Borrow checker unhelpful diagnostic

Code

fn confusing_diagnostic() {
    let r0_members = std::collections::BTreeSet::from(["m0", "m1", "m2", "m3"]);
    let mut rooms = std::collections::BTreeMap::from([(0, ("m0", r0_members))]);
    let for_removal = vec!["m1", "m3"];

    let room = rooms.get_mut(&0).unwrap();
    for_removal
        .iter()
        .filter(|&member| *member != (*room).0)
        .for_each(|member| {
            rooms.get_mut(&0).unwrap().1.remove(member);
        });
}

Current output

error[E0499]: cannot borrow `rooms` as mutable more than once at a time
  --> src/main.rs:24:19
   |
20 |     let room = rooms.get_mut(&0).unwrap();
   |                ----- first mutable borrow occurs here
...
24 |         .for_each(|member| {
   |          -------- ^^^^^^^^ second mutable borrow occurs here
   |          |
   |          first borrow later used by call
25 |             rooms.get_mut(&0).unwrap().1.remove(member);
   |             ----- second borrow occurs due to use of `rooms` in closure

Desired output

error[E0499]: cannot borrow `rooms` as mutable more than once at a time
  --> src/main.rs:24:19
   |
20 |     let room = rooms.get_mut(&0).unwrap();
   |                ----- first mutable borrow occurs here
...
23 |        .filter(|&member| *member != (*room).0)
   |         ------                        ^^^^ first borrow used here
   |         |
   |         first borrow later used by call
...
24 |         .for_each(|member| {
   |                   ^^^^^^^^ second mutable borrow occurs here
25 |             rooms.get_mut(&0).unwrap().1.remove(member);
   |             ----- second borrow occurs due to use of `rooms` in closure

Rationale and extra context

The mention "first borrow later used by call" to for_each is misleading. It would be more helpful to point to the filter closure, as a fix consists in using a reference to the field of room we need in filter instead:

fn fixed() {
    let r0_members = std::collections::BTreeSet::from(["m0", "m1", "m2", "m3"]);
    let mut rooms = std::collections::BTreeMap::from([(0, ("m0", r0_members))]);
    let for_removal = vec!["m1", "m3"];

    let room = rooms.get_mut(&0).unwrap();
    let owner = (*room).0;
    for_removal
        .iter()
        .filter(|&member| *member != owner)
        .for_each(|member| {
            rooms.get_mut(&0).unwrap().1.remove(member);
        });
}

@nirbheek
@sdroege

Other cases

No response

Anything else?

No response

Start by compiling the reproducer in src/main.rs and inspect the E0499 borrow-checker diagnostic shown in the issue. Compare the reported spans with the desired output, especially the filter closure; the work is done when the diagnostic points to the first borrow's use there and the existing example is covered by an appropriate compiler test.