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ptr::metadata: update comment on vtable_ptr work-around

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This commit is contained in:
Ralf Jung 2024-07-17 13:50:17 +02:00
parent 21dc49c587
commit f9c0d3370f
1 changed files with 7 additions and 4 deletions
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11
library/core/src/ptr/metadata.rs
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@ -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.