…iaskrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#126476 (Fix running bootstrap tests with a local Rust toolchain as the stage0)
 - rust-lang#127094 (E0191 suggestion correction, inserts turbofish)
 - rust-lang#127554 ( do not run test where it cannot run)
 - rust-lang#127564 (Temporarily remove me from review rotation.)
 - rust-lang#127568 (instantiate higher ranked goals in candidate selection again)
 - rust-lang#127569 (Fix local download of Docker caches from CI)
 - rust-lang#127570 ( small normalization improvement)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#127568 - lcnr:undo-leakcheck, r=oli-obk

instantiate higher ranked goals in candidate selection again

This reverts rust-lang#119820 as that PR has a significant impact and breaks code which *feels like it should work*. The impact ended up being larger than we expected during the FCP and we've ended up with some ideas for how we can work around this issue in the next solver. This has been discussed in the previous high bandwidth t-types meeting: https://rust-lang.zulipchat.com/#narrow/stream/326132-t-types.2Fmeetings/topic/2024-07-09.20high.20bandwidth.20meeting.

We'll therefore keep this inconsistency between the two solvers for now and will have to deal with it before stabilizating the use of the new solver outside of coherence: rust-lang/trait-system-refactor-initiative#120.

fixes rust-lang#125194 after a beta-backport.

The pattern which is more widely used than expected and feels like it should work, especially without deep knowledge of the type system is
```rust
trait Trait<'a> {}
impl<'a, T> Trait<'a> for T {}

fn trait_bound<T: for<'a> Trait<'a>>() {}

// A function with a where-bound which is more restrictive than the impl.
fn function1<T: Trait<'static>>() {
    // stable: ok
    // with rust-lang#119820: error as we prefer the where-bound over the impl
    // with this PR: back to ok
    trait_bound::<T>();
}

```

r? `@rust-lang/types`

`-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 #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 #119820 in #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 #136997. Closes rust-lang/trait-system-refactor-initiative#120.

`-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 rust-lang/rust#136997. Closes rust-lang/trait-system-refactor-initiative#120.

`-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 rust-lang/rust#136997. Closes rust-lang/trait-system-refactor-initiative#120.