fix(rt): implement Read/Write for Pin<P> (#3413)

Without these impls, something like Pin<Box<T: Read>> does not actually
implement Read.

Closes #3412

BREAKING CHANGE: Pin is #[fundamental], so this theoretically conflicts
  with existing user Read/Write for Pin<...> impls. However, those impls
  probably don't exist yet.

src/rt/io.rs

@@ -1,5 +1,6 @@
 use std::fmt;
 use std::mem::MaybeUninit;
+use std::ops::DerefMut;
 use std::pin::Pin;
 use std::task::{Context, Poll};
 
@@ -294,6 +295,20 @@ impl<T: ?Sized + Read + Unpin> Read for &mut T {
     deref_async_read!();
 }
 
+impl<P> Read for Pin<P>
+where
+    P: DerefMut,
+    P::Target: Read,
+{
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: ReadBufCursor<'_>,
+    ) -> Poll<std::io::Result<()>> {
+        pin_as_deref_mut(self).poll_read(cx, buf)
+    }
+}
+
 macro_rules! deref_async_write {
     () => {
         fn poll_write(
@@ -336,3 +351,45 @@ impl<T: ?Sized + Write + Unpin> Write for Box<T> {
 impl<T: ?Sized + Write + Unpin> Write for &mut T {
     deref_async_write!();
 }
+
+impl<P> Write for Pin<P>
+where
+    P: DerefMut,
+    P::Target: Write,
+{
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        buf: &[u8],
+    ) -> Poll<std::io::Result<usize>> {
+        pin_as_deref_mut(self).poll_write(cx, buf)
+    }
+
+    fn poll_write_vectored(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        bufs: &[std::io::IoSlice<'_>],
+    ) -> Poll<std::io::Result<usize>> {
+        pin_as_deref_mut(self).poll_write_vectored(cx, bufs)
+    }
+
+    fn is_write_vectored(&self) -> bool {
+        (**self).is_write_vectored()
+    }
+
+    fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<std::io::Result<()>> {
+        pin_as_deref_mut(self).poll_flush(cx)
+    }
+
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<std::io::Result<()>> {
+        pin_as_deref_mut(self).poll_shutdown(cx)
+    }
+}
+
+/// Polyfill for Pin::as_deref_mut()
+/// TODO: use Pin::as_deref_mut() instead once stabilized
+fn pin_as_deref_mut<P: DerefMut>(pin: Pin<&mut Pin<P>>) -> Pin<&mut P::Target> {
+    // SAFETY: we go directly from Pin<&mut Pin<P>> to Pin<&mut P::Target>, without moving or
+    // giving out the &mut Pin<P> in the process. See Pin::as_deref_mut() for more detail.
+    unsafe { pin.get_unchecked_mut() }.as_mut()
+}