Compiler error could suggest using an associated type for e.g. Iterator<T>

[tomjakubowski]

If you accidentally write a bound like:

pub struct Foo<T, I: Iterator<T>> { /* ... */ }

The error message is:

err.rs:3:22: 3:33 error: wrong number of type arguments: expected 0, found 1
err.rs:3 pub struct Foo<T, I: Iterator<T>> { x: I }
                              ^~~~~~~~~~~

The compiler could be more helpful in these cases where a trait takes 0 arguments but has an associated type and suggest something like: Iterator has an associated type. Did you mean: I: Iterator<Item=T>?.

[nikomatsakis]

cc @Manishearth this could be a good place for a custom error attribute like the one you recently implemented

[Manishearth]

Ooh, this looks doable.

Unfortunately I don't see any opportunity for sharing much code here, but I can try to move some of the code out.

[frewsxcv]

Visiting for triage: nothing has changed here

playpen link

[birkenfeld]

Current situation: the original error hasn't changed, but we get a second one that complains about the missing Item associated type:

<anon>:1:25: 1:40 error: wrong number of type arguments: expected 0, found 1 [E0244]
<anon>:1 pub struct Foo { i: Box<Iterator<isize>> }
                                 ^~~~~~~~~~~~~~~
<anon>:1:25: 1:40 help: see the detailed explanation for E0244
<anon>:1:25: 1:40 error: the value of the associated type `Item` (from the trait `core::iter::Iterator`) must be specified [E0191]
<anon>:1 pub struct Foo { i: Box<Iterator<isize>> }
                                 ^~~~~~~~~~~~~~~
<anon>:1:25: 1:40 help: see the detailed explanation for E0191

... which should clear it up. (But probably not if the associated type has a default?)

[estebank]

librustc_typeck/astconv.rs, methods create_substs_for_ast_path, check_type_argument_count and conv_object_ty_poly_trait_ref will need to be changed to not emit both errors, but rather condense them into one:

error[E0191]: the value of the associated type `Item` (from the trait `std::iter::Iterator`) must be specified
 --> src/main.rs:1:25
  |
1 | pub struct Foo { i: Box<Iterator<isize>> }
  |                         ^^^^^^^^^-----^ missing associated type `Item` value, found 1 type argument instead
  |                                  |
  |                                  help: did you mean to set the associated type?: `Item = isize`

error: aborting due to previous error

Care should be taken when some type arguments and associated items are expected. In those case, it's probably better to keep the current output to avoid confusion with the tradeoff of verbosity.