diff options
| -rw-r--r-- | library/core/src/ptr/metadata.rs | 11 |
1 files changed, 7 insertions, 4 deletions
diff --git a/library/core/src/ptr/metadata.rs b/library/core/src/ptr/metadata.rs index 21d1b7ea0ce..06f205c0f26 100644 --- a/library/core/src/ptr/metadata.rs +++ b/library/core/src/ptr/metadata.rs @@ -167,10 +167,13 @@ extern "C" { } impl<Dyn: ?Sized> DynMetadata<Dyn> { - /// One of the things that rustc_middle does with this being a lang item is - /// give it `FieldsShape::Primitive`, which means that as far as codegen can - /// tell, it *is* a reference, and thus doesn't have any fields. - /// That means we can't use field access, and have to transmute it instead. + /// When `DynMetadata` appears as the metadata field of a wide pointer, the rustc_middle layout + /// computation does magic and the resulting layout is *not* a `FieldsShape::Aggregate`, instead + /// it is a `FieldsShape::Primitive`. This means that the same type can have different layout + /// depending on whether it appears as the metadata field of a wide pointer or as a stand-alone + /// type, which understandably confuses codegen and leads to ICEs when trying to project to a + /// field of `DynMetadata`. To work around that issue, we use `transmute` instead of using a + /// field projection. #[inline] fn vtable_ptr(self) -> *const VTable { // SAFETY: this layout assumption is hard-coded into the compiler. |