Use the new solver in the `impossible_predicates`
The old solver is unsound for many reasons. One of which was weaponized by `@lcnr` in #140212, where the old solver was incompletely considering a dyn vtable method to be impossible and replacing its vtable entry with a null value. This null function could be called post-mono.
The new solver is expected to be less incomplete due to its correct handling of higher-ranked aliases in relate. This PR switches the `impossible_predicates` query to use the new solver, which patches this UB.
r? lcnr
Merge typeck loop with static/const item eval loop
r? `@ghost`
Let's try a small one first. Doing this in general has some bad cache coherence issues because the query caches are laid out in `Vec<QueryResult>` lists per query where each index refers to a `DefId` in the same order as we're iterating. Iterating two or more lists at the same time does have cache issues, so I want to poke a bit at it to see if we can't merge just a few of them at a time.
Only include `dyn Trait<Assoc = ...>` associated type bounds for `Self: Sized` associated types if they are provided
Since #136458, we began filtering out associated types with `Self: Sized` bounds when constructing the list of associated type bounds to put into our `dyn Trait` types. For example, given:
```rust
trait Trait {
type Assoc where Self: Sized;
}
```
After #136458, even if a user writes `dyn Trait<Assoc = ()>`, the lowered ty would have an empty projection list, and thus be equivalent to `dyn Trait`. However, this has the side effect of no longer constraining any types in the RHS of `Assoc = ...`, not implying any WF implied bounds, and not requiring that they hold when unsizing.
After this PR, we include these bounds, but (still) do not require that they are provided. If the are not provided, they are skipped from the projections list.
This results in `dyn Trait` types that have differing numbers of projection bounds. This will lead to re-introducing type mismatches e.g. between `dyn Trait` and `dyn Trait<Assoc = ()>`. However, this is expected and doesn't suffer from any of the deduplication unsoundness from before #136458.
We may want to begin to ignore thse bounds in the future by bumping `unused_associated_type_bounds` to an FCW. I don't want to tangle that up into the fix that was originally intended in #136458, so I'm doing a "fix-forward" in this PR and deferring thinking about this for the future.
Fixes#140645
r? lcnr
Don't ICE on pending obligations from deep normalization in a loop
See the comment I left inline in `compiler/rustc_trait_selection/src/traits/normalize.rs`.
Fixes https://github.com/rust-lang/rust/issues/133868
r? lcnr
Fix replacing supertrait aliases in `ReplaceProjectionWith`
The new solver has a procedure called `predicates_for_object_candidate`, which elaborates the super-bounds and item-bounds that are required to hold for a dyn trait to implement something via a built-in object impl.
In that procedure, there is a folder called `ReplaceProjectionWith` which is responsible for replacing projections that reference `Self`, so that we don't encounter cycles when we then go on to normalize those projections in the process of proving these super-bounds.
That folder had a few problems: Firstly, it wasn't actually checking that this was a super bound originating from `Self`. Secondly, it only accounted for a *single* projection type def id, but trait objects can have multiple (i.e. `trait Foo<A, B>: Bar<A, Assoc = A> + Bar<B, Assoc = B>`).
To fix the first, it's simple enough to just add an equality check for the self ty. To fix the second, I implemented a matching step that's very similar to the `projection_may_match` check we have for upcasting, since on top of having multiple choices, we need to deal with both non-structural matches and ambiguity.
This probably lacks a bit of documentation, but I think it works pretty well.
Fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/171
r? lcnr
`hir::AssocItem` currently has a boolean `fn_has_self_parameter` field,
which is misplaced, because it's only relevant for associated fns, not
for associated consts or types. This commit moves it (and renames it) to
the `AssocKind::Fn` variant, where it belongs.
This requires introducing a new C-style enum, `AssocTag`, which is like
`AssocKind` but without the fields. This is because `AssocKind` values
are passed to various functions like `find_by_ident_and_kind` to
indicate what kind of associated item should be searched for, and having
to specify `has_self` isn't relevant there.
New methods:
- Predicates `AssocItem::is_fn` and `AssocItem::is_method`.
- `AssocItem::as_tag` which converts `AssocItem::kind` to `AssocTag`.
Removed `find_by_name_and_kinds`, which is unused.
`AssocItem::descr` can now distinguish between methods and associated
functions, which slightly improves some error messages.
