auto merge of #12980 : cmr/rust/overhaul-stdio, r=thestinger

this comes from a discussion on IRC where the split between stdin and stdout
seemed unnatural, and the fact that reading on stdin won't flush stdout, which
is unlike every other language (including C's stdio).

src/libstd/io/stdio.rs

@@ -296,7 +296,13 @@ pub struct StdReader {
 impl Reader for StdReader {
     fn read(&mut self, buf: &mut [u8]) -> IoResult<uint> {
         let ret = match self.inner {
-            TTY(ref mut tty) => tty.read(buf),
+            TTY(ref mut tty) => {
+                // Flush the task-local stdout so that weird issues like a
+                // print!'d prompt not being shown until after the user hits
+                // enter.
+                flush();
+                tty.read(buf)
+            },
             File(ref mut file) => file.read(buf).map(|i| i as uint),
         };
         match ret {