vtable: fix UB detected as miri errors

We can't create references for things that aren't represented by the
reference. Even if we never dereference that reference.

Also add a miri test in the CI that runs the test on crate which are
error free.

.github/workflows/ci.yaml

@@ -553,3 +553,15 @@ jobs:
                   sed -i "s/#wasm# //" Cargo.toml
                   cargo apk build -p printerdemo --target aarch64-linux-android --lib
               working-directory: examples/printerdemo/rust
+
+    miri:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/checkout@v4
+            - uses: ./.github/actions/setup-rust
+              with:
+                  toolchain: nightly
+                  key: miri
+                  components: miri
+            - name: Run Miri
+              run: cargo miri test -p vtable -p const-field-offset -p i-slint-common

helper_crates/vtable/macro/macro.rs

@@ -439,8 +439,8 @@ pub fn vtable(_attr: TokenStream, item: TokenStream) -> TokenStream {
                             // Safety: The vtable is valid and inner is a type corresponding to the vtable,
                             // which was allocated such that drop is expected.
                             unsafe {
-                                let ptr = &*ptr;
-                                (ptr.vtable.as_ref().#ident)(VRefMut::from_raw(ptr.vtable, ptr.ptr)) }
+                                let (vtable, ptr) = ((*ptr).vtable, (*ptr).ptr);
+                                (vtable.as_ref().#ident)(VRefMut::from_raw(vtable, ptr)) }
                         }
                         fn new_box<X: HasStaticVTable<#vtable_name>>(value: X) -> VBox<#vtable_name> {
                             // Put the object on the heap and get a pointer to it

helper_crates/vtable/src/vrc.rs

@@ -73,9 +73,8 @@ struct VRcInner<'vt, VTable: VTableMeta, X> {
 }
 
 impl<'vt, VTable: VTableMeta, X> VRcInner<'vt, VTable, X> {
-    fn data_ptr(&self) -> *const X {
-        let ptr = self as *const Self as *const u8;
-        unsafe { ptr.add(self.data_offset as usize) as *const X }
+    unsafe fn data_ptr(s: *const Self) -> *const X {
+        (s as *const u8).add(*core::ptr::addr_of!((*s).data_offset) as usize) as *const X
     }
     fn as_ref(&self) -> &X {
         let ptr = self as *const Self as *const u8;
@@ -102,25 +101,27 @@ pub struct VRc<VTable: VTableMetaDropInPlace + 'static, X = Dyn> {
 impl<VTable: VTableMetaDropInPlace + 'static, X> Drop for VRc<VTable, X> {
     fn drop(&mut self) {
         unsafe {
-            let inner = self.inner.as_ref();
-            if inner.strong_ref.fetch_sub(1, Ordering::SeqCst) == 1 {
-                let data = inner.data_ptr() as *const _ as *const u8 as *mut u8;
-                let mut layout = VTable::drop_in_place(inner.vtable, data);
+            let inner = self.inner.as_ptr();
+            if (*inner).strong_ref.fetch_sub(1, Ordering::SeqCst) == 1 {
+                let data =
+                    (inner as *mut u8).add(*core::ptr::addr_of!((*inner).data_offset) as usize);
+                let vtable = core::ptr::addr_of!((*inner).vtable);
+                let mut layout = VTable::drop_in_place(&*vtable, data);
                 layout = core::alloc::Layout::new::<VRcInner<VTable, ()>>()
                     .extend(layout.try_into().unwrap())
                     .unwrap()
                     .0
                     .pad_to_align()
                     .into();
-                if inner.weak_ref.load(Ordering::SeqCst) > 1 {
+                if (*core::ptr::addr_of!((*inner).weak_ref)).load(Ordering::SeqCst) > 1 {
                     // at this point we are sure that no other thread can access the data
                     // since we still hold a weak reference, so the other weak references
                     // in other thread won't start destroying the object.
-                    self.inner.cast::<VRcInner<VTable, Layout>>().as_mut().data = layout;
+                    *(VRcInner::data_ptr(self.inner.cast::<VRcInner<VTable, Layout>>().as_ptr())
+                        as *mut Layout) = layout;
                 }
-                if inner.weak_ref.fetch_sub(1, Ordering::SeqCst) == 1 {
-                    let vtable = inner.vtable;
-                    VTable::dealloc(vtable, self.inner.cast().as_ptr(), layout);
+                if (*core::ptr::addr_of!((*inner).weak_ref)).fetch_sub(1, Ordering::SeqCst) == 1 {
+                    VTable::dealloc(&*vtable, self.inner.cast().as_ptr(), layout);
                 }
             }
         }
@@ -220,8 +221,8 @@ impl<VTable: VTableMetaDropInPlace, X> VRc<VTable, X> {
         unsafe {
             let inner = this.inner.cast::<VRcInner<VTable, u8>>();
             VRef::from_raw(
-                NonNull::from(inner.as_ref().vtable),
-                NonNull::from(&*inner.as_ref().data_ptr()),
+                NonNull::from(*::core::ptr::addr_of!((*inner.as_ptr()).vtable)),
+                NonNull::new_unchecked(VRcInner::data_ptr(inner.as_ptr()) as *mut u8),
             )
         }
     }
@@ -303,13 +304,14 @@ impl<VTable: VTableMetaDropInPlace + 'static, X> Clone for VWeak<VTable, X> {
 impl<T: VTableMetaDropInPlace + 'static, X> Drop for VWeak<T, X> {
     fn drop(&mut self) {
         if let Some(i) = self.inner {
-            let inner = unsafe { i.as_ref() };
-            if inner.weak_ref.fetch_sub(1, Ordering::SeqCst) == 1 {
-                let vtable = inner.vtable;
-                unsafe {
+            unsafe {
+                if (*core::ptr::addr_of!((*i.as_ptr()).weak_ref)).fetch_sub(1, Ordering::SeqCst)
+                    == 1
+                {
                     // Safety: while allocating, we made sure that the size was big enough to
                     // hold a VRcInner<T, Layout>.
-                    let layout = i.cast::<VRcInner<T, Layout>>().as_ref().data;
+                    let vtable = &*core::ptr::addr_of!((*i.as_ptr()).vtable);
+                    let layout = *(VRcInner::data_ptr(i.cast::<VRcInner<T, Layout>>().as_ptr()));
                     T::dealloc(vtable, i.cast().as_ptr(), layout);
                 }
             }