Unboxed closures with mutable reference parameter

[cgaebel]

It is either very hard (I don't know how to do it) or impossible to create an unboxed closure that takes a mutable reference. Here are my attempts, with the resulting error messages inline:

#![feature(unboxed_closures)]
#![feature(overloaded_calls)]

// test.rs:4:23: 4:31 error: missing lifetime specifier [E0106]
// test.rs:4 fn doit<T, F: FnOnce<(&mut int,), T>>(f: F) -> T {
//                                 ^~~~~~~~
// error: aborting due to previous error
fn doit<T, F: FnOnce<(&mut int,), T>>(f: F) -> T {
  let x = 4;
  f(&mut x,)
}

fn main() {
  let r: int = doit(|: i: &mut int| i + 1);
  println!("x = {}", r);
}

/////////////////////////////////////////////////////////////////////////

#![feature(unboxed_closures)]
#![feature(overloaded_calls)]

fn doit<'a, T, F: FnOnce<(&'a mut int,), T>>(f: F) -> T {
  let x = 4;
// test.rs:26:10: 26:11 error: `x` does not live long enough
// test.rs:26   f(&mut x,)
//                     ^
// test.rs:24:57: 27:2 note: reference must be valid for the lifetime 'a as defined on the block at 24:56...
// test.rs:24 fn doit<'a, T, F: FnOnce<(&'a mut int,), T>>(f: F) -> T {
// test.rs:25   let x = 4;
// test.rs:26   f(&mut x,)
// test.rs:27 }
// test.rs:24:57: 27:2 note: ...but borrowed value is only valid for the block at 24:56
// test.rs:24 fn doit<'a, T, F: FnOnce<(&'a mut int,), T>>(f: F) -> T {
// test.rs:25   let x = 4;
// test.rs:26   f(&mut x,)
// test.rs:27 }
// error: aborting due to previous error
  f(&mut x,)
}

fn main() {
  let r: int = doit(|: i: &mut int| *i + 1);
  println!("x = {}", r);
}

[japaric]

AIUI, this kind of usage requires higher-rank lifetimes, once we get that, doit would have this signature: fn doit<T, F: <'a>|: &'a mut int| -> T>(f: F), and your first attempt will work.

If you (desperately) need this code to work right now, you can throw a mem::transmute around the &mut x in your second attempt. But that's very hacky [1], so I recommend waiting for higher-rank lifetimes.

[1] Because 'a shouldn't appear in the generic parameter list of the doit function.

[aturon]

I'm going to close this as a duplicate of #17661

Thanks for reporting it!