Cannot borrow immutable 'Box' content as mutable - static

Question

I'm trying to provide a closure via a C callback using a static variable. I was able to get things working with a Fn type, but I'd like to make it work via FnMut to provide users of the library with more versatility.
Here's what I have:
lazy_static! {
static ref CALLBACK: Mutex<RefCell<Box<FnMut(Result<&str>) + Send>>> = Mutex::new(RefCell::new(Box::new(|_|())));
}
fn wrap_cb<F: Fn(Result<&str>)>(f: Option<F>) -> Option<unsafe extern "C" fn(*mut c_char, size_t)> {
match f {
Some(_) => {
unsafe extern "C" fn wrapped(msg: *mut c_char, len: size_t) {
let s = std::str::from_utf8(std::slice::from_raw_parts(msg as *const u8, len))
.map_err(Error::from);
let x = CALLBACK.lock().unwrap();
x.borrow_mut()(s);
}
Some(wrapped)
}
None => None,
}
}
This gives the error:
error[E0596]: cannot borrow immutable `Box` content as mutable
--> src/wpactrl.rs:56:17
|
56 | x.borrow_mut()(s);
| ^^^^^^^^^^^^^^ cannot borrow as mutable

Answer 1

It looks like the "cannot borrow immutable Box content as mutable" problem reduces to:
fn invoke(m: &Mutex<RefCell<Box<FnMut()>>>) {
let r = m.lock().unwrap();
r.borrow_mut()();
}
I haven't yet figured out why this works, but it does work if changed to:
fn invoke(m: &Mutex<RefCell<Box<FnMut()>>>) {
let r = m.lock().unwrap();
let f = &mut *r.borrow_mut();
f();
}