mutably captured default int emits type inference error in closure

[blaenk]

Really had no idea what to call this, feel free to change. As soon as I annotate the type of i when I create it or when I add to it, it works. I of course intended to annotate the type, but I did notice this and figured I'd post it in case it's not intentional.

extern crate libc;

fn main() {
    fn f<F>(f: F) where F: FnMut(&str) {
      let down = &f as *const _ as *mut libc::c_void;
      let up = down as *mut F;
      let s = "lol what";
      unsafe { (*up)(s); }
    }

    let mut i = 3;

    f(|&mut: data: &str| {
        println!("data is: {}", data);
        i += 3;
    });

    println!("i is now: {}", i);
}

Error:

<anon>:13:5: 13:6 error: unable to infer enough type information about `closure[<anon>:13:7: 16:6]`; type annotations required
<anon>:13     f(|&mut: data: &str| {
              ^
error: aborting due to previous error
playpen: application terminated with error code 101

playpen

[Aatch]

Dupe of #20558