Simple TT muncher

munch.rs

// switch to nightly and uncomment these for debugging
//#![feature(trace_macros)]
//trace_macros!(true);

/// Trick the macro parser into outputting an item without checking its syntax
macro_rules! as_item( ($i:item) => ($i) );

macro_rules! foo(
    // first rule: define the invocation syntax and call myself again with some
    //             easily parseable parameters (notice everything is bracketed)
    (
        pub struct $name:ident $body:tt
    ) => (
        foo!(parse [pub struct $name] [] $body);
    );
    
    // the following rules are all internal to the macro
    
    // this is the base case: all struct members have been munched,
    // so proceed to output
    (parse $decl:tt [$($member:tt)*] {})
        => ( foo!(output $decl [$($member)*]); );
        
    // parse one public struct member, then call myself to parse the rest
    (parse $decl:tt [$($member:tt)*] { pub $name:ident: $typ:ty, $($t:tt)* })
        => ( foo!(parse $decl [$($member)* pub $name: $typ,] { $($t)* }); );
        
    // parse one private struct member, then call myself to parse the rest
    (parse $decl:tt [$($member:tt)*] { $name:ident: $typ:ty, $($t:tt)* })
        => ( foo!(parse $decl [$($member)* $name: $typ,] { $($t)* }); );
        
    // output the finished struct
    (output [$($decls:tt)*] [$($member:tt)*])
        => ( as_item!(#[derive(Debug)] $($decls)* { $($member)* }); );
);

foo! {
    pub struct Foo {
        pub bar: usize,
        baz: isize, // <-- Suppose, this should be private.
    }
}

fn main() {
    println!("{:#?}", Foo { bar: 0, baz: 0 });
}