Validate the special layout restriction on DynMetadata

[scottmcm]

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:

rust/compiler/rustc_middle/src/ty/layout.rs

Lines 861 to 864 in 7601adc

	// Map `Metadata = DynMetadata<dyn Trait>` back to a vtable, since it
	// offers better information than `std::ptr::metadata::VTable`,
	// and we rely on this layout information to trigger a panic in
	// `std::mem::uninitialized::<&dyn Trait>()`, for example.

That explains the very confusing codegen ICEs I was getting in #124251 (comment)

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, 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.

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.

[rustbot]

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[workingjubilee]

Library changes look fine to me but I think I want to ask

r? compiler

[Noratrieb]

@bors r+ rollup

[bors]

📌 Commit d83f3ca has been approved by Nilstrieb

It is now in the queue for this repository.

[bors]

⌛ Testing commit d83f3ca with merge 4649877...

[bors]

☀️ Test successful - checks-actions
Approved by: Nilstrieb
Pushing 4649877 to master...

[rust-timer]

Finished benchmarking commit (4649877): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary -3.4%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.4%, secondary 4.9%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.0%, 2.7%]	5
Regressions ❌ (secondary)	4.9%	[2.1%, 9.3%]	11
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.0%, 2.7%]	5

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 673.551s -> 673.01s (-0.08%)
Artifact size: 315.70 MiB -> 315.73 MiB (0.01%)