ -134,6 +167,7 @@ error: layout_of(TwoVariants) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -171,6 +205,7 @@ error: layout_of(TwoVariants) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -216,6 +251,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -233,7 +269,9 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            abi: Align(8 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Memory {
+                           sized: true,
+                       },
                        fields: Arbitrary {
                            offsets: [
                                Size(8 bytes),
@@ -245,6 +283,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -272,6 +311,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
diff --git a/tests/ui/repr/repr-c-dead-variants.x86_64-unknown-linux-gnu.stderr b/tests/ui/repr/repr-c-dead-variants.x86_64-unknown-linux-gnu.stderr
index 8e8f1d159b740..204db30786ee3 100644
--- a/tests/ui/repr/repr-c-dead-variants.x86_64-unknown-linux-gnu.stderr
+++ b/tests/ui/repr/repr-c-dead-variants.x86_64-unknown-linux-gnu.stderr
@@ -4,7 +4,15 @@ error: layout_of(Univariant) = Layout {
                abi: Align(4 bytes),
                pref: $SOME_ALIGN,
            },
-           abi: Uninhabited,
+           abi: Scalar(
+               Initialized {
+                   value: Int(
+                       I32,
+                       false,
+                   ),
+                   valid_range: 0..=0,
+               },
+           ),
            fields: Arbitrary {
                offsets: [
                    Size(0 bytes),
@@ -23,6 +31,7 @@ error: layout_of(Univariant) = Layout {
                    valid_range: 0..=0,
                },
            ),
+           uninhabited: true,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -40,7 +49,15 @@ error: layout_of(Univariant) = Layout {
                            abi: Align(4 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Scalar(
+                           Initialized {
+                               value: Int(
+                                   I32,
+                                   false,
+                               ),
+                               valid_range: 0..=0,
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(4 bytes),
@@ -50,6 +67,7 @@ error: layout_of(Univariant) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -107,6 +125,7 @@ error: layout_of(TwoVariants) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -119,12 +138,26 @@ error: layout_of(TwoVariants) = Layout {
                tag_field: 0,
                variants: [
                    Layout {
-                       size: Size(4 bytes),
+                       size: Size(8 bytes),
                        align: AbiAndPrefAlign {
                            abi: Align(4 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: ScalarPair(
+                           Initialized {
+                               value: Int(
+                                   I32,
+                                   false,
+                               ),
+                               valid_range: 0..=1,
+                           },
+                           Union {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(4 bytes),
@@ -134,6 +167,7 @@ error: layout_of(TwoVariants) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -171,6 +205,7 @@ error: layout_of(TwoVariants) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -216,6 +251,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -233,7 +269,9 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            abi: Align(8 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Memory {
+                           sized: true,
+                       },
                        fields: Arbitrary {
                            offsets: [
                                Size(8 bytes),
@@ -245,6 +283,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -272,6 +311,7 @@ error: layout_of(DeadBranchHasOtherField) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
diff --git a/tests/ui/repr/repr-c-int-dead-variants.stderr b/tests/ui/repr/repr-c-int-dead-variants.stderr
index fa08b323dec2a..f852212deb908 100644
--- a/tests/ui/repr/repr-c-int-dead-variants.stderr
+++ b/tests/ui/repr/repr-c-int-dead-variants.stderr
@@ -4,7 +4,15 @@ error: layout_of(UnivariantU8) = Layout {
                abi: Align(1 bytes),
                pref: $SOME_ALIGN,
            },
-           abi: Uninhabited,
