Implement read_exact for the Read trait

This implements the proposed "read_exact" RFC
(https://github.com/rust-lang/rfcs/pull/980).

src/libstd/io/impls.rs

@@ -38,6 +38,11 @@ impl<'a, R: Read + ?Sized> Read for &'a mut R {
     fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
         (**self).read_to_string(buf)
     }
+
+    #[inline]
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        (**self).read_exact(buf)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, W: Write + ?Sized> Write for &'a mut W {
@@ -97,6 +102,11 @@ impl<R: Read + ?Sized> Read for Box<R> {
     fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
         (**self).read_to_string(buf)
     }
+
+    #[inline]
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        (**self).read_exact(buf)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<W: Write + ?Sized> Write for Box<W> {
@@ -153,6 +163,17 @@ impl<'a> Read for &'a [u8] {
         *self = b;
         Ok(amt)
     }
+
+    #[inline]
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        if buf.len() > self.len() {
+            return Err(Error::new(ErrorKind::UnexpectedEOF, "failed to fill whole buffer"));
+        }
+        let (a, b) = self.split_at(buf.len());
+        slice::bytes::copy_memory(a, buf);
+        *self = b;
+        Ok(())
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]