Function fill_region 

pub(crate) fn fill_region<T>(
    start: usize,
    len: usize,
    total_len: usize,
    out: &mut [T],
    elt: impl FnMut(usize) -> T,
) -> usize

Helper for ALTREP get_region implementations.

R guarantees that the caller-provided buffer is at least len long. This helper clamps the requested range to the vector’s total length and the actual buffer length, then fills out with values from the provided element accessor.