Conditionally select usize

[nickray]

Background is an RSA implementation I'm writing, where I'm using type Digit = usize.

I presume lack of implementation for usize/isize was an oversight, not intentional?

[hdevalence]

The lack of implementation for usize was because usize is most commonly used to index arrays, and providing ConditionallySelectable for usize could lead someone to think that doing

let index = usize::conditional_select(&a, &b, choice);
let val = array[index];

was doing a constant-time selection out of the array, when in fact using secret array indices can leak information into caches.

For a constant-time RSA implementation (I'm assuming Digit is used for a big-integer representation), it seems better to use Digit = u64, so that you know the size of your digits.