Mismatched signature in trait implementation with lifetimes not caught

[Ms2ger]

I think this shouldn't compile, but it does (in rustc 1.3.0-dev (fddfd08 2015-07-10)):

struct Foo;

trait Bar {
    fn get_style<'a>(&'a self, r: &'a Foo) -> &'a Foo;
}

impl Bar for Foo {
    fn get_style<'a>(&self, r: &'a Foo) -> &'a Foo {
        r
    }
}

fn main() {}

[arielb1]

@Ms2ger

Could you extend it into a real unsoundness? I don't think this is a real problem: the trait type is for<'a> fn(&'a Foo, &'a Foo) -> &'a Foo, which is a subtype of the impl type for<'a,'s> fn(&'s Foo, &'a Foo) -> &'a Foo - it requires both of the argument lifetimes to be the same as the output lifetime, not just the second argument's lifetime, and we allow impl-types to be subtypes of the trait-type (e.g. you can even replace the return type with &'static Foo - of course only if you also adjust get_style's body - and it would still work).

[Ms2ger]

I'm not sure anything's unsound here; I found this pattern in Servo and was surprised that it built at all, given the different signatures.

[arielb1]

I'll close this then.