Macro rc_zip::fields

macro_rules! fields {
    (from $input:ident { $($name:ident : $combinator:expr),+ $(,)* } do $body:expr) => { ... };
    ({ $($name:ident : $combinator:expr),+ $(,)* } chain $next:expr) => { ... };
    ({ $($name:ident : $combinator:expr),+ $(,)* } map $body:expr) => { ... };
    ($struct:ident { $($name:ident : $combinator:expr),+ $(,)* }) => { ... };
}