NLL diagnostics for `ref mut z @ &mut Some(ref a)` uses `_` as binding name

[Centril]

Context: #66296 (comment)

The _ below should be z instead.

error[E0502]: cannot borrow `_` as immutable because it is also borrowed as mutable
  --> $DIR/borrowck-pat-ref-mut-and-ref.rs:11:31
   |
LL |         ref mut z @ &mut Some(ref a) => {
   |         ----------------------^^^^^-
   |         |                     |
   |         |                     immutable borrow occurs here
   |         mutable borrow occurs here
...
LL |             **z = None;
   |             ---------- mutable borrow later used here

cc @mark-i-m @pnkfelix @matthewjasper

cc #65490

---

Minimal reproducer:

#![feature(bindings_after_at)]

fn foo() {
    let ref mut _z @ ref _a = &mut 1;
    **_z = 0;
}

[mark-i-m]

I feel like the internal use of _ as a default name for things is a bit undisciplined, and it's leaking into diagnostics in annoying ways. There is also this: #66679

I feel like internally, we should keep a strong separation between names that we get from the source code being compiled and names being generated by the compiler...

[Centril]

I think this issue is about the narrow issue of why this happens in the case of ref mut x @ Some(ref y) though whereas it doesn't happen in other cases.

[mark-i-m]

@Centril Sure, but I think it stems from a larger problem of undisciplined use of _ in various places... If we don't fix that, then we will get weird bugs like this showing up again later even if we fix this particular example....

[Centril]

@mark-i-m I don't mind fixing this by fixing the root cause, but can we discuss the bigger issue in a issue made for that and cc this one?