Type Alias LandingGuardFuture 

pub type LandingGuardFuture = Pin<Box<dyn Future<Output = Result<(), String>>>>;

The answer of a LandingGuard: Ok lets the body land, Err says why it may not (the topic waits).

Aliased Type

#[repr(transparent)]
pub struct LandingGuardFuture { /* private fields */ }