I was trying out problem https://mirror.codeforces.com/contest/1526/problem/D, and its implementation required us to have next_permutation. I couldn't find any reference for it in rust std lib, and since we can't import crates, I wrote a generic implementation that I could reuse across problems.

Just sharing this in case someone needs this:

/// Finds next lexographic permutation of given slice. If next larger permutation exists,
/// the slice would then have this and the return value would be true. If the slice
/// represents the largest lexicographic sequence, it updates the slice with smallest one
/// and returns false.
/// The type whose permutation is required must implement [Ord](std::cmp::Ord) trait.
pub fn next_permutation<T>(arr: &mut [T]) -> bool
where
    T: std::cmp::Ord,
{
    use std::cmp::Ordering;

    // find 1st pair (x, y) from back which satisfies x < y
    let last_ascending = match arr.windows(2).rposition(|w| w[0] < w[1]) {
        Some(i) => i,
        None => {
            arr.reverse();
            return false;
        }
    };

    // In the remaining later segment, find the one which is just
    // larger that the index found above.
    // SAFETY: unwrap_err whill not panic since binary search will
    // will never succeed since we never return equal ordering
    let swap_with = arr[last_ascending + 1..]
        .binary_search_by(|n| match arr[last_ascending].cmp(n) {
            Ordering::Equal => Ordering::Greater,
            ord => ord,
        })
        .unwrap_err();
    arr.swap(last_ascending, last_ascending + swap_with);
    arr[last_ascending + 1..].reverse();
    true
}

I added this on github: https://github.com/satylogin/cp-lib

I will probably add couple more algo and ds there as and when needed.

Submission for above problem in case someone is interested: https://mirror.codeforces.com/contest/1526/submission/119132920