Function parallel_reduce 

pub fn parallel_reduce<T>(
    items: &[T],
    f: impl Fn(T, T) -> T + Send + Sync + 'static,
    identity: T,
) -> T
where
    T: Clone + Send + Sync + 'static,

Reduce items to a single value using f with the given identity element.

Fans out the reduction into chunks (one per available hardware thread), reduces each chunk sequentially, then combines the chunk results.

Type constraints

• T: Clone + Send + Sync – items and identity must be cloneable and thread-safe.
• f: Fn(T, T) -> T + Send + Sync – the combining function must be thread-safe.