Support sized-to-unsized transmute_{ref,mut}!

joshlf · joshlf · commit 194830728e67 · 2026-01-28T04:10:24.000Z
Closes #2721 gherrit-pr-id: G73f67c103188ed404d0051bc140c4d0711fe0753
diff --git a/src/macros.rs b/src/macros.rs
@@ -1196,6 +1196,13 @@ mod tests {
         const X: &'static [[u8; 2]; 4] = transmute_ref!(&ARRAY_OF_U8S);
         assert_eq!(*X, ARRAY_OF_ARRAYS);
 
+        // Test sized -> unsized transmutation.
+        let array_of_u8s = [0u8, 1, 2, 3, 4, 5, 6, 7];
+        let array_of_arrays = [[0, 1], [2, 3], [4, 5], [6, 7]];
+        let slice_of_arrays = &array_of_arrays[..];
+        let x: &[[u8; 2]] = transmute_ref!(&array_of_u8s);
+        assert_eq!(x, slice_of_arrays);
+
         // Before 1.61.0, we can't define the `const fn transmute_ref` function
         // that we do on and after 1.61.0.
         #[cfg(no_zerocopy_generic_bounds_in_const_fn_1_61_0)]
diff --git a/src/util/macro_util.rs b/src/util/macro_util.rs
@@ -820,7 +820,7 @@ pub trait TransmuteRefDst<'a> {
 
 impl<'a, Src: ?Sized, Dst: ?Sized> TransmuteRefDst<'a> for Wrap<&'a Src, &'a Dst>
 where
-    Src: KnownLayout<PointerMetadata = usize> + IntoBytes + Immutable,
+    Src: KnownLayout + IntoBytes + Immutable,
     Dst: KnownLayout<PointerMetadata = usize> + FromBytes + Immutable,
 {
     type Dst = Dst;
@@ -853,7 +853,7 @@ pub trait TransmuteMutDst<'a> {
 
 impl<'a, Src: ?Sized, Dst: ?Sized> TransmuteMutDst<'a> for Wrap<&'a mut Src, &'a mut Dst>
 where
-    Src: KnownLayout<PointerMetadata = usize> + FromBytes + IntoBytes,
+    Src: KnownLayout + FromBytes + IntoBytes,
     Dst: KnownLayout<PointerMetadata = usize> + FromBytes + IntoBytes,
 {
     type Dst = Dst;
