Prefer lower `TraitUpcasting` candidates in selection
Fixes#135463. The underlying cause is this ambiguity, but it's more clear (and manifests as a coercion error, rather than a MIR validation error) when it's written the way I did in the UI test.
Sorry this is cursed r? lcnr
Improve diagnostics for `HostEffectPredicate` in the new solver
Adds derived cause for host effect predicates. Some diagnostics regress, but that's connected to the fact that our predicate visitor doesn't play well with aliases just yet.
finish `Reveal` removal
After #133212 changed the `TypingMode` to be the only source of truth, this entirely rips out `Reveal`.
cc #132279
r? `@compiler-errors`
Try to improve error messages involving aliases in the solver
1. Treat aliases as rigid only if it may not be defined and it's well formed (i.e. for projections, its trait goal is satisfied).
2. Record goals that are related to alias normalization under a new `GoalKind`, so we can look into them in the `BestObligation` visitor.
3. Try to deduplicate errors due to self types of goals that are un-normalizable aliases.
r? lcnr
`-Znext-solver` caching
This PR has two major changes while also fixing multiple issues found via fuzzing.
The main optimization is the ability to not discard provisional cache entries when popping the highest cycle head the entry depends on. This fixes the hang in Fuchsia with `-Znext-solver=coherence`.
It also bails if the result of a fixpoint iteration is ambiguous, even without reaching a fixpoint. This is necessary to avoid exponential blowup if a coinductive cycle results in ambiguity, e.g. due to unknowable candidates in coherence.
Updating stack entries pretty much exclusively happens lazily now, so `fn check_invariants` ended up being mostly useless and I've removed it. See https://gist.github.com/lcnr/8de338fdb2685581e17727bbfab0622a for the invariants we would be able to assert with it.
For a general overview, see the in-process update of the relevant rustc-dev-guide chapter: https://hackmd.io/1ALkSjKlSCyQG-dVb_PUHw
r? ```@compiler-errors```
