`Send`, `Sync`, and `Unpin` are special because they're so-called 'auto traits': The compiler automatically implements them for all compound types whose fields also implement those traits. That turns out to be a double-edged sword: The automatic implementation makes them pervasive in a way that `Clone` or `Debug` could never be, but it also means that changes which might be otherwise private can have unintended far-reaching effects.
In your case, what happens is that async code conceptually generates an `enum` with one variant per await point which contains the locals held across that point, and it's this enum that actually gets returned from the async method/block and implements the `Future` trait.
In your case, what happens is that async code conceptually generates an `enum` with one variant per await point which contains the locals held across that point, and it's this enum that actually gets returned from the async method/block and implements the `Future` trait.