auto merge of #18108 : mahkoh/rust/buffered_reader, r=alexcrichton

This optimizes `read` for the case in which the number of bytes
requested is larger than the internal buffer. Note that the first
comparison occurs again right afterwards and should thus be free. The
second comparison occurs only in the cold branch.

Author: bors

src/libstd/io/buffered.rs

@@ -104,6 +104,9 @@ impl<R: Reader> Buffer for BufferedReader<R> {
 
 impl<R: Reader> Reader for BufferedReader<R> {
     fn read(&mut self, buf: &mut [u8]) -> IoResult<uint> {
+        if self.pos == self.cap && buf.len() >= self.buf.capacity() {
+            return self.inner.read(buf);
+        }
         let nread = {
             let available = try!(self.fill_buf());
             let nread = cmp::min(available.len(), buf.len());
@@ -409,13 +412,19 @@ mod test {
 
     #[test]
     fn test_buffered_reader() {
-        let inner = MemReader::new(vec!(0, 1, 2, 3, 4));
+        let inner = MemReader::new(vec!(5, 6, 7, 0, 1, 2, 3, 4));
         let mut reader = BufferedReader::with_capacity(2, inner);
 
         let mut buf = [0, 0, 0];
         let nread = reader.read(buf);
+        assert_eq!(Ok(3), nread);
+        let b: &[_] = &[5, 6, 7];
+        assert_eq!(buf.as_slice(), b);
+
+        let mut buf = [0, 0];
+        let nread = reader.read(buf);
         assert_eq!(Ok(2), nread);
-        let b: &[_] = &[0, 1, 0];
+        let b: &[_] = &[0, 1];
         assert_eq!(buf.as_slice(), b);
 
         let mut buf = [0];