diff --git a/src/libstd/fs.rs b/src/libstd/fs.rs
index b07733d3c803c..563cce598fe80 100644
--- a/src/libstd/fs.rs
+++ b/src/libstd/fs.rs
@@ -449,6 +449,11 @@ impl Read for File {
         self.inner.read(buf)
     }
 
+    fn size_hint(&self) -> io::Result<usize> {
+        let position = self.inner.seek(SeekFrom::Current(0))?;
+        Ok(self.metadata()?.len().saturating_sub(position) as usize)
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
@@ -473,6 +478,10 @@ impl<'a> Read for &'a File {
         self.inner.read(buf)
     }
 
+    fn size_hint(&self) -> io::Result<usize> {
+        (**self).size_hint()
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
diff --git a/src/libstd/io/buffered.rs b/src/libstd/io/buffered.rs
index 6d3fbc9d26822..1f8113575b8e7 100644
--- a/src/libstd/io/buffered.rs
+++ b/src/libstd/io/buffered.rs
@@ -211,6 +211,12 @@ impl<R: Read> Read for BufReader<R> {
         Ok(nread)
     }
 
+    #[inline]
+    fn size_hint(&self) -> io::Result<usize> {
+        let buffered_len = self.cap - self.pos;
+        Ok(buffered_len.saturating_add(self.inner.size_hint()?))
+    }
+
     // we can't skip unconditionally because of the large buffer case in read.
     unsafe fn initializer(&self) -> Initializer {
         self.inner.initializer()
diff --git a/src/libstd/io/cursor.rs b/src/libstd/io/cursor.rs
index 32a92145aafed..1bb00d6e25061 100644
--- a/src/libstd/io/cursor.rs
+++ b/src/libstd/io/cursor.rs
@@ -230,6 +230,10 @@ impl<T> Read for Cursor<T> where T: AsRef<[u8]> {
         Ok(n)
     }
 
+    fn size_hint(&self) -> io::Result<usize> {
+        Ok((self.inner.as_ref().len() as u64).saturating_sub(self.pos) as usize)
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
diff --git a/src/libstd/io/impls.rs b/src/libstd/io/impls.rs
index fe1179a3b4a18..ce4af0db395aa 100644
--- a/src/libstd/io/impls.rs
+++ b/src/libstd/io/impls.rs
@@ -23,6 +23,11 @@ impl<'a, R: Read + ?Sized> Read for &'a mut R {
         (**self).read(buf)
     }
 
+    #[inline]
+    fn size_hint(&self) -> io::Result<usize> {
+        (**self).size_hint()
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         (**self).initializer()
@@ -92,6 +97,11 @@ impl<R: Read + ?Sized> Read for Box<R> {
         (**self).read(buf)
     }
 
+    #[inline]
+    fn size_hint(&self) -> io::Result<usize> {
+        (**self).size_hint()
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         (**self).initializer()
@@ -181,6 +191,11 @@ impl<'a> Read for &'a [u8] {
         Ok(amt)
     }
 
+    #[inline]
+    fn size_hint(&self) -> io::Result<usize> {
+        Ok(self.len())
+    }
+
     #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
diff --git a/src/libstd/io/mod.rs b/src/libstd/io/mod.rs
index b7a3695b47096..e43f49fe337ba 100644
--- a/src/libstd/io/mod.rs
+++ b/src/libstd/io/mod.rs
@@ -366,7 +366,17 @@ fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
 fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
     let start_len = buf.len();
     let mut g = Guard { len: buf.len(), buf: buf };
-    let ret;
+
+    let size_hint = r.size_hint()?;
+    if size_hint > 0 {
+        unsafe {
+            g.buf.reserve(size_hint.saturating_add(1));
+            let capacity = g.buf.capacity();
+            g.buf.set_len(capacity);
+            r.initializer().initialize(&mut g.buf[g.len..]);
+        }
+    }
+
     loop {
         if g.len == g.buf.len() {
             unsafe {
@@ -378,20 +388,12 @@ fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize>
         }
 
         match r.read(&mut g.buf[g.len..]) {
-            Ok(0) => {
-                ret = Ok(g.len - start_len);
-                break;
-            }
+            Ok(0) => return Ok(g.len - start_len),
             Ok(n) => g.len += n,
             Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
-            Err(e) => {
-                ret = Err(e);
-                break;
-            }
+            Err(e) => return Err(e),
         }
     }
-
-    ret
 }
 
 /// The `Read` trait allows for reading bytes from a source.
@@ -553,6 +555,19 @@ pub trait Read {
         Initializer::zeroing()
     }
 
+    /// Return an estimate of how many bytes would be read from this source until EOF,
+    /// or zero if that is unknown.
+    ///
+    /// This is used by [`read_to_end`] and [`read_to_string`] to pre-allocate a memory buffer.
+    ///
+    /// [`read_to_end`]: #method.read_to_end
+    /// [`read_to_string`]: #method.read_to_string
+    #[unstable(feature = "read_size_hint", issue = /* FIXME */ "0")]
+    #[inline]
+    fn size_hint(&self) -> Result<usize> {
+        Ok(0)
+    }
+
     /// Read all bytes until EOF in this source, placing them into `buf`.
     ///
     /// All bytes read from this source will be appended to the specified buffer
@@ -1729,6 +1744,14 @@ impl<T: Read, U: Read> Read for Chain<T, U> {
         self.second.read(buf)
     }
 
+    fn size_hint(&self) -> Result<usize> {
+        if self.done_first {
+            self.second.size_hint()
+        } else {
+            Ok(self.first.size_hint()?.saturating_add(self.second.size_hint()?))
+        }
+    }
+
     unsafe fn initializer(&self) -> Initializer {
         let initializer = self.first.initializer();
         if initializer.should_initialize() {
@@ -1927,6 +1950,14 @@ impl<T: Read> Read for Take<T> {
         Ok(n)
     }
 
+    fn size_hint(&self) -> Result<usize> {
+        if self.limit == 0 {
+            Ok(0)
+        } else {
+            Ok(cmp::min(self.limit, self.inner.size_hint()? as u64) as usize)
+        }
+    }
+
     unsafe fn initializer(&self) -> Initializer {
         self.inner.initializer()
     }