net: add TcpStream::ready and non-blocking ops (#3130)

Adds function to await for readiness on the TcpStream and non-blocking read/write functions.

`async fn TcpStream::ready(Interest)` waits for socket readiness satisfying **any** of the specified
interest. There are also two shorthand functions, `readable()` and `writable()`.

Once the stream is in a ready state, the caller may perform non-blocking operations on it using
`try_read()` and `try_write()`. These function return `WouldBlock` if the stream is not, in fact, ready.

The await readiness function are similar to `AsyncFd`, but do not require a guard. The guard in
`AsyncFd` protect against a potential race between receiving the readiness notification and clearing
it. The guard is needed as Tokio does not control the operations. With `TcpStream`, the `try_read()`
and `try_write()` function handle clearing stream readiness as needed.

This also exposes `Interest` and `Ready`, both defined in Tokio as wrappers for Mio types. These
types will also be useful for fixing #3072 .

Other I/O types, such as `TcpListener`, `UdpSocket`, `Unix*` should get similar functions, but this
is left for later PRs.

Refs: #3130

1 files changed, 13 insertions, 0 deletions

diff --git a/tokio/src/macros/cfg.rs b/tokio/src/macros/cfg.rs
index edf681a4..15216560 100644
--- a/tokio/src/macros/cfg.rs
+++ b/tokio/src/macros/cfg.rs
@@ -79,6 +79,19 @@ macro_rules! cfg_io_driver {
     }
 }
 
+macro_rules! cfg_io_driver_impl {
+    ( $( $item:item )* ) => {
+        $(
+            #[cfg(any(
+                feature = "net",
+                feature = "process",
+                all(unix, feature = "signal"),
+            ))]
+            $item
+        )*
+    }
+}
+
 macro_rules! cfg_not_io_driver {
     ($($item:item)*) => {
         $(