Rust: Error with autoderef on newtype wrapping FnMut

[eefriedman]

Testcase:

use std::ops::*;
struct S<'a, T:FnMut() + 'static + ?Sized>(&'a mut T);
impl<'a, T:?Sized + FnMut() + 'static> DerefMut for S<'a, T> {
    fn deref_mut(&mut self) -> &mut Self::Target { &mut self.0 }
}
impl<'a, T:?Sized + FnMut() + 'static> Deref for S<'a, T> {
    type Target = FnMut() + 'a;
    fn deref(&self) -> &Self::Target { &self.0 }
}
fn main() {
    let mut f = ||{};
    let mut s = S(&mut f);
    s();
}

Yields:

<anon>:13:5: 13:6 error: cannot borrow immutable borrowed content as mutable
<anon>:13     s();
              ^

Auto-deref should be picking DerefMut here, not Deref.

[Diggsey]

I just ran into this problem too, here's a shorter example:

use std::cell::RefCell;
use std::rc::Rc;

struct Foo(Rc<RefCell<FnMut()>>);

fn main() {
    let a = Foo(Rc::new(RefCell::new(||{
        println!("Hello, world!");
    })));
    a.0.borrow_mut()();
}

And playpen link: https://is.gd/KQJ9ij

[Mark-Simulacrum]

Further reduced:

use std::cell::RefCell;
use std::rc::Rc;

fn main() {
    let a: Rc<RefCell<FnMut()>> = Rc::new(RefCell::new(||{}));
    a.borrow_mut()();
}

[shepmaster]

Another example from Stack Overflow:

use std::ops::{Deref, DerefMut};

struct Foo;

impl Deref for Foo {
    type Target = FnMut() + 'static;
    fn deref(&self) -> &Self::Target {
        unimplemented!()
    }
}

impl DerefMut for Foo {
    fn deref_mut(&mut self) -> &mut Self::Target {
        unimplemented!()
    }
}

fn main() {
    let mut t = Foo;
    t();
}

[shepmaster]

And again experienced on Stack Overflow

[frankmcsherry]

Just got this today, with what is almost identical to Mark-Simulacrum's reduced version.

[Osspial]

I've also run into this issue, with this example:

use std::sync::Mutex;

struct FunctionIcon {
    get_icon: Mutex<Box<dyn FnMut() -> u32>>,
}

impl FunctionIcon {
    fn get_icon(&self) -> impl '_ + std::ops::DerefMut<Target=Box<dyn FnMut() -> u32>> {
        self.get_icon.lock().unwrap()
    }

    fn load_icon(&self)  {
        let mut get_icon = self.get_icon();
        let rgba_icon = (*get_icon)();
    }
}

and this error:

error[E0596]: cannot borrow data in a dereference of `impl std::ops::DerefMut` as mutable
  --> src/lib.rs:14:25
   |
14 |         let rgba_icon = (*get_icon)();
   |                         ^^^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `DerefMut` is required to modify through a dereference, but it is not implemented for `impl std::ops::DerefMut`