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Rollup merge of #127859 - RalfJung:ptr-dyn-metadata, r=scottmcm

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ptr::metadata: avoid references to extern types

References to `extern types` are somewhat dubious entities, since generally we say that references must be dereferenceable for their size as determined via `size_of_val`, but with `extern type` that is an ill-defined statement. I'd like to make Miri warn for such cases since it interacts poorly with Stacked Borrows. To avoid warnings people can't fix, this requires not using references to `extern type` in the standard library, and I think `DynMetadata` is the only currently remaining use. so this changes `DynMetadata` to use a NonNull raw pointer instead. Given that the alignment was 1, this shouldn't really change anything meaningful.

I also updated a comment added by `@scottmcm` in https://github.com/rust-lang/rust/pull/125479, since I think the old comment is wrong. The `DynMetadata` type itself is not special, it is a normal aggregate. But computing field types for wide pointers (including references) is special.
This commit is contained in:
Trevor Gross 2024-07-17 19:53:28 -05:00 committed by GitHub
commit c36a39cd1f
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1 changed files with 10 additions and 6 deletions
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16
library/core/src/ptr/metadata.rs
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@ -5,6 +5,7 @@ use crate::hash::{Hash, Hasher};
use crate::intrinsics::aggregate_raw_ptr;
use crate::intrinsics::ptr_metadata;
use crate::marker::Freeze;
use crate::ptr::NonNull;
/// Provides the pointer metadata type of any pointed-to type.
///
@ -153,7 +154,7 @@ pub const fn from_raw_parts_mut<T: ?Sized>(
/// compare equal (since identical vtables can be deduplicated within a codegen unit).
#[lang = "dyn_metadata"]
pub struct DynMetadata<Dyn: ?Sized> {
_vtable_ptr: &'static VTable,
_vtable_ptr: NonNull<VTable>,
_phantom: crate::marker::PhantomData<Dyn>,
}
@ -166,15 +167,18 @@ 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.
// If it's somehow not a size match, the transmute will error.
unsafe { crate::mem::transmute::<Self, &'static VTable>(self) }
unsafe { crate::mem::transmute::<Self, *const VTable>(self) }
}
/// Returns the size of the type associated with this vtable.