There's nothing `select!` could do here to force `future1` to drop, because it doesn't receive ownership of `future1`. If we wanted to force this, we'd have to forbid `select!` from polling futures by reference, but that's a pretty fundamental capability that we often rely on to `select!` in a loop for example. The blanket `impl<F> Future for &mut F where F: Future ...` isn't a Tokio thing either; that's in the standard library.