std: Add an experimental connect_timeout function

This adds a `TcpStream::connect_timeout` function in order to assist opening
connections with a timeout (cc #13523). There isn't really much design space for
this specific operation (unlike timing out normal blocking reads/writes), so I
am fairly confident that this is the correct interface for this function.

The function is marked #[experimental] because it takes a u64 timeout argument,
and the u64 type is likely to change in the future.

src/libstd/io/mod.rs

@@ -430,6 +430,8 @@ pub enum IoErrorKind {
     IoUnavailable,
     /// A parameter was incorrect in a way that caused an I/O error not part of this list.
     InvalidInput,
+    /// The I/O operation's timeout expired, causing it to be canceled.
+    TimedOut,
 }
 
 /// A trait for objects which are byte-oriented streams. Readers are defined by

src/libstd/io/net/tcp.rs

@@ -22,6 +22,7 @@ use io::IoResult;
 use io::net::ip::SocketAddr;
 use io::{Reader, Writer, Listener, Acceptor};
 use kinds::Send;
+use option::{None, Some};
 use rt::rtio::{IoFactory, LocalIo, RtioSocket, RtioTcpListener};
 use rt::rtio::{RtioTcpAcceptor, RtioTcpStream};
 
@@ -57,7 +58,21 @@ impl TcpStream {
     /// If no error is encountered, then `Ok(stream)` is returned.
     pub fn connect(addr: SocketAddr) -> IoResult<TcpStream> {
         LocalIo::maybe_raise(|io| {
-            io.tcp_connect(addr).map(TcpStream::new)
+            io.tcp_connect(addr, None).map(TcpStream::new)
+        })
+    }
+
+    /// Creates a TCP connection to a remote socket address, timing out after
+    /// the specified number of milliseconds.
+    ///
+    /// This is the same as the `connect` method, except that if the timeout
+    /// specified (in milliseconds) elapses before a connection is made an error
+    /// will be returned. The error's kind will be `TimedOut`.
+    #[experimental = "the timeout argument may eventually change types"]
+    pub fn connect_timeout(addr: SocketAddr,
+                           timeout_ms: u64) -> IoResult<TcpStream> {
+        LocalIo::maybe_raise(|io| {
+            io.tcp_connect(addr, Some(timeout_ms)).map(TcpStream::new)
         })
     }
 

src/libstd/rt/rtio.rs

@@ -146,7 +146,8 @@ impl<'a> LocalIo<'a> {
 
 pub trait IoFactory {
     // networking
-    fn tcp_connect(&mut self, addr: SocketAddr) -> IoResult<~RtioTcpStream:Send>;
+    fn tcp_connect(&mut self, addr: SocketAddr,
+                   timeout: Option<u64>) -> IoResult<~RtioTcpStream:Send>;
     fn tcp_bind(&mut self, addr: SocketAddr) -> IoResult<~RtioTcpListener:Send>;
     fn udp_bind(&mut self, addr: SocketAddr) -> IoResult<~RtioUdpSocket:Send>;
     fn unix_bind(&mut self, path: &CString)