Summary

Bounds on trait objects should be separated with +.

Motivation

With DST there is an ambiguity between the following two forms:

trait X {
    fn f(foo: b);
}

and

trait X {
    fn f(Trait: Share);
}

See Rust issue #12778 for details.

Also, since kinds are now just built-in traits, it makes sense to treat a bounded trait object as just a combination of traits. This could be extended in the future to allow objects consisting of arbitrary trait combinations.

Detailed design

Instead of : in trait bounds for first-class traits (e.g. &Trait:Share + Send), we use + (e.g. &Trait + Share + Send).

+ will not be permitted in as without parentheses. This will be done via a special restriction in the type grammar: the special TYPE production following as will be the same as the regular TYPE production, with the exception that it does not accept + as a binary operator.

Drawbacks

It may be that + is ugly.
Adding a restriction complicates the type grammar more than I would prefer, but the community backlash against the previous proposal was overwhelming.

Alternatives

The impact of not doing this is that the inconsistencies and ambiguities above remain.

Unresolved questions