Cannot call a Vec<Box<Fn()>> without explicit dereferencing

[shepmaster]

This code works:

fn main() {
    let func: Box<Fn()> = Box::new(|| println!("called"));
    func();
}

As does placing it in an array:

fn main() {
    let func: [Box<Fn()>; 1] = [Box::new(|| println!("called"))];
    func[0]();
}

However, using a Vec fails:

fn main() {
    let func: Vec<Box<Fn()>> = vec![Box::new(|| println!("called"))];
    func[0](); // error: expected function, found `Box<std::ops::Fn()>`
}

Instead, it must be explicitly dereferenced:

fn main() {
    let func: Vec<Box<Fn()>> = vec![Box::new(|| println!("called"))];
    (*func[0])();
}

It's even more annoying for Vec<Box<FnMut()>>:

fn main() {
    let mut func: Vec<Box<FnMut()>> = vec![Box::new(|| println!("called"))];
    // func[0](); // expected function, found `Box<std::ops::FnMut()>`
    // (*func[0])(); // cannot borrow immutable `Box` content as mutable
    (*&mut func[0])();
}

[Aatch]

The fact that this works for arrays, but not Vec suggests that it's probably related to associated types. It's possible that wherever the coercion from &Box<Fn()> to &Fn() is done sees &<Vec<Box<Fn()>> as Index<usize>>::Output instead.

[jonhoo]

This doesn't seem to be fully resolved:

use std::sync::Mutex;

pub struct Foo(Mutex<Box<FnMut()>>);

impl Foo {
    pub fn foo(&self) {
        // self.0.lock().unwrap()(); // cannot borrow immutable `Box` content as mutable
        // (*self.0.lock().unwrap())(); // cannot borrow immutable `Box` content as mutable
        // (&mut self.0.lock().unwrap())(); // cannot borrow immutable `Box` content as mutable
        (&mut *self.0.lock().unwrap())();
    }
}

fn main() {
    Foo(Mutex::new(Box::new(|| println!("called")))).foo()
}

[shepmaster]

@jonhoo That may be #26186 instead.