ptr::metadata: update comment on vtable_ptr work-around · model-checking/verify-rust-std@89cd225 (original) (raw)

`@@ -167,10 +167,13 @@ extern "C" {

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`impl<Dyn: ?Sized> DynMetadata {

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/// One of the things that rustc_middle does with this being a lang item is

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`` -

/// give it FieldsShape::Primitive, which means that as far as codegen can

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/// tell, it is a reference, and thus doesn't have any fields.

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/// That means we can't use field access, and have to transmute it instead.

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`` +

/// When DynMetadata appears as the metadata field of a wide pointer, the rustc_middle layout

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`` +

/// computation does magic and the resulting layout is not a FieldsShape::Aggregate, instead

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`` +

/// it is a FieldsShape::Primitive. This means that the same type can have different layout

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/// depending on whether it appears as the metadata field of a wide pointer or as a stand-alone

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/// type, which understandably confuses codegen and leads to ICEs when trying to project to a

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`` +

/// field of DynMetadata. To work around that issue, we use transmute instead of using a

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/// field projection.

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`#[inline]

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`fn vtable_ptr(self) -> *const VTable {

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`// SAFETY: this layout assumption is hard-coded into the compiler.