rough kwargs macro sketch

kwargs.rs

use std::borrow::Cow;

#[macro_export]
macro_rules! kwargs {
    // pass no arguments
    () => { |_| {} };
    // in-progress variants
    (@struct [$($attr:meta)*] $name:ident [$($decls:tt)*] $field:ident: $typ:ty, $($rest:tt)*) => {
        kwargs!(@struct [$($attr)*] $name [$($decls)* $field: ::std::option::Option<$typ>,] $($rest)*);
    };
    // version that does not end with a comma
    (@struct [$($attr:meta)*] $name:ident [$($decls:tt)*] $field:ident: $typ:ty) => {
        kwargs!(@struct [$($attr)*] $name [$($decls)* $field: ::std::option::Option<$typ>,]);
    };
    // base case
    (@struct [$($attr:meta)*] $name:ident [$($field:ident: $typ:ty,)*] $(,)*) => {
        $(#[$attr])*
        #[derive(Default)]
        pub struct $name<'kw> {
            $(pub $field: $typ,)*
            __marker: ::std::marker::PhantomData<&'kw ()>,
        }
    };
    // resolve kwargs
    (@resolve: $kw:ident) => {
        { let mut a = Default::default(); $kw(&mut a); a };
    };
    // declare kwargs receiver struct
    ($($(#[$attr:meta])* struct $name:ident { $($inner:tt)* };)*) => {
        $(kwargs!(@struct [$($attr)*] $name [] $($inner)*);)*
    };
    // pass some arguments, colon variant
    ($($field:ident: $value:expr),* $(,)*) => {
        |x| { $(x.$field = Some(From::from($value));)* }
    };
}

// example usage

kwargs! {
    #[doc(hidden)]
    struct FooArgs { unused: u32, first: Cow<'kw, str>, second: &'kw str };
}

pub fn foo<KW: FnOnce(&mut FooArgs)>(param: u32, kw: KW) {
    let kwargs = kwargs!(@resolve: kw);
    println!("param: {}", param);
    println!("  first: {:?}", kwargs.first);
    println!("  second: {:?}", kwargs.second);
}

fn main() {
    foo(1, kwargs!(first: "First", second: "Second"));
    //foo(2, kwargs!(second = "Second", first = "First".into()));
    foo(3, kwargs!(first: "First"));
    foo(4, kwargs!(second: "Second"));
    foo(4, kwargs!());
}