Unhelpful help for E0387 "consider changing this closure to take self by mutable reference"

[killercup]

I just got this error message (on current stable, beta, and the nightly mentioned below):

error[E0387]: cannot borrow data mutably in a captured outer variable in an `Fn` closure
  --> src/main.rs:30:57
   |
30 |         .for_each(|filename| tally_words(filename, &mut words).unwrap());
   |                                                         ^^^^^
   |
help: consider changing this closure to take self by mutable reference
  --> src/main.rs:30:19
   |
30 |         .for_each(|filename| tally_words(filename, &mut words).unwrap());
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

As you can see it suggests to do no change at all:

-          .for_each(|filename| tally_words(filename, &mut words).unwrap());
+          .for_each(|filename| tally_words(filename, &mut words).unwrap());

You can reproduce this by checking out hello-rust/show@d495abe and running cd episode/9/rust/pascal/; cargo build.

Rust version

rustc 1.30.0-nightly (f8d34596f 2018-08-30)
binary: rustc
commit-hash: f8d34596ff74da91e0877212a9979cb9ca13eb7e
commit-date: 2018-08-30
host: x86_64-apple-darwin
release: 1.30.0-nightly
LLVM version: 7.0

[hcs64]

As noted in #31701 the text of the message isn't particularly helpful, either.

[ibraheemdev]

I think this was fixed:

use rayon::prelude::*;

fn main() {
    let mut y = 0;
    (0..10).into_par_iter().for_each(|x| println!("{}{}", x, &mut y));
}

error[E0596]: cannot borrow `y` as mutable, as it is a captured variable in a `Fn` closure
 --> src/main.rs:5:62
  |
5 |     (0..10).into_par_iter().for_each(|x| println!("{}{}", x, &mut y));
  |                                                              ^^^^^^ cannot borrow as mutable