FnBox is not callable with function call syntax if it has a bound lifetime argument

[bluss]

The following does not compile, but I expect it to! If the argument type is changed from &str to i32 (and the call site adapted), then it compiles. Playground

#![feature(fnbox)]

use std::boxed::FnBox;

struct Func {
    out: Box<FnBox(&str)>
}

impl Func {
    fn callme(self) {
        let s = String::new();
        (self.out)(&s);
    }
}

error: expected function, found `Box<for<'r> std::boxed::FnBox<(&'r str,), Output=()> + 'static>`
  --> <anon>:12:9
12 |>         (self.out)(&s);
   |>         ^^^^^^^^^^^^^^

[alexcrichton]

cc #28796 (tracking issue)

[eefriedman]

The relevant impl block looks like this:

impl<'a, A, R> FnOnce<A> for Box<FnBox<A, Output = R> + 'a> {

There is no type A which would allow this impl be used for FnBox(&str): there isn't any appropriate lifetime for the reference. Therefore, the FnBox can't be called.

[bluss]

Aha, thanks for the explanation! I had assumed FnBox had direct compiler support like the other closure traits. That means FnBox is stuck in the same hole as function pointers -- we can't reach all possible types from trait impls.

[jwilm]

Heh, ran into this first time needing a FnBox.

[steveklabnik]

If #28796 (comment) succeeds, fnbox will go away

[cjgillot]

Now works with a Box<dyn FnOnce(&str)>.