Less accurate span is used when projection occurs in associated type position

[Aaron1011]

The following code:

trait MyTrait {
    type Item;
}

trait OtherTrait {
    type OtherItem;
}

/*impl OtherTrait for bool {
    type OtherItem = (<u8 as MyTrait>::Item, bool);
}*/

fn main() {
    let a: (<u8 as MyTrait>::Item, bool);
}

produces the following error:

error[E0277]: the trait bound `u8: MyTrait` is not satisfied
  --> src/main.rs:14:13
   |
14 |     let a: (<u8 as MyTrait>::Item, bool);
   |             ^^^^^^^^^^^^^^^^^^^^^ the trait `MyTrait` is not implemented for `u8`

error: aborting due to previous error

Note that the span points directly at <u8 as MyTrait>::Item

However, if the same projection <u8 as MyTrait>::Item is used in associated type position:

trait MyTrait {
    type Item;
}

trait OtherTrait {
    type OtherItem;
}

impl OtherTrait for bool {
    type OtherItem = (<u8 as MyTrait>::Item, bool);
}

/*fn main() {
    let a: (<u8 as MyTrait>::Item, bool);
}*/

produces this error:

error[E0277]: the trait bound `u8: MyTrait` is not satisfied
  --> src/lib.rs:10:5
   |
10 |     type OtherItem = (<u8 as MyTrait>::Item, bool);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `MyTrait` is not implemented for `u8`

error: aborting due to previous error

Here, the span points to the entire associated type, instead of just the projection.
This can lead to unhelpful errors in macro-generated code, since the span of the associated type may be completely different from the span of the projection.

[Aaron1011]

As of the latest nightly, this now produces:

error[E0277]: the trait bound `u8: MyTrait` is not satisfied
  --> src/lib.rs:10:23
   |
1  | trait MyTrait {
   | ------------- required by this bound in `MyTrait`
...
10 |     type OtherItem = (<u8 as MyTrait>::Item, bool);
   |                       ^^^^^^^^^^^^^^^^^^^^^ the trait `MyTrait` is not implemented for `u8`

pointing directly to the affected projection.