Function create_dir_all_beneath 

pub fn create_dir_all_beneath(root: &Path, rel: &Path) -> Result<(), Error>

Confined mkdir -p for a root-relative directory path: creates each missing component beneath root, refusing to descend through a symlink that escapes (#77). On Linux: mkdirat + openat2(RESOLVE_BENEATH) per component; otherwise the contain_within_canonical + std::fs fallback.