supertrait methods should be callable on object types

[brson]

Bound type parameters can call supertrait methods, objects should be able to as well.

trait A { fn f() }
trait B: A { }

fn foo(b: &B) {
    b.f()
}

[nikomatsakis]

Bumping to 0.6

[catamorphism]

Reproduced as of eed2ca6

[catamorphism]

Not critical for 0.6, de-milestoning

[Sodel-the-Vociferous]

catamorphism, are you actively working on this one, or do you mind if I take a crack at it?

[catamorphism]

@Sodel-the-Vociferous I'm not actively working on it, so please do! Ask on IRC if you have any questions :-)

[Sodel-the-Vociferous]

Problems found so far:

• LookupContext::push_inherent_impl_candidates_for_type() wasn't checking supertraits; seems to be fixed on my branch.
• LookupContext::push_inherent_items_from_trait() wasn't checking supertraits, either. This is taking a bit longer because of the next problem.
• ty::lookup_item_type() crashes the compiler when I give it a supertrait's def_id, and says there's no matching type in the tcache. I'm going to do a bit more testing to see what else does and does not aggravate this problem.

Also, by analogy to the method_origins "method_self" and "method_super", I'm going to split "method_trait" into "method_trait" and "method_trait_super". We might as well be explicit when we're trying to use trait inheritance with dynamic dispatch.

[Sodel-the-Vociferous]

Objects need some refactoring first. Waiting on #5762.

[catamorphism]

Nominating for milestone 5, production-ready

[graydon]

accepted for feature-complete milestone

[huonw]

@msullivan has been doing work in this area recently, maybe he has some light to shed?

[msullivan]

Yeah, I'll take this.

[vi]

It works, but casting b as &A not. It can be worked around:

struct ProxyObject<'q> (&'q B);

impl<'q> A for ProxyObject<'q> {
   fn f(&self) {let &ProxyObject(x) = self;  x.f();}
}

// &ProxyObject(b) as &A

Maybe the compiler can do something like this automatically?

[nikomatsakis]

@vi indeed, adding more general upcasts is definitely something we'd like to do soon-ish. It's a touch tricky when you dig into the details.