Wrong trait in "use the fully qualified path for the potential candidates" suggestion

[dtolnay]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ee16c31890addb9bd9e64b3d393a2de3

struct A<T>(T);
struct B;

trait I<T> {}
impl I<i32> for B {}
impl I<u32> for B {}

trait V<U> {
    fn method(self) -> U;
}

impl<T, U> V<U> for A<T>
where
    T: I<U>,
{
    fn method(self) -> U { unimplemented!() }
}

fn main() {
    let a = A(B);
    a.method();
}

The current output is:

error[E0283]: type annotations needed
  --> src/main.rs:21:7
   |
21 |     a.method();
   |     --^^^^^^--
   |     | |
   |     | cannot infer type for type parameter `U`
   |     this method call resolves to `U`
   |
note: multiple `impl`s satisfying `B: I<_>` found
  --> src/main.rs:5:1
   |
5  | impl I<i32> for B {}
   | ^^^^^^^^^^^^^^^^^
6  | impl I<u32> for B {}
   | ^^^^^^^^^^^^^^^^^
note: required because of the requirements on the impl of `V<_>` for `A<B>`
  --> src/main.rs:12:12
   |
12 | impl<T, U> V<U> for A<T>
   |            ^^^^     ^^^^
help: use the fully qualified path for the potential candidates
   |
21 |     <B as I<i32>>::method(a);
   |     ~~~~~~~~~~~~~~~~~~~~~~~~
21 |     <B as I<u32>>::method(a);
   |     ~~~~~~~~~~~~~~~~~~~~~~~~

The two suggestions <B as I<i32>>::method(a) and <B as I<u32>>::method(a) are nonsensical because a is not of type B and the trait I<T> does not have a method called method. This is not how the ambiguous type would need to be disambiguated.

error[E0576]: cannot find method or associated constant `method` in trait `I`
  --> src/main.rs:21:20
   |
21 |     <B as I<i32>>::method(a);
   |                    ^^^^^^ not found in `I`

Instead a correct suggestion would look like one of the following:

• <A<B> as V<i32>>::method(a)
• <A<_> as V<i32>>::method(a)
• <_ as V<i32>>::method(a)
• <V<i32>>::method(a)
• V::<i32>::method(a)

Exactly which one of the above 5 correct possibilities is chosen is related to #96292, but this issue focuses on the fact that the current suggestion's trait is wrong (I vs V).

[estebank]

I think the only short term solution we can have here is not to emit the suggestion at all for these cases. A generalized solution to provide the right suggestion might require some more extensive development work than I can tackle at the moment.

[estebank]

error[[E0282]](https://doc.rust-lang.org/stable/error_codes/E0282.html): type annotations needed
  --> src/main.rs:21:7
   |
21 |     a.method();
   |       ^^^^^^
   |
help: try using a fully qualified path to specify the expected types
   |
21 |     <A<B> as V<U>>::method(a);
   |     +++++++++++++++++++++++ ~

error[[E0283]](https://doc.rust-lang.org/stable/error_codes/E0283.html): type annotations needed
  --> src/main.rs:21:7
   |
21 |     a.method();
   |       ^^^^^^
   |
note: multiple `impl`s satisfying `B: I<_>` found
  --> src/main.rs:5:1
   |
5  | impl I<i32> for B {}
   | ^^^^^^^^^^^^^^^^^
6  | impl I<u32> for B {}
   | ^^^^^^^^^^^^^^^^^
note: required for `A<B>` to implement `V<_>`
  --> src/main.rs:12:12
   |
12 | impl<T, U> V<U> for A<T>
   |            ^^^^     ^^^^
13 | where
14 |     T: I<U>,
   |        ---- unsatisfied trait bound introduced here
help: try using a fully qualified path to specify the expected types
   |
21 |     <A<B> as V<U>>::method(a);
   |     +++++++++++++++++++++++ ~

error[[E0282]](https://doc.rust-lang.org/stable/error_codes/E0282.html): type annotations needed
  --> src/main.rs:21:5
   |
21 |     <A<B> as V<_>>::method(a);
   |     ^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `U` declared on the trait `V`

error[[E0283]](https://doc.rust-lang.org/stable/error_codes/E0283.html): type annotations needed
  --> src/main.rs:21:5
   |
21 |     <A<B> as V<_>>::method(a);
   |     ^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `U` declared on the trait `V`
   |
note: multiple `impl`s satisfying `B: I<_>` found
  --> src/main.rs:5:1
   |
5  | impl I<i32> for B {}
   | ^^^^^^^^^^^^^^^^^
6  | impl I<u32> for B {}
   | ^^^^^^^^^^^^^^^^^
note: required for `A<B>` to implement `V<_>`
  --> src/main.rs:12:12
   |
12 | impl<T, U> V<U> for A<T>
   |            ^^^^     ^^^^
13 | where
14 |     T: I<U>,
   |        ---- unsatisfied trait bound introduced here