fix: Do not ICE when missing match arm with ill-formed subty is met

[ShoyuVanilla]

Fixes #148192

The ICE comes from the following line, calling normalize_erasing_regions to a projection type whose trait bound is not met:

rust/compiler/rustc_pattern_analysis/src/rustc.rs

Lines 185 to 194 in 2fcbda6

	pub(crate) fn variant_sub_tys(
	&self,
	ty: RevealedTy<'tcx>,
	variant: &'tcx VariantDef,
	) -> impl Iterator<Item = (&'tcx FieldDef, RevealedTy<'tcx>)> {
	let ty::Adt(_, args) = ty.kind() else { bug!() };
	variant.fields.iter().map(move |field| {
	let ty = field.ty(self.tcx, args);
	// `field.ty()` doesn't normalize after instantiating.
	let ty = self.tcx.normalize_erasing_regions(self.typing_env, ty);

The above function is called while trying to lint missing match arms, or scrutinize ctors of missing(not necessary error) match arms.

So, the following code can trigger ICEs.

trait WhereTrait {
    type Type;
}

fn foo(e: Enum) {
    match e {
        Enum::Map(_) => (), // ICE, while trying to lint missing arms
    }

    if let Enum::Map(_) = e {} // ICE, while trying to scrutinize missing ctors (even worse)
}

enum Enum {
    Map(()),
    Map2(<() as WhereTrait>::Type),
}

This ICE won't be triggered with the following code, as this is filtered out before check_match as the existence of ill-formed type inside the variant marks the body as tainted by error in hir_typeck, but for the above code, the hir_typeck complains nothing because everything it sees is locally correct.

fn foo(e: Enum) {
    match e {
        Enum::Map2(_) => (), // No ICE
    }
}

I've considered visiting and wf checking for the match scrutinee before entering check_match, but that might regress the perf and I think just emitting delayed bug would enough as the normalization failure would be originated by other errors like ill-formdness.

[rustbot]

Some changes occurred in exhaustiveness checking

cc @Nadrieril

[rustbot]

r? @chenyukang

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[chenyukang]

Thanks!
@bors r=chenyukang

[bors]

📌 Commit dae003b has been approved by chenyukang

It is now in the queue for this repository.