Cannot borrow as mutable despite DerefMut

[smcmurray]

May be related to #17501

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

struct Test(Box<FnMut()>);

impl Deref for Test {
    type Target = Box<FnMut()>;
    fn deref(&self)->&Self::Target {
        &self.0
    }
}

impl DerefMut for Test {
    fn deref_mut(&mut self)-> &mut Box<FnMut()> {
        &mut self.0
    }
}

fn main() {
    let mut t = Test(Box::new(move || ()));
    (t.0)();
    t();
}

Rust 2015:

error[E0596]: cannot borrow immutable `Box` content as mutable
  --> src/main.rs:21:5
   |
21 |     t();
   |     ^ cannot borrow as mutable

Rust 2018:

error[E0596]: cannot borrow data in a `&` reference as mutable
  --> src/main.rs:21:5
   |
21 |     t();
   |     ^ cannot borrow as mutable

[cuviper]

FWIW, I tried to hide the Box aspect, but it also fails with type Target = dyn FnMut().

[kuviman]

It looks like anything but Box<T: FnMut()> and T: FnMut() itself, cause this error:

fn foo<F: FnMut(), W: Deref<Target = F> + DerefMut>(mut f: W) {
    f()
}

warning: variable does not need to be mutable
 --> src/lib.rs:3:53
  |
3 | fn foo<F: FnMut(), W: Deref<Target = F> + DerefMut>(mut f: W) {
  |                                                     ----^
  |                                                     |
  |                                                     help: remove this `mut`
  |
  = note: #[warn(unused_mut)] on by default

error[E0596]: cannot borrow data in a `&` reference as mutable
 --> src/lib.rs:4:5
  |
4 |     f()
  |     ^ cannot borrow as mutable

Based on the warning, it looks like only Deref is being considered, and DerefMut is not

[cuviper]

It's happy if you force DerefMut like this: (&mut *t)()

[rylev]

Triage: This no longer seems to be an issue as both code samples from above compile successfully now. This seems to have been fixed in 1.46.