Auto merge of #112682 - spastorino:new-rpitit-21, r=compiler-errors

Add bidirectional where clauses on RPITIT synthesized GATs

Given the following:

```rust
struct MyStruct<'a, T>(&'a T);

trait MyTrait<'a, T> {
    fn my_fn<'b, 'c, 'd, V>(item: &'c String) -> impl Sized + 'a + 'b + 'c where V: 'b, V: 'd;
}

impl<'a, T> MyTrait<'a, T> for MyStruct<'a, T> {
    fn my_fn<'b, 'c, 'd, V>(_: &'c String) -> impl Sized + 'a + 'b + 'c
    where
        V: 'b,
        V: 'd,
    {
        unimplemented!();
    }
}
```

We need the desugaring to be:

```rust
trait MyTrait<'a, T> {
    type MyFn<'bf, 'df, Vf, 'a2, 'b2, 'c2>: Sized + 'a2 + 'b2 + 'c2 where Vf: 'b2, 'a2: 'a, 'a: 'a2, 'b2: 'bf, 'bf: 'b2;

    fn my_fn<'b, 'c, 'd, V>(item: &'c String) -> MyStruct<'a>::MyFn<'b, 'd, V, 'a, 'b, 'c> where V: 'b, V: 'd {
        type opaque<'a3, 'b3, 'c3>;
    };
}

impl<'a, T> MyIter<'a, T> for MyStruct<'a, T> {
    type MyFn<'bf, 'df, Vf, 'a2, 'b2, 'c2> = impl Sized + 'a2 + 'b2 + 'c2 where Vf: b2, 'a2: 'a, 'a: 'a2, 'b2: 'bf, 'bf: 'b2;

    fn my_fn<'b, 'c, 'd, V>(_: &'c String) -> MyStruct<'a>::MyFn<'a, 'b, 'c, V> where V: 'b, V: 'd {
        type opaque<'a3, 'b3, 'c3>;
        unimplemented!();
    }
}
```

This PR adds the where clauses for the `MyFn` generated GATs.

This is a draft with a very ugly solution so we can make comments over concrete code.

r? `@compiler-errors`

0 files changed, 0 insertions, 0 deletions