Cannot declare a generic fn with a constraint containing a region

[erickt]

We cannot declare a generic function where one of the tyvars is constrained by a type that contains a region. Here's an example:

trait Trait<'self> {
    fn foo(&self) -> &'self str;
}

fn foo<'a, T: Trait<'a>>(_x: T) {}

fn main() {}

This errors with:

a.rs:5:22: 5:24 error: Illegal lifetime 'a: only 'self is allowed as part of a type declaration
a.rs:5 fn foo<'a, T: Trait<'a>>(_x: T) {}

There is a workaround though. Instead we can use a vtable, which compiles fine.

trait Trait<'self> {
    fn foo(&self) -> &'self str;
}

fn foo<'a>(_x: &Trait<'a>) {}

fn main() {}

[erickt]

After poking around a bit in collect.rs, I suspect the problem is in instantiate_trait_ref on this line. Bound traits use the type_rscope, which only permits the 'self region. Instead I think it should probably use a MethodRscope-like scope to allow the bounds to use any user-declared regions. However, there are a couple different users of instantiate_trait_ref so I'm not sure if this is the right thing to do in all cases.

@nikomatsakis: you wrote most of the region code. Is this the right way to solve this bug?

[huonw]

Similar to #6551 (which is the ICE one gets if one does what it asks and uses foo<T: Trait<'self>> or foo<'self, T: Trait<'self>>).

[alexcrichton]

This is working today due to many changes to lifetimes, and I've seen tests for this in the testsuite so closing.