Auto merge of rust-lang#137728 - Darksonn:no-tuple-unsize, r=<try>

Remove unsizing coercions for tuples See rust-lang#42877 (comment) and below comments for justification. Tracking issue: rust-lang#42877 Fixes: rust-lang#135217
rust-lang-ci · Feb 27, 2025 · 802c9b5 · 802c9b5
2 parents 96cfc75 + 8cab0ba
commit 802c9b5
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diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
@@ -655,8 +655,6 @@ declare_features! (
     (internal, unsized_fn_params, "1.49.0", Some(48055)),
     /// Allows unsized rvalues at arguments and parameters.
     (incomplete, unsized_locals, "1.30.0", Some(48055)),
-    /// Allows unsized tuple coercion.
-    (unstable, unsized_tuple_coercion, "1.20.0", Some(42877)),
     /// Allows using the `#[used(linker)]` (or `#[used(compiler)]`) attribute.
     (unstable, used_with_arg, "1.60.0", Some(93798)),
     /// Allows use of x86 `AMX` target-feature attributes and intrinsics

diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -51,15 +51,13 @@ use rustc_infer::traits::{
     PredicateObligations,
 };
 use rustc_middle::span_bug;
-use rustc_middle::traits::BuiltinImplSource;
 use rustc_middle::ty::adjustment::{
     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCoercion,
 };
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt};
-use rustc_session::parse::feature_err;
-use rustc_span::{BytePos, DUMMY_SP, DesugaringKind, Span, sym};
+use rustc_span::{BytePos, DUMMY_SP, DesugaringKind, Span};
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{
@@ -610,8 +608,6 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target])
         )];
 
-        let mut has_unsized_tuple_coercion = false;
-
         // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
         // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
         // inference might unify those two inner type variables later.
@@ -690,31 +686,10 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                     // be silent, as it causes a type mismatch later.
                 }
 
-                Ok(Some(impl_source)) => {
-                    // Some builtin coercions are still unstable so we detect
-                    // these here and emit a feature error if coercion doesn't fail
-                    // due to another reason.
-                    match impl_source {
-                        traits::ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, _) => {
-                            has_unsized_tuple_coercion = true;
-                        }
-                        _ => {}
-                    }
-                    queue.extend(impl_source.nested_obligations())
-                }
+                Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
             }
         }
 
-        if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion() {
-            feature_err(
-                &self.tcx.sess,
-                sym::unsized_tuple_coercion,
-                self.cause.span,
-                "unsized tuple coercion is not stable enough for use and is subject to change",
-            )
-            .emit();
-        }
-
         Ok(coercion)
     }
 

diff --git a/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs b/compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
@@ -787,13 +787,6 @@ where
                     )
                 }
 
-                //  `(A, B, T)` -> `(A, B, U)` where `T: Unsize<U>`
-                (ty::Tuple(a_tys), ty::Tuple(b_tys))
-                    if a_tys.len() == b_tys.len() && !a_tys.is_empty() =>
-                {
-                    result_to_single(ecx.consider_builtin_tuple_unsize(goal, a_tys, b_tys))
-                }
-
                 _ => vec![],
             }
         })
@@ -1085,48 +1078,6 @@ where
             .enter(|ecx| ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes))
     }
 
