Function visit_leaves 

unsafe fn visit_leaves(
    sexp: SEXP,
    borrow: NativeBorrow,
    path: &mut Vec<R_xlen_t>,
    visit: &mut impl FnMut(SEXP, &[R_xlen_t], NativeBorrow) -> bool,
) -> bool

Keep each current ALTREP/list element rooted while the other argument is visited. Protection depth follows type nesting, not the number of elements.