Fix incorrect diagnostics for expected type in E0271 with an associated type

[ohadravid]

With code like the following code:

#[derive(Debug)] struct Data {} fn do_stuff<'a>(iterator: impl Iterator<Item = &'a Data>) { for item in iterator { println!("{:?}", item) } } fn main() { let v = vec![Data {}]; do_stuff(v.into_iter()); }

the diagnostic (in nightly & stable) wrongly complains about the expected type:

error[E0271]: type mismatch resolving `<std::vec::IntoIter<Data> as std::iter::Iterator>::Item == &Data` --> src/main.rs:15:5 | 5 | fn do_stuff<'a>(iterator: impl Iterator<Item = &'a Data>) { | -------- --------------- required by this bound in `do_stuff` ... 15 | do_stuff(v.into_iter()); | ^^^^^^^^ expected struct `Data`, found &Data | = note: expected type `Data` found type `&Data` 

This PR fixes this issue by flipping the expected/actual values where appropriate, so it looks like this:

error[E0271]: type mismatch resolving `<std::vec::IntoIter<Data> as std::iter::Iterator>::Item == &Data` --> main.rs:15:5 | 5 | fn do_stuff<'a>(iterator: impl Iterator<Item = &'a Data>) { | -------- --------------- required by this bound in `do_stuff` ... 15 | do_stuff(v.into_iter()); | ^^^^^^^^ expected &Data, found struct `Data` | = note: expected type `&Data` found type `Data` 

This improves the output of a lot of existing tests (check out associated-types-binding-to-type-defined-in-supertrait!).

The only change which I wasn't too sure about is in the test associated-types-overridden-binding-2, but I think it's an improvement and the underlying problem is with handling of trait_alias.

Fix #57226, fix #64760, fix #58092.

[estebank]

It looks good but I am also slightly concerned with associated-types-overridden-binding-2, but it is a weird construct where we don't point at trait I32Iterator = Iterator<Item = i32>; and it is a leveraging a nightly feature for it. I think the error was already not great, so r=me once CI is green.

[ohadravid]

@estebank CI is green 😄

[estebank]

@bors r+

[bors]

📌 Commit 7689aac618c61457d52b69b40c84fa7229c0e1d1 has been approved by estebank

[ohadravid]

@estebank updated the code according to your comment, CI is green again

[estebank]

@bors r+ rollup

[bors]

📌 Commit 8bb5450 has been approved by estebank

[gralpli]

Does this also fix #63211 ?