Cannot move a value of type dyn for<'r> FnOnce(&'r mut [u8]): the size of dyn for<'r> FnOnce(&'r mut [u8]) cannot be statically determined

[guerlando]

I'm trying to have a function shared between threads, that can be called with another function on it:

fn main() {
    let on_virtual_tun_write = std::sync::Arc::new(|f: &dyn FnOnce(&mut [u8]), size: usize|-> std::result::Result<(),()>{
        let mut buffer = vec![0; size];
        f(buffer.as_mut_slice());
        Ok(())
    });
}

Once it's called, it uses the f passed as an argument, to retrieve a buffer. As you can see, I do not intend to copy the function, I just use it as soon as someone calls the closure, and discards it. However, Rust thinks that I want to copy it. What can I do to tell Rust that I just want to use a reference to the function? I thought &dyn was sufficient.

Error:

error[E0161]: cannot move a value of type dyn for<'r> FnOnce(&'r mut [u8]): the size of dyn for<'r> FnOnce(&'r mut [u8]) cannot be statically determined
 --> src/main.rs:4:9
  |
4 |         f(buffer.as_mut_slice());
  |         ^

error[E0507]: cannot move out of `*f` which is behind a shared reference
 --> src/main.rs:4:9
  |
4 |         f(buffer.as_mut_slice());
  |         ^ move occurs because `*f` has type `dyn for<'r> FnOnce(&'r mut [u8])`, which does not implement the `Copy` trait

error: aborting due to 2 previous errors; 1 warning emitted

[quinedot]

See the definition of call, in particular the flavor of each self receiver, in the following links:

A FnOnce closure consumes it's closure data. You could use Box<dyn FnOnce(&mut [u8])> at the cost of boxing.

A FnMut closure mutates it's closure data. If it's not too restrictive, you could use &mut dyn FnMut(&mut [u8]).

A Fn closure does neither. If it's still not too restrictive, you could use &dyn Fn(&mut [u8]).