With newtypes you can also emulate named args with a single struct. This also allows them to be passed on together.

struct RectArgs{
    x:i32,
    y:32,
    w:32,
    h:32,
}
impl Default for RectArgs{...}

Then the call site looks like this:

rect(RectArgs{
    x:0,
    y:0,
    w:30,
    h:20,
}
// with default arguments
rect(RectArgs{
    w:30,
    h:20,
    .. RectArgs::default ()
}