Clarify error on ambiguous associated type ("Try using Iterator" instead of "X as Trait")

[binary132]

Hi, I think this error message is unclear.

https://play.rust-lang.org/?gist=5b08c55cf63a625ced0cbaf83b4db0de&version=stable

The solution is here:

https://play.rust-lang.org/?gist=3ce26cc2d8cbd16533b884a1f8f76900&version=stable

@oli-obk suggested mentoring a PR to add a suggestion for possible trait types in the impl. This Issue serves as a starting point.

[binary132]

Thought: if Self implements Iterator, and Iterator fulfills the requirement, then the ambiguity can be resolved automatically, right?

[oli-obk]

The idea is that both

error[E0576]: cannot find associated type `Item` in trait `Trait`
  --> src/main.rs:23:54
   |
23 |     fn get_next(&mut self) -> Option<<Foo as Trait>::Item> {
   |                                                      ^^^^ not found in `Trait`

and

error[E0433]: failed to resolve. Use of undeclared type or module `Trait`
  --> src/main.rs:21:46
   |
21 |     fn get_next(&mut self) -> Option<<Foo as Trait>::Item> {
   |                                              ^^^^^ Use of undeclared type or module `Trait`

crawl the list of traits that Foo implements, find any with the appropriate item, and suggest them.

The error is reported in

rust/src/librustc_resolve/lib.rs

Line 2547 in e0d11f3

(format!("cannot find {} `{}` in {}{}", expected, item_str, mod_prefix, mod_str),

Since this error is reported during name resolution, I don't think it is possible to figure out all the traits that the type implements. Instead we can crawl the AST to find all impl SomeTrait for Foo, check their item names and then suggest them.

[Pulkit07]

Hey, I am going to try fix this.

[estebank]

Current output:

• When using <Self as Trait>::Item, only one error:

error[E0433]: failed to resolve: use of undeclared type or module `Trait`
  --> src/main.rs:23:46
   |
23 |     fn get_next(&mut self) -> Option<<Foo as Trait>::Item> {
   |                                              ^^^^^ use of undeclared type or module `Trait`

• When using Self or Foo:

error[E0223]: ambiguous associated type
  --> src/main.rs:23:38
   |
23 |     fn get_next(&mut self) -> Option<Self::Item> {
   |                                      ^^^^^^^^^^ help: use fully-qualified syntax: `<Foo as Trait>::Item`

@oli-obk's idea to crawl the traits implemented for Foo and suggest the one(s) that have Item defined is still applicable.

[andronat]

Any updates on this? I recently stumbled upon this issue when I was trying to implement some trait for mockall.

[estebank]

@andronat #106585