pub fn get_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut T>, Pin<&mut U>) {
        let this = self.project();
        (this.first, this.second)
    }
Gets pinned mutable references to the underlying readers in this `Chain`.

Care should be taken to avoid modifying the internal I/O state of the
underlying readers as doing so may corrupt the internal state of this
`Chain`.
futures-util/src/io/chain.rs