Suggest Stream<Item = String> when specifying the item using the trait syntax.

[Darksonn]

This issue describes a suggestion I wish the compiler made. Given the following code:

use futures::stream::Stream;

struct Foo<T>
where
    T: Stream,
    <T as Stream>::Item: String,
{
    t: T,
}

The compiler produces this error:

error[E0404]: expected trait, found struct `String`
    --> src/lib.rs:6:26
     |
6    |       <T as Stream>::Item: String,
     |                            ^^^^^^ help: a trait with a similar name exists: `ToString`

It would be nice if the compiler suggested changing it to

use futures::stream::Stream;

struct Foo<T>
where
    T: Stream<Item = String>,
{
    t: T,
}

The same applies to impl blocks, that also don't produce the suggestion.

[estebank]

I have a hacky prototype with the following output:

Funnily enough, there was no existing test that triggered this.

For the (nightly only) where <T as Bar>::Baz = String case, we already provide a suggestion: