def is_vararg(leaf: Leaf, within: set[NodeType]) -> bool:
    """Return True if `leaf` is a star or double star in a vararg or kwarg.

    If `within` includes VARARGS_PARENTS, this applies to function signatures.
    If `within` includes UNPACKING_PARENTS, it applies to right hand-side
    extended iterable unpacking (PEP 3132) and additional unpacking
    generalizations (PEP 448).
    """
    if leaf.type not in VARARGS_SPECIALS or not leaf.parent:
        return False

    p = leaf.parent
    if p.type == syms.star_expr:
        # Star expressions are also used as assignment targets in extended
        # iterable unpacking (PEP 3132).  See what its parent is instead.
        if not p.parent:
            return False

        p = p.parent

    return p.type in within
Return True if `leaf` is a star or double star in a vararg or kwarg.

    If `within` includes VARARGS_PARENTS, this applies to function signatures.
    If `within` includes UNPACKING_PARENTS, it applies to right hand-side
    extended iterable unpacking (PEP 3132) and additional unpacking
    generalizations (PEP 448).
    
src/black/nodes.py