RFC 0087: Use + instead of : for trait bounds

lang (syntax | traits | trait-object)

• Start Date: 2014-05-22
• RFC PR: rust-lang/rfcs#87
• Rust Issue: rust-lang/rust#12778

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

Where does the 'static bound fit into all this?