-    /// We generate the following builtin impl for tuples of all sizes.
-    ///
-    /// This impl is still unstable and we emit a feature error when it
-    /// when it is used by a coercion.
-    /// ```ignore (builtin impl example)
-    /// impl<T: ?Sized, U: ?Sized, V: ?Sized> Unsize<(T, V)> for (T, U)
-    /// where
-    ///     U: Unsize<V>,
-    /// {}
-    /// ```
-    fn consider_builtin_tuple_unsize(
-        &mut self,
-        goal: Goal<I, (I::Ty, I::Ty)>,
-        a_tys: I::Tys,
-        b_tys: I::Tys,
-    ) -> Result<Candidate<I>, NoSolution> {
-        let cx = self.cx();
-        let Goal { predicate: (_a_ty, b_ty), .. } = goal;
-
-        let (&a_last_ty, a_rest_tys) = a_tys.split_last().unwrap();
-        let b_last_ty = b_tys.last().unwrap();
-
-        // Instantiate just the tail field of B., and require that they're equal.
-        let unsized_a_ty = Ty::new_tup_from_iter(cx, a_rest_tys.iter().copied().chain([b_last_ty]));
-        self.eq(goal.param_env, unsized_a_ty, b_ty)?;
-
-        // Similar to ADTs, require that we can unsize the tail.
-        self.add_goal(
-            GoalSource::ImplWhereBound,
-            goal.with(
-                cx,
-                ty::TraitRef::new(
-                    cx,
-                    cx.require_lang_item(TraitSolverLangItem::Unsize),
-                    [a_last_ty, b_last_ty],
-                ),
-            ),
-        );
-        self.probe_builtin_trait_candidate(BuiltinImplSource::TupleUnsizing)
-            .enter(|ecx| ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes))
-    }
-
     // Return `Some` if there is an impl (built-in or user provided) that may
     // hold for the self type of the goal, which for coherence and soundness
     // purposes must disqualify the built-in auto impl assembled by considering

diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
@@ -1,5 +1,4 @@
-//! An "interner" is a data structure that associates values with usize tags and
-//! allows bidirectional lookup; i.e., given a value, one can easily find the
+//! An "interner" is a data structure that associates values with usize tags and allows bidirectional lookup; i.e., given a value, one can easily find the
 //! type, and vice versa.
 
 use std::hash::{Hash, Hasher};
@@ -2165,7 +2164,6 @@ symbols! {
         unsized_const_params,
         unsized_fn_params,
         unsized_locals,
-        unsized_tuple_coercion,
         unstable,
         unstable_location_reason_default: "this crate is being loaded from the sysroot, an \
                           unstable location; did you mean to load this crate \

diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -1232,8 +1232,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                 // why we special case object types.
                 false
             }
-            ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { .. }, _)
-            | ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, _) => {
+            ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { .. }, _) => {
                 // These traits have no associated types.
                 selcx.tcx().dcx().span_delayed_bug(
                     obligation.cause.span,
@@ -1325,8 +1324,7 @@ fn confirm_select_candidate<'cx, 'tcx>(
         }
         ImplSource::Builtin(BuiltinImplSource::Object { .. }, _)
         | ImplSource::Param(..)
-        | ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { .. }, _)
-        | ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, _) => {
+        | ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { .. }, _) => {
             // we don't create Select candidates with this kind of resolution
             span_bug!(
                 obligation.cause.span,

diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -1017,13 +1017,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
             }
 
-            // `(.., T)` -> `(.., U)`
-            (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
-                if tys_a.len() == tys_b.len() {
-                    candidates.vec.push(BuiltinUnsizeCandidate);
-                }
-            }
-
             _ => {}
         };
     }

diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -7,7 +7,6 @@
 //! [rustc dev guide]:
 //! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
 
-use std::iter;
 use std::ops::ControlFlow;
 
 use rustc_ast::Mutability;
@@ -1320,34 +1319,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ImplSource::Builtin(BuiltinImplSource::Misc, nested)
             }
 
