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Auto merge of #125479 - scottmcm:validate-vtable-projections, r=Nilstrieb

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Validate the special layout restriction on `DynMetadata`

If you look at <https://stdrs.dev/nightly/x86_64-unknown-linux-gnu/std/ptr/struct.DynMetadata.html>, you'd think that `DynMetadata` is a struct with fields.

But it's actually not, because the lang item is special-cased in rustc_middle layout:

7601adcc76/compiler/rustc_middle/src/ty/layout.rs (L861-L864)

That explains the very confusing codegen ICEs I was getting in https://github.com/rust-lang/rust/pull/124251#issuecomment-2128543265

> Tried to extract_field 0 from primitive OperandRef(Immediate((ptr:  %5 = load ptr, ptr %4, align 8, !nonnull !3, !align !5, !noundef !3)) @ TyAndLayout { ty: DynMetadata<dyn Callsite>, layout: Layout { size: Size(8 bytes), align: AbiAndPrefAlign { abi: Align(8 bytes), pref: Align(8 bytes) }, abi: Scalar(Initialized { value: Pointer(AddressSpace(0)), valid_range: 1..=18446744073709551615 }), fields: Primitive, largest_niche: Some(Niche { offset: Size(0 bytes), value: Pointer(AddressSpace(0)), valid_range: 1..=18446744073709551615 }), variants: Single { index: 0 }, max_repr_align: None, unadjusted_abi_align: Align(8 bytes) } })

because there was a `Field` projection despite the layout clearly saying it's [`Primitive`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/abi/enum.FieldsShape.html#variant.Primitive).

Thus this PR updates the MIR validator to check for such a projection, and changes `libcore` to not ever emit any projections into `DynMetadata`, just to transmute the whole thing when it wants a pointer.
This commit is contained in:
bors 2024-05-24 08:53:27 +00:00
commit 464987730a
2 changed files with 28 additions and 8 deletions
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9
compiler/rustc_const_eval/src/transform/validate.rs
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@ -685,6 +685,15 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
check_equal(self, location, *f_ty);
}
ty::Adt(adt_def, args) => {
// see <https://github.com/rust-lang/rust/blob/7601adcc764d42c9f2984082b49948af652df986/compiler/rustc_middle/src/ty/layout.rs#L861-L864>
if Some(adt_def.did()) == self.tcx.lang_items().dyn_metadata() {
self.fail(
location,
format!("You can't project to field {f:?} of `DynMetadata` because \
layout is weird and thinks it doesn't have fields."),
);
}
let var = parent_ty.variant_index.unwrap_or(FIRST_VARIANT);
let Some(field) = adt_def.variant(var).fields.get(f) else {
fail_out_of_bounds(self, location);