Idiomatic rust for sorting a pair of integers (a,b) such that a < b

All these require mut, how about let (a, b) = if .. else .. ?

I would instinctively follow this anyway since I would typically want to rename from thinga and thingb to thingmin and thingmax.

For completeness, let mut ab = [a, b]; ab.sort(); (ab[0], ab[1])

While I'm not as familiar with Rust, if there's an rust-esque way of writing (a, b) = (min(a, b), max(a, b)) it'd be the most clear imo. It looks like you effectively have that via a.min(b) and so on.

FWIW, there is nightly minimax that sorts pairs of values. Notably, it does the following:

pub const fn minmax(v1: T, v2: T) -> [T; 2]
where
    T: [const] Ord,
{
    if v2 < v1 { [v2, v1] } else { [v1, v2] }
}

I'm not sure any language would have a single idiomatic philosophy at this level of specificity. That said, I personally find the first approach most readable. rustfmt will turn it into a single line, which I find very easy to read.