Higher-kinded lifetime bounds could be smarter

[apasel422]

The following code fails to compile despite i32: 'static:

fn foo<T>() where for<'a> T: 'a {}

fn main() {
    foo::<i32>();
}

The exact error is:

foo.rs:4:5: 4:15 error: the requirement `for<'a> i32 : 'a` is not satisfied [E0280]
foo.rs:4     foo::<i32>();
             ^~~~~~~~~~

Per @nikomatsakis's comment here, it seems that T: 'static could imply for<'a> T: 'a.

[apasel422]

rust/src/librustc/middle/traits/fulfill.rs

Lines 416 to 419 in 560b1da

	// For now, we just check that there are no higher-ranked
	// regions. If there are, we will call this obligation an
	// error. Eventually we should be able to support some
	// cases here, I imagine (e.g., `for<'a> int : 'a`).

contains the following comment:

// For now, we just check that there are no higher-ranked
// regions. If there are, we will call this obligation an
// error. Eventually we should be able to support some
// cases here, I imagine (e.g., for<'a> int : 'a).

[apasel422]

@nikomatsakis I'd be interested in digging into this if you can give me some advice on how to get started.

[nikomatsakis]

@apasel422 hmm, probably best would be to privmsg me on IRC, if that's possible (nmatsakis). It should be fairly easy to write a targeted patch covering cases like for<'a> int : 'a, where the bound region only appears on the RHS; doing a more general fix (e.g., one that handles for<'a> &'a i32 : 'a) would be somewhat harder, though some of the recent refactoring I've been doing would help there.