Rollup merge of #127859 - RalfJung:ptr-dyn-metadata, r=scottmcm · model-checking/verify-rust-std@3d50720 (original) (raw)
`@@ -5,6 +5,7 @@ use crate::hash::{Hash, Hasher};
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`use crate::intrinsics::aggregate_raw_ptr;
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`use crate::intrinsics::ptr_metadata;
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`use crate:📑:Freeze;
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use crate::ptr::NonNull;
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``
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`/// Provides the pointer metadata type of any pointed-to type.
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`///
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`@@ -153,7 +154,7 @@ pub const fn from_raw_parts_mut<T: ?Sized>(
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`/// compare equal (since identical vtables can be deduplicated within a codegen unit).
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`#[lang = "dyn_metadata"]
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`pub struct DynMetadata<Dyn: ?Sized> {
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_vtable_ptr: &'static VTable,
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_vtable_ptr: NonNull,
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`_phantom: crate:📑:PhantomData,
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`}
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`@@ -166,15 +167,18 @@ extern "C" {
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`}
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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.
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`// If it's somehow not a size match, the transmute will error.
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``
`-
unsafe { crate::mem::transmute::<Self, &'static VTable>(self) }
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unsafe { crate::mem::transmute::<Self, *const VTable>(self) }
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`}
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`/// Returns the size of the type associated with this vtable.
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