Function difft::diff::graph::set_neighbours

pub(crate) fn set_neighbours<'s, 'b>(
    v: &Vertex<'s, 'b>,
    alloc: &'b Bump,
    seen: &mut HashMap<&Vertex<'s, 'b>, SmallVec<[&'b Vertex<'s, 'b>; 2]>, BuildHasherDefault<FxHasher>>
)

Compute the neighbours of v if we haven’t previously done so, and write them to the .neighbours cell inside v.