Only zero at most 64k at a time. We still use the doubling

reallocation strategy since extend() calls reserve() and/or
push() for us.

src/libstd/io/mod.rs

@@ -101,18 +101,14 @@ 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 len = start_len;
-    let min_cap_bump = 16;
+    let mut new_write_size = 16;
     let ret;
     loop {
         if len == buf.len() {
-            if buf.capacity() == buf.len() {
-                // reserve() rounds up our request such that every request
-                // (with maybe the exception of the first request) for the
-                // same amount of space doubles our capacity.
-                buf.reserve(min_cap_bump);
+            if new_write_size < DEFAULT_BUF_SIZE {
+                new_write_size *= 2;
             }
-            let new_area = buf.capacity() - buf.len();
-            buf.extend(iter::repeat(0).take(new_area));
+            buf.extend(iter::repeat(0).take(new_write_size));
         }
 
         match r.read(&mut buf[len..]) {