Function rank_into 

pub fn rank_into(
    values: &[f64],
    idx_scratch: &mut [usize],
    ranks_out: &mut [f64],
) -> Option<()>

Rank values (1-based, ties averaged) into the caller-owned ranks_out. idx_scratch is a caller-owned index buffer; both must equal values.len(). Returns None on a length mismatch. No allocation.