-            // `(.., T)` -> `(.., U)`
-            (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
-                assert_eq!(tys_a.len(), tys_b.len());
-
-                // The last field of the tuple has to exist.
-                let (&a_last, a_mid) = tys_a.split_last().ok_or(Unimplemented)?;
-                let &b_last = tys_b.last().unwrap();
-
-                // Check that the source tuple with the target's
-                // last element is equal to the target.
-                let new_tuple =
-                    Ty::new_tup_from_iter(tcx, a_mid.iter().copied().chain(iter::once(b_last)));
-                let InferOk { mut obligations, .. } = self
-                    .infcx
-                    .at(&obligation.cause, obligation.param_env)
-                    .eq(DefineOpaqueTypes::Yes, target, new_tuple)
-                    .map_err(|_| Unimplemented)?;
-
-                // Add a nested `T: Unsize<U>` predicate.
-                let last_unsize_obligation = obligation.with(
-                    tcx,
-                    ty::TraitRef::new(tcx, obligation.predicate.def_id(), [a_last, b_last]),
-                );
-                obligations.push(last_unsize_obligation);
-
-                ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, obligations)
-            }
-
             _ => bug!("source: {source}, target: {target}"),
         })
     }

diff --git a/compiler/rustc_ty_utils/src/instance.rs b/compiler/rustc_ty_utils/src/instance.rs
@@ -381,8 +381,7 @@ fn resolve_associated_item<'tcx>(
             }
         }
         traits::ImplSource::Param(..)
-        | traits::ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { .. }, _)
-        | traits::ImplSource::Builtin(BuiltinImplSource::TupleUnsizing, _) => None,
+        | traits::ImplSource::Builtin(BuiltinImplSource::TraitUpcasting { .. }, _) => None,
     })
 }
 

diff --git a/compiler/rustc_type_ir/src/solve/mod.rs b/compiler/rustc_type_ir/src/solve/mod.rs
@@ -181,11 +181,6 @@ pub enum BuiltinImplSource {
     ///
     /// The index is only used for winnowing.
     TraitUpcasting(usize),
-    /// Unsizing a tuple like `(A, B, ..., X)` to `(A, B, ..., Y)` if `X` unsizes to `Y`.
-    ///
-    /// This can be removed when `feature(tuple_unsizing)` is stabilized, since we only
-    /// use it to detect when unsizing tuples in hir typeck.
-    TupleUnsizing,
 }
 
 #[derive_where(Clone, Copy, Hash, PartialEq, Eq, Debug; I: Interner)]

diff --git a/library/coretests/tests/lib.rs b/library/coretests/tests/lib.rs
@@ -88,7 +88,6 @@
 #![feature(try_find)]
 #![feature(try_trait_v2)]
 #![feature(unsize)]
-#![feature(unsized_tuple_coercion)]
 #![feature(unwrap_infallible)]
 // tidy-alphabetical-end
 #![allow(internal_features)]

diff --git a/library/coretests/tests/ptr.rs b/library/coretests/tests/ptr.rs
@@ -525,31 +525,24 @@ fn ptr_metadata() {
     assert_eq!(metadata("foo"), 3_usize);
     assert_eq!(metadata(&[4, 7][..]), 2_usize);
 
-    let dst_tuple: &(bool, [u8]) = &(true, [0x66, 0x6F, 0x6F]);
     let dst_struct: &Pair<bool, [u8]> = &Pair(true, [0x66, 0x6F, 0x6F]);
-    assert_eq!(metadata(dst_tuple), 3_usize);
     assert_eq!(metadata(dst_struct), 3_usize);
     unsafe {
-        let dst_tuple: &(bool, str) = std::mem::transmute(dst_tuple);
         let dst_struct: &Pair<bool, str> = std::mem::transmute(dst_struct);
-        assert_eq!(&dst_tuple.1, "foo");
         assert_eq!(&dst_struct.1, "foo");
-        assert_eq!(metadata(dst_tuple), 3_usize);
         assert_eq!(metadata(dst_struct), 3_usize);
     }
 
