Macro string_handler 

macro_rules! string_handler {
    ($(#[$attr:meta])* $vis:vis fn $name:ident($input:ident, $out:ident, $data:ident) $body:block) => { ... };
    ($(#[$attr:meta])* $vis:vis fn $name:ident($action:ident, $input:ident, $out:ident, $data:ident) $body:block) => { ... };
}

Define a command or prompt callback with a &str input and &mut Output. The four-argument form also supports runtime-action trampolines. Both forms copy the input inside the ABI boundary and contain handler panics.