Add Read::initializer.

This is an API that allows types to indicate that they can be passed
buffers of uninitialized memory which can improve performance.

src/libstd/io/util.rs

@@ -11,7 +11,8 @@
 #![allow(missing_copy_implementations)]
 
 use fmt;
-use io::{self, Read, Write, ErrorKind, BufRead};
+use io::{self, Read, Initializer, Write, ErrorKind, BufRead};
+use mem;
 
 /// Copies the entire contents of a reader into a writer.
 ///
@@ -47,7 +48,12 @@ use io::{self, Read, Write, ErrorKind, BufRead};
 pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> io::Result<u64>
     where R: Read, W: Write
 {
-    let mut buf = [0; super::DEFAULT_BUF_SIZE];
+    let mut buf = unsafe {
+        let mut buf: [u8; super::DEFAULT_BUF_SIZE] = mem::uninitialized();
+        reader.initializer().initialize(&mut buf);
+        buf
+    };
+
     let mut written = 0;
     loop {
         let len = match reader.read(&mut buf) {
@@ -90,11 +96,19 @@ pub fn empty() -> Empty { Empty { _priv: () } }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Read for Empty {
+    #[inline]
     fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> { Ok(0) }
+
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        Initializer::nop()
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl BufRead for Empty {
+    #[inline]
     fn fill_buf(&mut self) -> io::Result<&[u8]> { Ok(&[]) }
+    #[inline]
     fn consume(&mut self, _n: usize) {}
 }
 
@@ -133,12 +147,18 @@ pub fn repeat(byte: u8) -> Repeat { Repeat { byte: byte } }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Read for Repeat {
+    #[inline]
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
         for slot in &mut *buf {
             *slot = self.byte;
         }
         Ok(buf.len())
     }
+
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        Initializer::nop()
+    }
 }
 
 #[stable(feature = "std_debug", since = "1.16.0")]
@@ -176,7 +196,9 @@ pub fn sink() -> Sink { Sink { _priv: () } }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Write for Sink {
+    #[inline]
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> { Ok(buf.len()) }
+    #[inline]
     fn flush(&mut self) -> io::Result<()> { Ok(()) }
 }