     let vtable_1: DynMetadata<dyn Debug> = metadata(&4_u16 as &dyn Debug);
     let vtable_2: DynMetadata<dyn Display> = metadata(&4_u16 as &dyn Display);
     let vtable_3: DynMetadata<dyn Display> = metadata(&4_u32 as &dyn Display);
-    let vtable_4: DynMetadata<dyn Display> = metadata(&(true, 7_u32) as &(bool, dyn Display));
-    let vtable_5: DynMetadata<dyn Display> =
+    let vtable_4: DynMetadata<dyn Display> =
         metadata(&Pair(true, 7_u32) as &Pair<bool, dyn Display>);
     unsafe {
         let address_1: *const () = std::mem::transmute(vtable_1);
         let address_2: *const () = std::mem::transmute(vtable_2);
         let address_3: *const () = std::mem::transmute(vtable_3);
         let address_4: *const () = std::mem::transmute(vtable_4);
-        let address_5: *const () = std::mem::transmute(vtable_5);
         // Different trait => different vtable pointer
         assert_ne!(address_1, address_2);
         // Different erased type => different vtable pointer
@@ -558,7 +551,6 @@ fn ptr_metadata() {
         // This is *not guaranteed*, so we skip it in Miri.
         if !cfg!(miri) {
             assert_eq!(address_3, address_4);
-            assert_eq!(address_3, address_5);
         }
     }
 }

diff --git a/src/doc/unstable-book/src/language-features/unsized-tuple-coercion.md b/src/doc/unstable-book/src/language-features/unsized-tuple-coercion.md
diff --git a/src/tools/miri/tests/pass/unsized.rs b/src/tools/miri/tests/pass/unsized.rs
@@ -1,22 +1,8 @@
 //@revisions: stack tree
 //@[tree]compile-flags: -Zmiri-tree-borrows
-#![feature(unsized_tuple_coercion)]
 #![feature(unsized_fn_params)]
 #![feature(custom_mir, core_intrinsics)]
 
-use std::mem;
-
-fn unsized_tuple() {
-    let x: &(i32, i32, [i32]) = &(0, 1, [2, 3]);
-    let y: &(i32, i32, [i32]) = &(0, 1, [2, 3, 4]);
-    let mut a = [y, x];
-    a.sort();
-    assert_eq!(a, [x, y]);
-
-    assert_eq!(&format!("{:?}", a), "[(0, 1, [2, 3]), (0, 1, [2, 3, 4])]");
-    assert_eq!(mem::size_of_val(x), 16);
-}
-
 fn unsized_params() {
     pub fn f0(_f: dyn FnOnce()) {}
     pub fn f1(_s: str) {}
@@ -56,7 +42,6 @@ fn unsized_field_projection() {
 }
 
 fn main() {
-    unsized_tuple();
     unsized_params();
     unsized_field_projection();
 }
diff --git a/src/tools/rust-analyzer/crates/ide-db/src/generated/lints.rs b/src/tools/rust-analyzer/crates/ide-db/src/generated/lints.rs
@@ -12311,40 +12311,6 @@ fn main() {
 ```
 
 will unnecessarily extend the stack frame.
-"##,
-        default_severity: Severity::Allow,
-        warn_since: None,
-        deny_since: None,
-    },
-    Lint {
-        label: "unsized_tuple_coercion",
-        description: r##"# `unsized_tuple_coercion`
-
-The tracking issue for this feature is: [#42877]
-
-[#42877]: https://github.com/rust-lang/rust/issues/42877
-
-------------------------
-
-This is a part of [RFC0401]. According to the RFC, there should be an implementation like this:
-
-```rust,ignore (partial-example)
-impl<..., T, U: ?Sized> Unsized<(..., U)> for (..., T) where T: Unsized<U> {}
-```
-
-This implementation is currently gated behind `#[feature(unsized_tuple_coercion)]` to avoid insta-stability. Therefore you can use it like this:
-
-```rust
-#![feature(unsized_tuple_coercion)]
-
-fn main() {
-    let x : ([i32; 3], [i32; 3]) = ([1, 2, 3], [4, 5, 6]);
-    let y : &([i32; 3], [i32]) = &x;
-    assert_eq!(y.1[0], 4);
-}
-```
-
-[RFC0401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
 "##,
         default_severity: Severity::Allow,
         warn_since: None,