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Improve diagnostics for pointer arithmetic += and -= (fixes #137391) **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 ```rust let mut arr = [0u8; 10]; let mut ptr = arr.as_mut_ptr(); ptr += 2; ``` it will say: ```rust 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)