Fn trait, lifetime, and HRTB

[user16251]

The Fn() -> T trait has a method (roughly speaking) of the type fn call(&self) -> T. I'd like to create a closure whose call method has the type fn call<'a>(&'a self) -> &'a T and name the trait it implements, but I can't: something like

let x = "three".to_string();
let f = move || &x;

fails to compile. Is there a way around this?

[nerditation]

@user16251

let f = move || &x;

this line moves x into the closure and the closure returns reference to a local variable, which will not pass the borrow checker. it's semantically same as

fn f() -> &'??? str {
    let x = "".to_string();
    &x
}

[geeklint]

This is not possible because the Fn trait is a subtrait of FnOnce, which means it has to additionally support being called as (self) -> Output.

It's theoretically possible to write a closure that supports returning borrowed data that originated outside it (e.g., if your example was not move) but then the closure itself will have a limited lifetime.

If you need precisely this signature, I suggest forgoing the closure sugar and just writing your own trait.

[nerditation]

what you described sounds like the Borrow trait to me, but I'm not sure it can be achieved with the Fn trait,