nll allows &'static Fn() to contain function local upvars

[oli-obk]

#![feature(nll)]

fn main() {
    let x = 42;
    let _: &'static _ = &|| { let x = x; x };
}

I don't think this should compile. I didn't test whether we can actually abuse this to do unsoundness, but this snippet seems very unintuitive

[matthewjasper]

I think this is #47184, as this doesn't compile

fn is_static<F: 'static>(f: F) -> F { f }

fn foo() {
    let x = 42;
    is_static(&|| { let x = x; x });
}

[nikomatsakis]

I don't think this is #47184 exactly -- we are enforcing user-given types in this particular case, but I think we are probably not requiring them to be well-formed. As a result, if the type of _ is &'static T, we don't end up requiring that T: 'static.

I suspect that you would have a similar error with something like struct Foo<T: 'static> and an annotation let x: Foo<_> = ....

This ought to be fixed for EP2. Nominating.

[nikomatsakis]

Assigning to myself as this intersects my branch for #48071 quite a bit.

[nikomatsakis]

Actually, I think @matthewjasper was right! In particular, I think we don't currently enforce type annotations that are placed on patterns that are not variable bindings. So e.g. this does not compile:

#![feature(nll)]
#![allow(warnings)]

fn main() {
    let x = 42;
    let y: &'static _ = &|| { let x = x; x };
}

[nikomatsakis]

Going to close as a duplicate of #47184