Macro callback

macro_rules! callback {
    (
        @capture<$tcx:lifetime $(,)? $($lft:lifetime),*> { $($name:ident: $type:ty),* $(,)? }
        @unblock = |$this:ident| $unblock:block
    ) => { ... };
    (
        @capture<$tcx:lifetime $(,)? $($lft:lifetime),*> { $($name:ident: $type:ty),* $(,)? }
        @unblock = |$this:ident| $unblock:block
        @timeout = |$this_timeout:ident| $timeout:block
    ) => { ... };
}