Incorrect suggestion when trying to implement Trait<Associated = ()> with impl Trait + 'static

[patrick-gu]

Given the following code (Playground):

trait Trait {
    type Associated;
}

fn returns_opaque() -> impl Trait + 'static {
    struct Impl;

    impl Trait for Impl {
        type Associated = ();
    }

    Impl
}

fn accepts_trait<T>(_: T)
where
    T: Trait<Associated = ()>,
{
}

fn main() {
    accepts_trait(returns_opaque());
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error[E0271]: type mismatch resolving `<impl Trait as Trait>::Associated == ()`
  --> src/main.rs:22:5
   |
5  | fn returns_opaque() -> impl Trait + 'static {
   |                        -------------------- the found opaque type
...
15 | fn accepts_trait<T>(_: T)
   |    ------------- required by a bound in this
16 | where
17 |     T: Trait<Associated = ()>,
   |              --------------- required by this bound in `accepts_trait`
...
22 |     accepts_trait(returns_opaque());
   |     ^^^^^^^^^^^^^ expected `()`, found associated type
   |
   = note:    expected unit type `()`
           found associated type `<impl Trait as Trait>::Associated`
help: consider constraining the associated type `<impl Trait as Trait>::Associated` to `()`
   |
5  | fn returns_opaque() -> impl Trait + 'static<Associated = ()> {
   |                                            ^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0271`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

The syntax impl Trait + 'static<Associated = ()> does not actually work.

Ideally the suggested fix should look like:

help: consider constraining the associated type `<impl Trait as Trait>::Associated` to `()`
   |
5  | fn returns_opaque() -> impl Trait<Associated = ()> + 'static {
   |                                  ^^^^^^^^^^^^^^^^^

This bug appears to be present in stable, beta, and nightly, on both Rust 2015 and 2018.