Function place 

pub fn place(
    phys: u64,
    arena: usize,
    used: usize,
    name: &'static str,
    len: usize,
    align: usize,
) -> Result<usize, NoRoom>

Where a part of len bytes, aligned to align, goes in an arena of arena bytes whose physical base is phys and whose first used bytes are taken: its offset, or why it cannot go anywhere.

Pure, so it is pinned by host tests; Layout::take is this and nothing more.