Rust: Add a method for computing the absolute difference between unsigned integers

[Centril]

See discussion in https://github.com/rust-lang/rust/pull/62103#discussion_r296988078.

[ExpHP]

I suggest not calling it abs_sub due to the confusion created by the (terribly misnamed) function on floats, whose tortured legacy continues to this day in num_traits::Signed.

[Centril]

@ExpHP abs_diff perhaps?

[leonardo-m]

abs_difference ?

[Centril]

What do you think about adding:

fn abs_difference<T: Sub<Output = T> + Ord>(x: T, y: T) -> T {
    if x < y {
        y - x
    } else {
        x - y
    }
}

instead of doing it on just the unsigned integer types?

[scalexm]

distance is a good name, I think, as it is the mathematical term for abs(x-y).

I dislike @centril’s above generalization, as what it does is unrelated with the name of the function in most cases (the first thing that comes to mind is that something called an « absolute difference » should be a non-negative real number).

[HeroicKatora]

The term distance seems too generic from the standpoint of a mathematical motivation, it is also used in the context of very different metric spaces than the normed space induced by the L1-norm. I think 'absolute difference' hits the nail on the head and with the given prior art of abbreviations that would make abs_diff my clear favorite. Similar to abs, checked_sub and other functions it doesn't seem necessary to add it in another way than as a method.