Rollup merge of #140094 - Kivooeo:raw-pointer-assignment-suggestion, r=compiler-errors

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)

tests/ui/typeck/pointer-arith-assign.fixed

@@ -0,0 +1,20 @@
+//@ run-rustfix
+#![allow(dead_code)]
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+
+fn test_add_assign_raw_pointer() {
+    let mut arr = [0u8; 10];
+    let mut _ptr = arr.as_mut_ptr();
+
+    _ptr = _ptr.wrapping_add(2); //~ ERROR binary assignment operation `+=` cannot be applied to type `*mut u8` [E0368]
+}
+
+fn test_sub_assign_raw_pointer() {
+    let mut arr = [0u8; 10];
+    let mut _ptr = arr.as_mut_ptr();
+
+    _ptr = _ptr.wrapping_sub(2); //~ ERROR binary assignment operation `-=` cannot be applied to type `*mut u8` [E0368]
+}
+
+fn main() {}

tests/ui/typeck/pointer-arith-assign.rs

@@ -0,0 +1,20 @@
+//@ run-rustfix
+#![allow(dead_code)]
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+
+fn test_add_assign_raw_pointer() {
+    let mut arr = [0u8; 10];
+    let mut _ptr = arr.as_mut_ptr();
+
+    _ptr += 2; //~ ERROR binary assignment operation `+=` cannot be applied to type `*mut u8` [E0368]
+}
+
+fn test_sub_assign_raw_pointer() {
+    let mut arr = [0u8; 10];
+    let mut _ptr = arr.as_mut_ptr();
+
+    _ptr -= 2; //~ ERROR binary assignment operation `-=` cannot be applied to type `*mut u8` [E0368]
+}
+
+fn main() {}

tests/ui/typeck/pointer-arith-assign.stderr

@@ -0,0 +1,31 @@
+error[E0368]: binary assignment operation `+=` cannot be applied to type `*mut u8`
+  --> $DIR/pointer-arith-assign.rs:10:5
+   |
+LL |     _ptr += 2;
+   |     ----^^^^^
+   |     |
+   |     cannot use `+=` on type `*mut u8`
+   |
+help: consider using `add` or `wrapping_add` to do pointer arithmetic
+   |
+LL -     _ptr += 2;
+LL +     _ptr = _ptr.wrapping_add(2);
+   |
+
+error[E0368]: binary assignment operation `-=` cannot be applied to type `*mut u8`
+  --> $DIR/pointer-arith-assign.rs:17:5
+   |
+LL |     _ptr -= 2;
+   |     ----^^^^^
+   |     |
+   |     cannot use `-=` on type `*mut u8`
+   |
+help: consider using `sub` or `wrapping_sub` to do pointer arithmetic
+   |
+LL -     _ptr -= 2;
+LL +     _ptr = _ptr.wrapping_sub(2);
+   |
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0368`.