Report unsatisfied opaque item bounds instead of a type mismatch under -Znext-solver

[yashksaini-coder]

LLM disclosure

I used Claude extensively on this PR: to explore the compiler source. The initial diagnosis it produced was wrong twice — the first patch it suggested compiled but was entirely dead code. The actual cause (FreeTy rather than OpaqueTy) came from debug output I added and ran locally at its suggestion. I have read and understand the final patch, and verified it against a local build and the full tests/ui suite.

Fixes #162093

Under -Znext-solver, a TAIT whose hidden type doesn't satisfy the opaque's item bounds reported a bare type mismatch rather than naming the unsatisfied bound. This also meant #[diagnostic::on_unimplemented] on that trait never applied.

Before

error[E0271]: type mismatch resolving `Ta == u32`
 --> src/main.rs:9:19
  |
9 | fn construct() -> Ta {
  |                   ^^ types differ

After

error[E0277]: my custom message
 --> src/main.rs:9:19
  |
9 | fn construct() -> Ta {
  |                   ^^ the trait `Marker` is not implemented for `u32`

What was happening

type Ta = impl Marker; is a free type alias whose value is the opaque, so the failing goal is a projection on a FreeTy alias rather than on the opaque directly.

normalize_free_alias normalizes the alias' value and propagates failure with ?, returning before evaluate_added_goals_and_make_canonical_response. No MakeCanonicalResponse step gets recorded, and candidates_recur only builds a candidate when shallow_certainty was set — so the goal ends up with no candidates at all. BestObligation has nothing to descend into and falls back to the projection obligation, which fulfillment_error_for_no_solution turns into E0271.

The fix recurses into the alias' value from detect_error_from_empty_candidates. That reaches the opaque, which does record a candidate and carries its item bounds as nested GoalSource::AliasWellFormed goals, so the real u32: Marker failure surfaces.

Test changes

Three errors in tests/ui/traits/next-solver/stalled-coroutine-obligations.rs improve from type mismatch resolving to naming the unsatisfied trait bound.

Known gaps

• RPIT (-> impl Future + Send) has no free alias in front of the opaque, so it still reports E0271 — this is the fourth error in stalled-coroutine-obligations, left unchanged.
• The ObligationCauseCode::OpaqueReturnType cause code isn't attached, so the return type was inferred to be u32 here note the old solver emits is still missing under next. Visible as a difference between the current and next revisions of the new test.

Full tests/ui run is clean (20484 passed, 0 failed).

r? lcnr

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @lcnr (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.