Function diff 

pub fn diff<T, K: Eq + Hash>(
    a: &[T],
    b: &[T],
    key: impl Fn(&T) -> K,
    opts: &DiffOptions,
) -> Vec<Op>

Diffs a against b, comparing items by key.

Moves and cross-hunk replacements are paired by index distance; use Differ to supply a visual cost or replacement classes.