pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut St1>, Pin<&mut St2>) {
        let this = self.project();
        this.inner.get_pin_mut()
    }
Acquires a pinned mutable reference to the underlying streams that this
combinator is pulling from.

Note that care must be taken to avoid tampering with the state of the
stream which may otherwise confuse this combinator.
futures-util/src/stream/select.rs