Report higher-ranked trait error when higher-ranked projection goal fails in new solver

[compiler-errors]

See HACK comment inline. Not actually sure if it should be marked as a HACK, b/c it's kinda a legitimate case we want to care about unless we're going to make the proof tree visitor smarter about the leak check than the actual trait solver itself.

Encountered this while battling with NiceRegionErrors in the old solver b/c I wondered what this code ended up giving us in the new solver as a comparison:

trait Foo {}

impl<T: FnOnce(&())> Foo for T {}

fn baz<T: Foo>() {}

fn main() {
    baz::<fn(&'static ())>();
}

On master it's pretty bad:

error[E0271]: type mismatch resolving `<fn(&()) as FnOnce<(&(),)>>::Output == ()`
 --> <source>:8:11
  |
8 |     baz::<fn(&'static ())>();
  |           ^^^^^^^^^^^^^^^ types differ
  |
note: required for `fn(&'static ())` to implement `Foo`
 --> <source>:3:22
  |
3 | impl<T: FnOnce(&())> Foo for T {}
  |         -----------  ^^^     ^
  |         |
  |         unsatisfied trait bound introduced here

After this PR it's much better:

error[E0277]: the trait bound `fn(&'static ()): Foo` is not satisfied
 --> /home/mgx/test.rs:8:11
  |
8 |     baz::<fn(&'static ())>();
  |           ^^^^^^^^^^^^^^^ the trait `for<'a> FnOnce(&'a ())` is not implemented for `fn(&'static ())`
  |
  = note: expected a closure with arguments `(&'static (),)`
             found a closure with arguments `(&(),)`
note: required for `fn(&'static ())` to implement `Foo`
 --> /home/mgx/test.rs:3:22
  |
3 | impl<T: FnOnce(&())> Foo for T {}
  |         -----------  ^^^     ^
  |         |
  |         unsatisfied trait bound introduced here
note: required by a bound in `baz`
 --> /home/mgx/test.rs:5:11
  |
5 | fn baz<T: Foo>() {}
  |           ^^^ required by this bound in `baz`

r? lcnr

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

[compiler-errors]

Pulled this into another detect_* helper method, so it's consistent with the others.

@bors r=lcnr rollup

[bors]

📌 Commit f6faaee has been approved by lcnr

It is now in the queue for this repository.

[lcnr]

Suggested change

	fn detect_error_in_higher_ranked_projection(
	fn detect_trait_error_in_higher_ranked_projection(

r=me with this change

[compiler-errors]

Already got rolled up, and I don't think this is a significant enough change to kick it out of the rollup. I'll fold this into #139513 which is based off this one.

[compiler-errors]

#139564 *

[compiler-errors]

Unless this fails to merge in which case I'll amend this. I'll keep an eye on the rollup lol