Closure arguments are inferred monomorphically

[eira-fransham]

Without an explicit annotation, closure arguments are inferred to have a concrete lifetime. You must manually annotate the argument in order to satisfy a HRTB.

#[derive(PartialEq)]
struct Foo;

fn use_a_foo(_: &Fn(&Foo)) {}

fn main() -> () {
    // This doesn't compile
    // let function = |foo_arg| {
    //   assert!(&foo == foo_arg);
    // };
    // explicit_for(&foo, &function);

    // This does
    let function = |foo_arg: &_| {};
    use_a_foo(&function);
}

[ExpHP]

Dupe of #41078. (at least, that's the issue that seems to have received the most attention; took a bit of searching to find though!)