`-Znext-solver` instantiate predicate binder without recanonicalizing goal
This strengthens the leak check to match the old trait solver. The new trait solver now also instantiates higher ranked goals in the same scope as candidate selection, so the leak check in each candidate detects placeholder errors involving this higher ranked goal.
E.g. let's look at tests/ui/higher-ranked/leak-check/leak-check-in-selection-2.rs
```rust
trait Trait<T, U> {}
impl<'a> Trait<&'a str, &'a str> for () {}
impl<'a> Trait<&'a str, String> for () {}
fn impls_trait<T: for<'a> Trait<&'a str, U>, U>() {}
fn main() {
impls_trait::<(), _>();
}
```
Here proving `(): for<'a> Trait<&'a str, ?u>` via `impl<'a> Trait<&'a str, &'a str> for ()` equates `?u` with `&'!a str` which results in a leak check error as `?u` cannot name `'a`. If this leak check error happens while considering candidates we drop the first impl and infer `?u` to `String`. If not, this remains ambiguous.
This behavior is a bit iffy, see the FCP proposal in rust-lang/rust#119820 for more details on why this current behavior is somewhat undesirable. However, considering placeholders from higher-ranked goals for candidate selection does allow more code to compile and a lot of the code *feels like it should compile*. **This caused us to revert the change of rust-lang/rust#119820 in rust-lang/rust#127568.**
I originally expected that we can avoid breakage with the new solver differently here, e.g. by considering OR-region constraints. However, doing so is a significant change and I don't have a great idea for how that should work. Matching the old solver behavior for now should not make this cleaner approach any more difficult in the future, so let's just go with what actually allows us to stabilize the new solver for now.
This PR changing the new solver to match the behavior of the old one wrt the leak check. As the new solver is already used by default in coherence, this allows more code to compile, see `tests/ui/higher-ranked/leak-check/leak-check-in-selection-7-coherence.rs`:
```rust
struct W<T, U>(T, U);
trait Trait<T> {}
// using this impl results in a higher-ranked region error.
impl<'a> Trait<W<&'a str, &'a str>> for () {}
impl<'a> Trait<W<&'a str, String>> for () {}
trait NotString {}
impl NotString for &str {}
impl NotString for u32 {}
trait Overlap<U> {}
impl<T: for<'a> Trait<W<&'a str, U>>, U> Overlap<U> for T {}
impl<U: NotString> Overlap<U> for () {}
fn main() {}
```
This behavior is quite arbitrary and not something I expect users to rely on in practice, however, it should still go through an FCP imo.
r? `@BoxyUwU` originally implemented by `@compiler-errors` in https://github.com/rust-lang/rust/pull/136997. Closes https://github.com/rust-lang/trait-system-refactor-initiative/issues/120.
For sizedness, default and auto trait predicates, now prefer non-param
candidates if any exist. As these traits do not have generic parameters,
it never makes sense to prefer an non-alias candidate, as there can
never be a more permissive candidate.
fix 2 search graph bugs
wooooooooops, i should really run the fuzzer even when not changing the structure of the search graph as a whole :3 fixes the `ml-kem` ICE in the next-solver crater run
r? ````@BoxyUwU````
fix rebasing cycle heads when not reaching a fixpoint
fixes https://github.com/rust-lang/trait-system-refactor-initiative/issues/232
annoyingly subtle, imagine the following proof tree
- A (no cycle head usages, final result Y)
- *ignored* B (depends on A with provisional result X)
- A (cycle, provisional result X)
- B (using the cache entry here incorrectly assumes A has final result X)
r? ``@BoxyUwU``
eagerly compute `sub_unification_table` again
Previously called `sub_relations`. We still only using them for diagnostics right now. This mostly reverts rust-lang/rust#119989. Necessary for type inference guidance due to not-yet defined opaque types, cc https://github.com/rust-lang/trait-system-refactor-initiative/issues/182.
We could use them for cycle detection in generalization and it seems desirable to do so in the future. However, this is unsound with the old trait solver as its cache does not track these `sub_unification_table` in any way.
We now properly track the `sub_unification_table` when canonicalizing so using them in the new solver is totally sound and the performance impact is far more manageable than I thought back in rust-lang/rust#119989.
r? `@compiler-errors`
Migrate more things in the new solver to specific `DefId`s
Continuation of https://github.com/rust-lang/rust/pull/145377. I migrated the rest of the types, except aliases.
Aliases are problematic because opaques and associated types share the same type in the new solver. `@jackh726,` `@lcnr,` `@ShoyuVanilla` I'd like to hear ideas here. Anyway, even if we do nothing with them we already got a substantial improvement.
r? types
Switch next solver to use a specific associated type for trait def id
The compiler just puts `DefId` in there, but rust-analyzer uses different types for each kind of item.
See [the Zulip discussion](https://rust-lang.zulipchat.com/#narrow/channel/185405-t-compiler.2Frust-analyzer/topic/Implmentating.20New.20Trait.20Solver/near/534329794). In short, it will be a tremendous help to r-a to use specific associated types, while for the solver and the compiler it's a small change. So I ported `TraitId`, as a proof of concept and it's also likely the most impactful.
r? types
Do not consider a `T: !Sized` candidate to satisfy a `T: !MetaSized` obligation.
This example should fail to compile (and does under this PR, with the old and new solvers), but currently compiles successfully ([playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=6e0e5d0ae0cdf0571dea97938fb4a86d)), because (IIUC) the old solver's `lazily_elaborate_sizedness_candidate`/callers and the new solver's `TraitPredicate::fast_reject_assumption`/`match_assumption` consider a `T: _ Sized` candidate to satisfy a `T: _ MetaSized` obligation, for either polarity `_`, when that should only hold for positive polarity.
```rs
#![feature(negative_bounds)]
#![feature(sized_hierarchy)]
use std::marker::MetaSized;
fn foo<T: !MetaSized>() {}
fn bar<T: !Sized + MetaSized>() {
foo::<T>();
//~^ ERROR the trait bound `T: !MetaSized` is not satisfied // error under this PR
}
```
Only observable with the internal-only `feature(negative_bounds)`, so might just be "wontfix".
This example is added as a test in this PR (as well as testing that `foo<()>` and `foo<str>` are disallowed for `fn foo<T: !MetaSized`).
cc `@davidtwco` for `feature(sized_hierarchy)`
Maybe similar to 91c53c9 from <https://github.com/rust-lang/rust/pull/143307>
