Code shared from the Rust Playground

playground.rs

macro_rules! or_else {
    (,$b:tt) => {
        stringify!($b)
    };
    ($a:tt, $b:tt) => {
        stringify!($a)
    };
}

macro_rules! func {
    ($export:ident $(: $native:ident)?($($args:tt),*)$(-> $ret:ident)?, $($u:tt)*) => {
        func!($export $(: $native)?($($args),*)$(-> $ret)?);
        func!($($u)*);
    };
    ($export:ident $(: $native:ident)?($($args:tt),*)$(-> $ret:ident)?) => {
        println!(
            r#"  "{}": extern "C" fn {}({}) -> {}"#,
            stringify!($export),
            or_else!($($native)?, $export),
            stringify!($($args),*),
            or_else!($($ret)?, ())
        );
    };
}

macro_rules! ns {
    ($ns:ident: {
        $($t:tt)*
    },$($u:tt)*) => {
        ns!($ns: {
            $($t)*
        });
        ns!($($u)*);
    };
    ($ns:ident: {
        $($t:tt)*
    }) => {
        println!(r#""{}": {{"#, stringify!($ns));
        func!($($t)*);
        println!("}}");  
    };
}

fn main() {
    ns!( wasi: {
        fd_read(qsdf,qsdf),
        fd_write: mbed_os_fd_write(txt, len),
        bye(qa, ppl)
    });
    ns!(mbed_os: {
        gpio_set: mbed_os_gpio_set(u32, bool),
        gpio_get: mbed_os_gpio_get(u32) -> bool
    });
}