+           abi: Scalar(
+               Initialized {
+                   value: Int(
+                       I8,
+                       false,
+                   ),
+                   valid_range: 0..=0,
+               },
+           ),
            fields: Arbitrary {
                offsets: [
                    Size(0 bytes),
@@ -23,6 +31,7 @@ error: layout_of(UnivariantU8) = Layout {
                    valid_range: 0..=0,
                },
            ),
+           uninhabited: true,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -40,7 +49,15 @@ error: layout_of(UnivariantU8) = Layout {
                            abi: Align(1 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Scalar(
+                           Initialized {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                               valid_range: 0..=0,
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(1 bytes),
@@ -50,6 +67,7 @@ error: layout_of(UnivariantU8) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -107,6 +125,7 @@ error: layout_of(TwoVariantsU8) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -119,12 +138,26 @@ error: layout_of(TwoVariantsU8) = Layout {
                tag_field: 0,
                variants: [
                    Layout {
-                       size: Size(1 bytes),
+                       size: Size(2 bytes),
                        align: AbiAndPrefAlign {
                            abi: Align(1 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: ScalarPair(
+                           Initialized {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                               valid_range: 0..=1,
+                           },
+                           Union {
+                               value: Int(
+                                   I8,
+                                   false,
+                               ),
+                           },
+                       ),
                        fields: Arbitrary {
                            offsets: [
                                Size(1 bytes),
@@ -134,6 +167,7 @@ error: layout_of(TwoVariantsU8) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -171,6 +205,7 @@ error: layout_of(TwoVariantsU8) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -216,6 +251,7 @@ error: layout_of(DeadBranchHasOtherFieldU8) = Layout {
                    valid_range: 0..=1,
                },
            ),
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -233,7 +269,9 @@ error: layout_of(DeadBranchHasOtherFieldU8) = Layout {
                            abi: Align(8 bytes),
                            pref: $SOME_ALIGN,
                        },
-                       abi: Uninhabited,
+                       abi: Memory {
+                           sized: true,
+                       },
                        fields: Arbitrary {
                            offsets: [
                                Size(8 bytes),
@@ -245,6 +283,7 @@ error: layout_of(DeadBranchHasOtherFieldU8) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: true,
                        variants: Single {
                            index: 0,
                        },
@@ -272,6 +311,7 @@ error: layout_of(DeadBranchHasOtherFieldU8) = Layout {
                            ],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
diff --git a/tests/ui/static/issue-24446.stderr b/tests/ui/static/issue-24446.stderr
index 033caf07d8e54..0e6e338c5efeb 100644
--- a/tests/ui/static/issue-24446.stderr
+++ b/tests/ui/static/issue-24446.stderr
@@ -14,6 +14,7 @@ LL |     static foo: dyn Fn() -> u32 = || -> u32 {
    |                 ^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn Fn() -> u32 + 'static)`
+   = note: statics and constants must have a statically known size
 
 error[E0308]: mismatched types
   --> $DIR/issue-24446.rs:2:35
diff --git a/tests/ui/statics/unsized_type2.stderr b/tests/ui/statics/unsized_type2.stderr
index ffbbe218c87a0..3f9b0879c166d 100644
--- a/tests/ui/statics/unsized_type2.stderr
+++ b/tests/ui/statics/unsized_type2.stderr
@@ -10,6 +10,7 @@ note: required because it appears within the type `Foo`
    |
 LL | pub struct Foo {
    |            ^^^
+   = note: statics and constants must have a statically known size
 
 error[E0308]: mismatched types
   --> $DIR/unsized_type2.rs:14:45
diff --git a/tests/ui/trait-bounds/ice-unsized-tuple-const-issue-121443.stderr b/tests/ui/trait-bounds/ice-unsized-tuple-const-issue-121443.stderr
