Structurally normalize weak and inherent in new solver
It seems pretty obvious to me that we should be normalizing weak and inherent aliases too, since they can always be normalized. This PR still leaves open the question of what to do with opaques, though 💀
**Also**, we need to structurally resolve the target of a coercion, for the UI test to work.
r? `@lcnr`
Bubble up nested goals from equation in `predicates_for_object_candidate`
This used to be needed for https://github.com/rust-lang/rust/pull/114036#discussion_r1273987510, but since it's no longer, I'm opening this as a separate PR. This also fixes one ICEing UI test: (`tests/ui/unboxed-closures/issue-53448.rs`)
r? `@lcnr`
Structurally normalize in selection
We need to do this because of the fact that we're checking the `Ty::kind` on a type during selection, but goals passed into select are not necessarily normalized.
Right now, we're (kinda) unnecessarily normalizing the RHS of a trait upcasting goal, which is broken for different reasons (#113393). But I'm waiting for this PR to land before discussing that one.
r? `@lcnr`
Implement selection for `Unsize` for better coercion behavior
In order for much of coercion to succeed, we need to be able to deal with partial ambiguity of `Unsize` traits during selection. However, I pessimistically implemented selection in the new trait solver to just bail out with ambiguity if it was a built-in impl:
9227ff28af/compiler/rustc_trait_selection/src/solve/eval_ctxt/select.rs (L126)
This implements a proper "rematch" procedure for dealing with built-in `Unsize` goals, so that even if the goal is ambiguous, we are able to get nested obligations which are used in the coercion selection-like loop:
9227ff28af/compiler/rustc_hir_typeck/src/coercion.rs (L702)
Second commit just moves a `resolve_vars_if_possible` call to fix a bug where we weren't detecting a trait upcasting to occur.
r? ``@lcnr``
Structurally normalize again for byte string lit pat checking
We need to structurally normalize the pointee of a match scrutinee when trying to match byte string patterns -- we used[^1] to call `structurally_resolve_type`, which errors for type vars[^2], but lcnr added `try_structurally_resolve_type`[^3] in the mean time, which is the right thing to use here since it's totally opportunistic.
Fixes rust-lang/trait-system-refactor-initiative#38
[^1]: #112428
[^2]: #112993
[^3]: #113086
Prefer object candidates in new selection
`dyn Any` shouldn't be using [this implementation](https://doc.rust-lang.org/std/any/trait.Any.html#impl-Any-for-T) during codegen.
Prefer object candidates over other candidates, except for other object candidates.
Revert "Structurally resolve correctly in check_pat_lit"
This reverts commit 54fb5a48b9. Also adds a couple of tests, and downgrades the existing `-Ztrait-solver=next` test to a known-bug.
Fixes#112993
[-Ztrait-solver=next, mir-typeck] instantiate hidden types in the root universe
Fixes an ICE in the test `member-constraints-in-root-universe`.
Main motivation is to make #112691 pass under the new solver.
r? ``@compiler-errors``
The only regression is one ambiguity in the new trait solver, having to
do with two param-env candidates that may apply. I think this is fine,
since the error message already kinda sucks.
Opportunistically resolve regions in new solver
Use `opportunistic_resolve_var` during canonicalization to collapse some regions.
We have to start using `CanonicalVarValues::is_identity_modulo_regions`. We also have to modify that function to consider responses like `['static, ^0, '^1, ^2]` to be an "identity" response, since because we opportunistically resolve regions, there's no longer a 1:1 mapping between canonical var values and bound var indices in the response...
There's one nasty side-effect -- one test (`tests/ui/dyn-star/param-env-infer.rs`) starts to ICE because the certainty goes from `Yes` to `Maybe(Overflow)`... Not exactly sure why, though? Putting this up for discussion/investigation.
r? ```@lcnr```
Instantiate closure synthetic substs in root universe
In the UI test example, we end up generalizing an associated type (something like `<Map<Option<i32>, [closure upvars=?0]> as IntoIterator>::Item` generalizes into `<Map<Option<i32>, [closure upvars=?1]> as IntoIterator>::Item`) then assigning it to itself, emitting an alias-relate goal. This trivially holds via one of the normalizes-to candidates, instead of relating substs, so when closure analysis eventually sets `?0` to the actual upvars, `?1` never gets constrained. This ends up being reported as an ambiguity error during writeback.
Instead, we can take advantage of the fact that we *know* the closure substs live in the root universe. This will prevent them being generalized, since they always can be named, and the alias-relate above never gets emitted at all.
We can probably do this to a handful of other `next_ty_var` calls in typeck for variables that are clearly associated with the body of the program, but I wanted to limit this for now. Eventually, if we end up representing universes more faithfully like a tree or whatever, we can remove this and turn it back to just a call to `next_ty_var`.
Note: This is incredibly order-dependent -- we need to be assigning a type variable that was created *before* the closure substs, and we also need to actually have an unnormalized type at the time of the assignment. This currently seems easiest to trigger during call argument analysis just due to the fact that we instantiate the call's substs, normalize, THEN check args.
r? ```@lcnr```
