Support builtin bounds on associated types and make them Sized by default

[kennytm]

(Moved from http://discuss.rust-lang.org/t/make-associated-types-sized-by-default/615)

Currently this program:

#![feature(associated_types)]
trait A {
    type T;
    fn foo(&self) -> <Self as A>::T;
    fn bar(&self) -> <Self as A>::T { self.foo() }
}
fn main() {}

Fails to compile as:

1.rs:5:39: 5:49 error: cannot move a value of type T: the size of T cannot be statically determined [E0161]
1.rs:5     fn bar(&self) -> <Self as A>::T { self.foo() }
                                             ^~~~~~~~~~

According to RFC 59 we should be able to fix it by (currently unimplemented)

trait A {
    type T : Sized;
    ...
}

But,

1. Shouldn't associated types be Sized by default? As associated types are to be used to replace "output" type parameters, it is more convenient to be Sized.

2. If associated types are Sized by default, what should be the syntax then if we want to allow unsized associated types? The generic spelling currently suggests something like

   type Sized? T;
   

[aturon]

cc @nikomatsakis @nick29581 @pcwalton

[aturon]

Nominating. Associated types are planned to be un-feature-gated by 1.0, and this change is not backwards-compatible.

Assuming that we keep our current rules/syntax for generics and unsized types, I think we need to take both of the suggestions in this issue to ensure consistency with the rest of the language.

[nrc]

Agreed on both suggestions.

[pnkfelix]

Assigning P-backcompat-lang, 1.0.

[nikomatsakis]

I'm sort of working on this

[nikomatsakis]

I updated the title to reflect the full work involved in this issue. I am targeting this as supporting builtin bounds like Sized or Copy, which is mildly easier than the general case. See #18178 for the general case.