Function num_of_fill_args

fn num_of_fill_args(
    spec: FnArgs,
    args: u16,
    name: &str,
    cx: &Context<'_>,
) -> Result<u16>

Number of arguments needed to fill out the remaining slots on the stack. If a function has 3 required args and 2 optional, and it is called with 4 arguments, then 1 will be returned. Indicating that 1 additional nil argument should be added to the stack.