Fixes a soundness bug in io::Take (#4428). The unsoundness is exposed when leaking memory in the given AsyncRead implementation and then overwriting the supplied buffer:

impl AsyncRead for Buggy {
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut ReadBuf<'_>
    ) -> Poll<Result<()>> {
      let new_buf = vec![0; 5].leak();
      *buf = ReadBuf::new(new_buf);
      buf.put_slice(b"hello");
      Poll::Ready(Ok(()))
    }
}

Also, this release includes improvements to the multi-threaded scheduler that can increase throughput by up to 20% in some cases (#4383).

Fixed

• io: soundness don't expose uninitialized memory when using io::Take in edge case (#4428)
• fs: ensure File::write results in a write syscall when the runtime shuts down (#4316)
• process: drop pipe after child exits in wait_with_output (#4315)
• rt: improve error message when spawning a thread fails (#4398)
• rt: reduce false-positive thread wakups in the multi-threaded scheduler (#4383)
• sync: don't inherit Send from parking_lot::*Guard (#4359)

Added

• net: TcpSocket::linger() and set_linger() (#4324)
• net: impl UnwindSafe for socket types (#4384)
• rt: impl UnwindSafe for JoinHandle (#4418)
• sync: watch::Receiver::has_changed() (#4342)
• sync: oneshot::Receiver::blocking_recv() (#4334)
• sync: RwLock blocking operations (#4425)

Unstable

The following changes only apply when building with --cfg tokio_unstable

• rt: breaking change overhaul runtime metrics API (#4373)