remove the :: after type names in "as" expressions

[nikomatsakis]

Currently, you must write foo as bar::<X> if the type you are casting to contains a type parameter. I find this very unintuitive. I think the rule "type names do not require ::, explicit function type parameters do" is easy enough to explain. But the rule ":: are needed in expressions, but only where it might cause ambiguity" is a bit more troublesome.

I realize there is a kind of ambiguity (foo as uint < 5) but I am ok with having to write (foo as uint) < 5 in this case. I think this will come up less often than casting to parameterized interfaces.

[graydon]

I don't particularly care, and doubt it's a big enough issue to warrant the RFC tag. Suspect if you just poke at the parser and change this nobody is likely to complain. You are already clearly in a type-expression context so it's just a matter of bumping precedence (unlike foo<int>( ... ) where we'd have to get resolver feedback mid-parse to realize we'd switched to type-expression grammar).

[sanxiyn]

@pcwalton, I think this was done?

[graydon]

see also #1429 and #1187

[nikomatsakis]

I think this was fixed by @pcwalton

[pcwalton]

Closing; I'm pretty sure I fixed this.