Better suggestion for closure that needs to capture bindings, but not all

[estebank]

Given

    let numbers: Vec<i32> = (1..100).collect();
    let len = numbers.len();

    let _sums_of_pairs: Vec<_> = (0..len)
        .map(|j| ((j + 1)..len).map(|k| numbers[j] + numbers[k]))
        .flatten()
        .collect();

we currently emit

error[E0373]: closure may outlive the current function, but it borrows `j`, which is owned by the current function
 --> src/main.rs:6:37
  |
6 |         .map(|j| ((j + 1)..len).map(|k| numbers[j] + numbers[k]))
  |                                     ^^^         - `j` is borrowed here
  |                                     |
  |                                     may outlive borrowed value `j`
  |
note: closure is returned here
 --> src/main.rs:6:18
  |
6 |         .map(|j| ((j + 1)..len).map(|k| numbers[j] + numbers[k]))
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: to force the closure to take ownership of `j` (and any other referenced variables), use the `move` keyword
  |
6 |         .map(|j| ((j + 1)..len).map(move |k| numbers[j] + numbers[k]))
  |                                     ^^^^^^^^

if we apply the suggestion we get the following

error[E0507]: cannot move out of `numbers`, a captured variable in an `FnMut` closure
 --> src/main.rs:6:37
  |
2 |     let numbers: Vec<i32> = (1..100).collect();
  |         ------- captured outer variable
...
6 |         .map(|j| ((j + 1)..len).map(move |k| numbers[j] + numbers[k]))
  |                                     ^^^^^^^^ -------
  |                                     |        |
  |                                     |        move occurs because `numbers` has type `Vec<i32>`, which does not implement the `Copy` trait
  |                                     |        move occurs due to use in closure
  |                                     move out of `numbers` occurs here

because we're moving numbers into the closure and consuming it.

Ideally, we would suggest to introduce a new binding that borrows numbers to avoid moving it.

Example taken from https://stackoverflow.com/questions/65258521/how-do-i-write-a-lazily-evaluated-double-for-loop-in-a-functional-style

[zbyrn]

@rustbot claim

[zbyrn]

I don't know whether anyone else is working on this, but I feel this personally. I was confused by this exact problem even after I had some experience in rust.

Do you think we should make the let-bound reference suggestion when we suggest to use move, or after the user puts in the move and E0507 occurs?

[estebank]

Do you think we should make the let-bound reference suggestion when we suggest to use move, or after the user puts in the move and E0507 occurs?

Ideally we would provide the suggestion in the first error, but it is likely quite hard to accomplish (and less general). I'll be more than happy @zbyrn if you can suggest it on the E0507.