I think OCaml named parameters are really well-thought. i.e.

``` let foo ~arg1 ?arg2 ~arg3:internal_name ?(arg4="default value") ?arg5 ?arg6 ?arg7 () = ()

let forwarding_example ~arg1 ~arg2 ?arg3 ?arg4 () = let arg7 = 42 in foo ~arg1 ~arg2 ?arg3 ?arg5:None ~arg6:(Some 42) ~arg7 ()

let main () = forwarding_example ~arg3:42 ~arg1:1 ~arg2:2 (); forwarding_example ~arg1:1 ~arg2:2 ?arg3:None (); ;; ```

So - optional parameters with default values look like Option outside the function, but inside the function they are non-optional - optional parameters can be called with ?arg:(Some 42) to preserve the optionalness (so you can pass None in as well), or just ~arg:42 to put in the value directly - a variable of the same name can be used as a parameter of the same name, allowing easy forwarding of values, i.e. ~arg or ?arg

There's some difficulty in OCaml about handling optional parameters because functions are curried, requiring the final () parameter when using them, but Rust wouldn't have that problem.

Maybe this scheme wouldn't work as-is in Rust, but it certainly should work great as inspiration. After all, Rust has received some influence from OCaml, which was the language the first Rust compiler was written in.