Macro message

macro_rules! message {
    ($name:ident, $value:ty, $docs:literal) => { ... };
    ($name:ident, $docs:literal) => { ... };
    ($name:ident, $value:ty => $result:ty, $docs:literal) => { ... };
    ($name:ident => $result:ty, $docs:literal) => { ... };
}