Add specializations of read_to_end for Stdin, TcpStream and File,

allowing them to read into a buffer containing uninitialized data,
rather than pay the cost of zeroing.

src/libstd/fs.rs

@@ -25,6 +25,7 @@ use io::{self, SeekFrom, Seek, Read, Write};
 use path::{Path, PathBuf};
 use sys::fs as fs_imp;
 use sys_common::{AsInnerMut, FromInner, AsInner};
+use sys_common::io::read_to_end_uninitialized;
 use vec::Vec;
 
 /// A reference to an open file on the filesystem.
@@ -328,6 +329,9 @@ impl Read for File {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
         self.inner.read(buf)
     }
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        unsafe { read_to_end_uninitialized(self, buf) }
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Write for File {