index 76e015a7238b5..4609e02716fdc 100644
--- a/tests/ui/trait-bounds/ice-unsized-tuple-const-issue-121443.stderr
+++ b/tests/ui/trait-bounds/ice-unsized-tuple-const-issue-121443.stderr
@@ -11,6 +11,7 @@ LL | const TEST: Fn = some_fn;
    |             ^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn FnOnce() -> u8 + 'static)`
+   = note: statics and constants must have a statically known size
 
 error[E0277]: the size for values of type `(dyn FnOnce() -> u8 + 'static)` cannot be known at compilation time
   --> $DIR/ice-unsized-tuple-const-issue-121443.rs:11:14
diff --git a/tests/ui/type/pattern_types/range_patterns.stderr b/tests/ui/type/pattern_types/range_patterns.stderr
index 7da8cfd4dbc2d..690592ba0b8da 100644
--- a/tests/ui/type/pattern_types/range_patterns.stderr
+++ b/tests/ui/type/pattern_types/range_patterns.stderr
@@ -31,6 +31,7 @@ error: layout_of(NonZero<u32>) = Layout {
                    valid_range: 1..=4294967295,
                },
            ),
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -69,6 +70,7 @@ error: layout_of((u32) is 1..=) = Layout {
                    valid_range: 1..=4294967295,
                },
            ),
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
@@ -105,6 +107,7 @@ error: layout_of(Option<(u32) is 1..=>) = Layout {
                ],
            },
            largest_niche: None,
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -134,6 +137,7 @@ error: layout_of(Option<(u32) is 1..=>) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -174,6 +178,7 @@ error: layout_of(Option<(u32) is 1..=>) = Layout {
                                valid_range: 1..=4294967295,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -216,6 +221,7 @@ error: layout_of(Option<NonZero<u32>>) = Layout {
                ],
            },
            largest_niche: None,
+           uninhabited: false,
            variants: Multiple {
                tag: Initialized {
                    value: Int(
@@ -245,6 +251,7 @@ error: layout_of(Option<NonZero<u32>>) = Layout {
                            memory_index: [],
                        },
                        largest_niche: None,
+                       uninhabited: false,
                        variants: Single {
                            index: 0,
                        },
@@ -285,6 +292,7 @@ error: layout_of(Option<NonZero<u32>>) = Layout {
                                valid_range: 1..=4294967295,
                            },
                        ),
+                       uninhabited: false,
                        variants: Single {
                            index: 1,
                        },
@@ -336,6 +344,7 @@ error: layout_of(NonZeroU32New) = Layout {
                    valid_range: 1..=4294967295,
                },
            ),
+           uninhabited: false,
            variants: Single {
                index: 0,
            },
diff --git a/tests/ui/uninhabited/uninhabited-transparent-return-abi.rs b/tests/ui/uninhabited/uninhabited-transparent-return-abi.rs
new file mode 100644
index 0000000000000..2c2788a3e5642
--- /dev/null
+++ b/tests/ui/uninhabited/uninhabited-transparent-return-abi.rs
@@ -0,0 +1,33 @@
+//@ run-pass
+//@ needs-unwind
+// See https://github.com/rust-lang/rust/issues/135802
+
+enum Void {}
+
+// Should be ABI-compatible with T, but wasn't prior to the PR adding this test.
+#[repr(transparent)]
+struct NoReturn<T>(T, Void);
+
+// Returned by invisible reference (in most ABIs)
+#[allow(dead_code)]
+struct Large(u64, u64, u64);
+
+// Prior to the PR adding this test, this function had a different ABI than
+// `fn() -> Large` (on `x86_64-unknown-linux-gnu` at least), so calling it as `fn() -> Large`
+// would pass the return place pointer in rdi and `correct` in rsi, but the function
+// would expect `correct` in rdi.
+fn never(correct: &mut bool) -> NoReturn<Large> {
+    *correct = true;
+    panic!("catch this")
+}
+
+fn main() {
+    let mut correct = false;
+    let never: fn(&mut bool) -> NoReturn<Large> = never;
+    // Safety: `NoReturn<Large>` is a `repr(transparent)` wrapper around `Large`,
+    // so they should be ABI-compatible.
+    let never: fn(&mut bool) -> Large = unsafe { std::mem::transmute(never) };
+    let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| never(&mut correct)));
+    assert!(result.is_err(), "function should have panicked");
+    assert!(correct, "function should have stored `true` into `correct`");
+}