Function partition_in_place 

pub(crate) fn partition_in_place<T>(
    slice: &mut [T],
    pred: impl FnMut(&T) -> bool,
) -> usize

Partitions slice in place so that elements matching pred come first, preserving the original relative order within each group. Returns the number of matching elements.

Stable replacement for the unstable Iterator::partition_in_place.