Improve diagnostics for pointer arithmetic += and -= (fixes #137391)

[Kivooeo]

Description:

This PR improves the diagnostic message for cases where a binary assignment operation like ptr += offset or ptr -= offset is attempted on *mut T. These operations are not allowed, and the compiler previously suggested calling .add() or .wrapping_add(), which is misleading if not assigned.

This PR updates the diagnostics to suggest assigning the result of .wrapping_add() or .wrapping_sub() back to the pointer, e.g.:

Examples

For this code

let mut arr = [0u8; 10];
let mut ptr = arr.as_mut_ptr();

ptr += 2;

it will say:

10 |     ptr += 2;
   |     ---^^^^^
   |     |
   |     cannot use `+=` on type `*mut u8`
   |
help: consider replacing `ptr += offset` with `ptr = ptr.wrapping_add(offset)` or `ptr.add(offset)`
   |
10 -     ptr += 2;
10 +     ptr = ptr.wrapping_add(2);

Related issue: #137391
cc @nabijaczleweli for context (issue author)

[rustbot]

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[Kivooeo]

Oh wait, I did something very wrong with brances...
(fixed now ^^)

[compiler-errors]

r? compiler-errors

[rustbot]

Some changes occurred in src/tools/cargo

cc @ehuss

[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit 834e476 has been approved by compiler-errors

It is now in the queue for this repository.