Fix ICE with unsized type in const pattern

[FabianWolff]

Fixes #87046. The deref_const() query currently contains the following check:

rust/compiler/rustc_mir/src/const_eval/mod.rs

Lines 191 to 204 in e9a387d

	let ty = match mplace.meta {
	MemPlaceMeta::None => mplace.layout.ty,
	MemPlaceMeta::Poison => bug!("poison metadata in `deref_const`: {:#?}", mplace),
	// In case of unsized types, figure out the real type behind.
	MemPlaceMeta::Meta(scalar) => match mplace.layout.ty.kind() {
	ty::Str => bug!("there's no sized equivalent of a `str`"),
	ty::Slice(elem_ty) => tcx.mk_array(elem_ty, scalar.to_machine_usize(&tcx).unwrap()),
	_ => bug!(
	"type {} should not have metadata, but had {:?}",
	mplace.layout.ty,
	mplace.meta
	),
	},
	};

i.e. this will cause an ICE for every unsized type except slices. An error is reported with my changes if such a type is used as a const pattern (this should not be a breaking change, since so far, this has caused an ICE).

[rust-highfive]

r? @oli-obk

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[oli-obk]

that certainly avoids the ICE, but we could also make such patterns legal. I think we should. @rust-lang/wg-const-eval what do you think about these?

[RalfJung]

This always does span_err or delay_span_bug... how does this make anything legal?

[oli-obk]

I said "we could ...", I'm fine doing this PR, it changes an ICE to an error, but why should the code in the new test not be permitted? It seems perfectly alright to me

[RalfJung]

I'm not familiar enough with pattern matching code to comment on this.

But I'd prefer we port that code to valtrees first, then I will be a lot more confident that what that code does makes sense in my mental model of rustc. :)

[kim]

@oli-obk What would be a good way to direct attention to this? I would find it incredibly useful to be able to use newtypes around str as const patterns.

[oli-obk]

Valtrees are being worked on right now, once we have them, this becomes trivial to do and we can ask T-lang whether they think this should br supported, too

[kim]

Cool thanks

[oli-obk]

@bors r+

[bors]

📌 Commit 79f0743 has been approved by oli-obk