Improve the help message for the error on using fully qualified syntax for nested associated types

[salmans]

Here is a small example to reproduce the error and its help message:

trait A {
    type Item;
}

trait B {
    type Item: A;

    fn trait_fn(x: Self::Item::Item);
}

Then I get the following error:

error[E0223]: ambiguous associated type
 --> src/main.rs:8:20
  |
8 |     fn trait_fn(x: Self::Item::Item);
  |                    ^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<<Self as B>::Item as Trait>::Item`

It looks like the word "Trait" in <<Self as B>::Item as Trait>::Item should be replaced with the trait name A so that the suggested expression compiles:
help: use fully-qualified syntax: <<Self as B>::Item as A>::Item.

[JohnTitor]

I think this error is occured here:

rust/src/librustc_typeck/astconv.rs

Lines 1393 to 1396 in 15a5dfa

	self.report_ambiguous_associated_type(span,
	&qself_ty.to_string(),
	"Trait",
	&assoc_ident.as_str());

We can replace this "Trait", but I don't know what to use.(If someone tells me that, I'll improve wording and open a PR.)

[pickfire]

I got something similar while playing with diesel. At the same time the error also seemed somewhat incorrect to me? Is this similar here or considered a different issue?

error[E0223]: ambiguous associated type
  --> src/main.rs:19:45
   |
19 |     let query = diesel::update(posts.filter(posts::columns::id.eq_any(subquery)))
   |                                             ^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<hello::schema::posts::table as Trait>::columns`

I was a bit confused what can I do here, it would be good to have some suggestions on what kind of traits could be use for as Trait, or maybe the error message is incorrect. posts::columns::id is a unit struct. I don't quite recall the step to reproduce since I got it working already.

[jmstudyacc]

On enums3 of the Rustlings exercises you can replicate this error with the following:

enum Message {
    // TODO: implement the message variant types based on their usage below
    ChangeColor(State::color),
    Echo(String),
    Move(Point),
    Quit,

}

struct Point {
    x: u8,
    y: u8,
}

struct State {
    color: (u8, u8, u8),
    position: Point,
    quit: bool,
}

Will produce:

error[E0223]: ambiguous associated type
  --> exercises/enums/enums3.rs:21:13
   |
21 |     color: (State::color),
   |             ^^^^^^^^^^^^ help: use fully-qualified syntax: `<State as Trait>::color`

It would be great to get a bit more guidance on the reasoning.

[estebank]

Current output:

error[E0223]: ambiguous associated type
 --> src/lib.rs:8:20
  |
8 |     fn trait_fn(x: Self::Item::Item);
  |                    ^^^^^^^^^^^^^^^^
  |
help: use fully-qualified syntax
  |
8 -     fn trait_fn(x: Self::Item::Item);
8 +     fn trait_fn(x: <<Self as B>::Item as IntoAsyncIterator>::Item);
  |
8 -     fn trait_fn(x: Self::Item::Item);
8 +     fn trait_fn(x: <<Self as B>::Item as IntoIterator>::Item);
  |

I don't know why it doesn't realize that it can be <<Self as B>::Item as A>::Item.

error[E0223]: ambiguous associated type
 --> src/lib.rs:3:17
  |
3 |     ChangeColor(State::color),
  |                 ^^^^^^^^^^^^
  |
help: if there were a trait named `Example` with associated type `color` implemented for `State`, you could use the fully-qualified path
  |
3 -     ChangeColor(State::color),
3 +     ChangeColor(<State as Example>::color),
  |