Emit delayed bug during wfck for stranded opaque

[tiif]

Fixes rust-lang/trait-system-refactor-initiative#235

Problem

The fundamental issue here is OpaqueTypeCollector operates on rustc_middle::Ty, but check_type_wf operates on HIR.

Since check_type_wf operates on HIR, it can see the stranded opaque and tries to infer it's hidden type. But OpaqueTypeCollector operates on rustc_middle::Ty, so the OpaqueTypeCollector can no longer see a stranded opaque, hence its hidden type could not be inferred.

As a result, the tests ICE'ed at

rust/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs

Line 253 in 34a8c73

assert!(!tcx.next_trait_solver_globally());

Proposed solution

This PR detects stranded opaque types during wf check and emit a delayed bug for it.

Alternative solution

@BoxyUwU and I had considered rewriting OpaqueTypeCollector to be a HIR visitor instead of a rustc_middle::Ty visitor, but we believe a HIR-based OpaqueTypeCollector will not work and might not worth the cost of rewriting.

Acknowledgement

This PR is a joint effort with @BoxyUwU :3

[rustbot]

r? @jackh726

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[tiif]

r? @BoxyUwU

[BoxyUwU]

can you add a test case for something like -> String<impl Trait> which should also be another way of getting stranded opaques

[tiif]

okay, done :)