Bad interaction of smart pointers, auto-deref and closures

[RalfJung]

The following code does not compile:

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

pub struct Callbacks {
    callbacks: Vec<Rc<RefCell<FnMut(i32)>>>,
}

impl Callbacks {
    pub fn call(&mut self, val: i32) {
        for callback in self.callbacks.iter() {
            let mut closure = callback.borrow_mut();
            closure(val);
        }
    }
}

(Playpen)

It says: "cannot borrow immutable borrowed content as mutable".

Instead, I have to explicitly dereference and borrow the closure in the loop, with (&mut *closure)(val). Typically, Rust performs this automatically, so I expected that to also happen in this case. At the very least, the error message should be improved, as currently, it doesn't help with the actual issue at all.

[eefriedman]

See master...eefriedman:fnmut-autoderef .

[Diggsey]

This looks like a duplicate of #26186

[Mark-Simulacrum]

Closing in favor of #26186.