Macro succeed 

macro_rules! succeed {
    (|$first_arg:ident $(, $arg:ident )*| $function_body:expr) => { ... };
    (|$first_arg:ident:$first_arg_type:ty $(, $arg:ident:$arg_type:ty )*| $function_body:expr) => { ... };
    ($value:expr) => { ... };
    () => { ... };
}

A parser that succeeds without chomping any characters.

Seems weird on its own, but it is very useful in combination with other functions. The docs for keep and and_then have some neat examples.