Minor tweaks to refcell logging

[nealsid]

• Fix the logging when debug_refcell is enabled so the location is output as a string rather than a sequence of bytes
• Avoid logging an empty struct when debug_refcell is disabled
• Make the panic error message consistent with the compiler borrow-checking error messages

[rustbot]

r? @tgross35

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[tgross35]

Thank you for the PR!

The wording isn't quite accurate because panic_already_borrowed now says "...because it is also borrowed as immutable", but borrow_mut will also fail if there is an existing mutable borrow. The current messages aren't ideal either because

let cell = RefCell::new(0u32);
let b = (cell.borrow_mut(), cell.borrow_mut());
drop(b);

also prints "already borrowed: BorrowMutError" rather than the more accurate "already mutably borrowed: BorrowMutError", but it still works.

So I think the original wording of "already (mutably) borrowed (here: {err:?})" is probably fine to keep. We could alternatively add track mutably/immutably in the error type, but I don't expect that to be worth it since the # Panics API docs are pretty easy to locate (by comparison, with the rustc error it might not be as obvious that you are mutably/immutably borrowing because e.g. it's in a function signature).

Fix the logging when debug_refcell is enabled so the location is output as a string rather than a sequence of bytes

Mind commenting the before and after here? Location's debug should just print the file, line, and column.

[tgross35]

Small nit, this can be _err -> err since it is used