Don't normalize to an un-revealed opaque when we hit the recursion limit

[compiler-errors]

Currently, we will normalize Opaque := Option<&Opaque> to something like Option<&Option<&Option<&...Opaque>>>, hitting a limit and bottoming out in an unnormalized opaque after the recursion limit gets hit.

Unfortunately, during layout_of, we'll simply recurse and try again if the type normalizes to something different than the type:

rust/compiler/rustc_ty_utils/src/layout.rs

Lines 58 to 60 in e6e931d

	if ty != unnormalized_ty {
	// Ensure this layout is also cached for the normalized type.
	return tcx.layout_of(param_env.and(ty));

That means then we'll try to normalize Option<&Option<&Option<&...Opaque>>> again, substituting Opaque into itself even deeper. Eventually this will get to the point that we're just stack-overflowing on a really deep type before even hitting an opaque again.

To fix this, we just bottom out into ty::Error instead of the unrevealed opaque type.

Fixes #117412

r? @oli-obk

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit c91f60e has been approved by oli-obk

It is now in the queue for this repository.