Finally block is a block that is executed when it's dropped. It helps a user write the deferred statements that should be executed, even when some statements return early.
function f(flag: &AtomicBool) -> Option<()> { if some_condition { let _f = finally(|| { flag.store(true, Ordering::SeqCst); }); some_function(flag)?; } else { another_function()?; } }