Provide more context when mutably borrowing an imutably borrowed value

[estebank]

Point at statics and consts being mutable borrowed or written to:

error[E0594]: cannot assign to immutable static item `NUM`
  --> $DIR/E0594.rs:4:5
   |
LL | static NUM: i32 = 18;
   | --------------- this `static` cannot be written to
...
LL |     NUM = 20;
   |     ^^^^^^^^ cannot assign

Point at the expression that couldn't be mutably borrowed from a pattern:

error[E0596]: cannot borrow data in a `&` reference as mutable
  --> $DIR/mut-pattern-of-immutable-borrow.rs:19:14
   |
LL |     match &arg.field {
   |           ---------- this cannot be borrowed as mutable
LL |         Some(ref mut s) => s.push('a'),
   |              ^^^^^^^^^ cannot borrow as mutable

Partially address #74617.

[rustbot]

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

[rustbot]

r? @nnethercote

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[hkBst]

This does not seem to have a testcase.

[estebank]

It's covered by a few for Boring like in tests/ui/nll/dont-print-desugared.stderr, and tests/ui/pattern/mut-pattern-of-immutable-borrow.rs covers the BlockTailTemp case.

[hkBst]

I checked for the output "this cannot be written to" and did not find it. Since you added the "to", the output should be in the diff, but I did not find it...

[estebank]

I see. It is a combination that can't happen. It would trigger with something like

struct S;
impl S {
    fn foo(&self) {
        *self = S;
    }
}

but the explicit check for no overlapping spans means that the output is unchanged:

error[E0594]: cannot assign to `*self`, which is behind a `&` reference
 --> f16.rs:4:9
  |
4 |         *self = S;
  |         ^^^^^^^^^ `self` is a `&` reference, so it cannot be written to
  |
help: consider changing this to be a mutable reference
  |
3 |     fn foo(&mut self) {
  |             +++

[nnethercote]

r=me with the two commits squashed. Thanks.

[estebank]

@bors r=nnethercote

[bors]

📌 Commit 71b0755 has been approved by nnethercote

It is now in the queue for this repository.

[bors]

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.