RFC: Bounds on trait objects should be separated with +

active/0000-trait-bounds-with-plus.md

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+- Start Date: 2014-05-22
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# 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?