Summary

For now, manual_rotate only lints when the shift amounts are constant. However, there is also the option of shifting by some expression x and either BIT_WIDTH - x or (BIT_WIDTH - 1) ^ x. This is a bit harder to detect, as we don't know whether the subtraction will be in the first or second part of the binary operation, but it should certainly be possible.

cc @frp who implemented the original lint.

Lint Name

manual_rotate

Reproducer

I tried this code:

let s = 5;
let x = 123456_u32;

let _ = (x << s) | (x >> (32 - s));
let _ = (x << s) | (x >> (31 ^ s));

The lint should be extended to catch both cases, suggesting x.rotate_left(s).

Version

No response