Confusing span in `FnMut` closure diagnostic

[varkor]

#![feature(unboxed_closures)]

fn to_fn<A, F:Fn<A>>(f: F) -> F { f }

fn main() {
    let mut x = 0;
    let _f = to_fn(|| x = 42); //~ ERROR cannot assign to `x`
}

help: consider changing this to accept closures that implement `FnMut`
  --> $DIR/borrow-immutable-upvar-mutation.rs:15:24
   |
LL |         let _f = to_fn(|| x = 42);
   |                        ^^^^^^^^^

This is confusing, as it appears to suggest the user change the closure to "accept closures". The span is incorrect: we should use the span of the function to_fn, not the span of the closure.