Traits extending kinds / built-in traits

[nikomatsakis]

You should be able to write:

trait Foo: Sized { ... }

and then have a parameter like

fn foo<T:Foo>(...) 

where this also implies that T is Sized.

Related to #5527

[nikomatsakis]

Nominating for milestone feature complete.

[emberian]

Testcase:

trait Foo: Send {
    pub fn sendit(self) {
        do spawn {
            printfln!("%?", self);
        }
    }
}

fn foo<T: Foo>(x: T) {
    x.sendit();
}

impl Foo for int;

fn main() {
    let x = 5i;
    foo(x);
}

fails with:

foo.rs:13:0: 13:17 error: failed to find an implementation of trait std::kinds::Send for int
foo.rs:13 impl Foo for int;
          ^~~~~~~~~~~~~~~~~

@nikomatsakis I think this error is a different bug?

[bblum]

In addition to inheriting from the builtin kinds, should we allow boxing up as the builtin kinds? I can't imagine what possible use this could have, as no vtable could be generated, but something like:

fn main() {
    let x = ~() as ~Send;
}

currently errors with

foo.rs:2:12: 2:24 error: failed to find an implementation of trait ~std::kinds::Send:Send for ()
foo.rs:2     let x = ~() as ~Send;
                     ^~~~~~~~~~~~

because of code very similar to the code I will be changing for this, in typeck/collect.rs.

[nikomatsakis]

I ... see no reason to allow that in particular, since the built-in kinds have no methods, but I guess if it's easy to do?

[bblum]

Probably it is not trivial to do.