Function nth_argument 

fn nth_argument<'hir>(
    hir: &'hir Hir<'hir>,
    function_id: FunctionId,
    args: &'hir CallArgs<'hir>,
    arg: usize,
    parameter: usize,
) -> Option<&'hir Expr<'hir>>

The argument at position arg of a positional call, or the one a named call binds to the callee’s parameter at position parameter. Named arguments come in source order, which is neither the parameter order nor, in the method form, the argument order.