rust/compiler/rustc_data_structures/src/binary_search_util/mod.rs

#[cfg(test)]
mod tests;

/// Uses a sorted slice `data: &[E]` as a kind of "multi-map". The
/// `key_fn` extracts a key of type `K` from the data, and this
/// function finds the range of elements that match the key. `data`
/// must have been sorted as if by a call to `sort_by_key` for this to
/// work.
pub fn binary_search_slice<'d, E, K>(data: &'d [E], key_fn: impl Fn(&E) -> K, key: &K) -> &'d [E]
where
    K: Ord,
{
    let size = data.len();
    let start = data.partition_point(|x| key_fn(x) < *key);
    // At this point `start` either points at the first entry with equal or
    // greater key or is equal to `size` in case all elements have smaller keys
    if start == size || key_fn(&data[start]) != *key {
        return &[];
    };

    // Find the first entry with key > `key`. Skip `start` entries since
    // key_fn(&data[start]) == *key
    let offset = start + 1;
    let end = data[offset..].partition_point(|x| key_fn(x) <= *key) + offset;

    &data[start..end]
}