Add vectored read and write support

This functionality has lived for a while in the tokio ecosystem, where
it can improve performance by minimizing copies.

src/libstd/io/impls.rs

@@ -1,5 +1,6 @@
 use cmp;
-use io::{self, SeekFrom, Read, Initializer, Write, Seek, BufRead, Error, ErrorKind};
+use io::{self, SeekFrom, Read, Initializer, Write, Seek, BufRead, Error, ErrorKind, IoVecMut,
+         IoVec};
 use fmt;
 use mem;
 
@@ -13,6 +14,11 @@ impl<'a, R: Read + ?Sized> Read for &'a mut R {
         (**self).read(buf)
     }
 
+    #[inline]
+    fn read_vectored(&mut self, bufs: &mut [IoVecMut<'_>]) -> io::Result<usize> {
+        (**self).read_vectored(bufs)
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         (**self).initializer()
@@ -38,6 +44,11 @@ impl<'a, W: Write + ?Sized> Write for &'a mut W {
     #[inline]
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> { (**self).write(buf) }
 
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoVec<'_>]) -> io::Result<usize> {
+        (**self).write_vectored(bufs)
+    }
+
     #[inline]
     fn flush(&mut self) -> io::Result<()> { (**self).flush() }
 
@@ -82,6 +93,11 @@ impl<R: Read + ?Sized> Read for Box<R> {
         (**self).read(buf)
     }
 
+    #[inline]
+    fn read_vectored(&mut self, bufs: &mut [IoVecMut<'_>]) -> io::Result<usize> {
+        (**self).read_vectored(bufs)
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         (**self).initializer()
@@ -107,6 +123,11 @@ impl<W: Write + ?Sized> Write for Box<W> {
     #[inline]
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> { (**self).write(buf) }
 
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoVec<'_>]) -> io::Result<usize> {
+        (**self).write_vectored(bufs)
+    }
+
     #[inline]
     fn flush(&mut self) -> io::Result<()> { (**self).flush() }
 
@@ -171,6 +192,19 @@ impl<'a> Read for &'a [u8] {
         Ok(amt)
     }
 
+    #[inline]
+    fn read_vectored(&mut self, bufs: &mut [IoVecMut<'_>]) -> io::Result<usize> {
+        let mut nread = 0;
+        for buf in bufs {
+            nread += self.read(buf.as_mut_slice())?;
+            if self.is_empty() {
+                break;
+            }
+        }
+
+        Ok(nread)
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
@@ -231,6 +265,19 @@ impl<'a> Write for &'a mut [u8] {
         Ok(amt)
     }
 
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoVec<'_>]) -> io::Result<usize> {
+        let mut nwritten = 0;
+        for buf in bufs {
+            nwritten += self.write(buf.as_slice())?;
+            if self.is_empty() {
+                break;
+            }
+        }
+
+        Ok(nwritten)
+    }
+
     #[inline]
     fn write_all(&mut self, data: &[u8]) -> io::Result<()> {
         if self.write(data)? == data.len() {
@@ -254,6 +301,16 @@ impl Write for Vec<u8> {
         Ok(buf.len())
     }
 
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoVec<'_>]) -> io::Result<usize> {
+        let len = bufs.iter().map(|b| b.as_slice().len()).sum();
+        self.reserve(len);
+        for buf in bufs {
+            self.extend_from_slice(buf.as_slice());
+        }
+        Ok(len)
+    }
+
     #[inline]
     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
         self.extend_from_slice(buf);