Suggest `Trait<Ty: Trait2>` syntax when user typed `Trait<Ty = Trait2>`

[jdahlstrom]

Consider the following code:

trait Foo {}

fn bar(_: impl Iterator<Item = Foo>) {}

// Or another similar case:

trait Bar {
    type X: Iterator<Item = Foo>;
}

The current output is:

error[E0782]: trait objects must include the `dyn` keyword
 --> src/lib.rs:3:32
  |
3 | fn bar(_: impl Iterator<Item = Foo>) {}
  |                                ^^^
  |
help: add `dyn` keyword before this trait
  |
3 | fn bar(_: impl Iterator<Item = dyn Foo>) {}
  |                                +++

This is reasonable especially in light of the ongoing migration from the old dyn-less trait object syntax. However, an alternative interpretation of the syntax error is that the user accidentally used a type equality bound = when they actually meant a trait bound :. The error message could include a suggestion to replace = with :.

help: or if you meant to write a trait bound, replace `=` with `:`
  |
3 | fn bar(_: impl Iterator<Item = Foo>) {}
  |                             --
3 | fn bar(_: impl Iterator<Item: Foo>) {}
  |                             +

See also related #99304 which proposes that impl Foo should be suggested rather than dyn Foo.

[fmease]

This should only be suggested on nightly as associated type bounds are still unstable.

[jdahlstrom]

@fmease Oops, good point. Not sure how I thought that compiled on stable, mea culpa. I guess the compiler could suggest using the stable where X: Trait, X::Ty: Trait2 syntax if the user attempts to use ATBs, but that should probably be its own issue.