diff --git a/src/libstd/io/buffered.rs b/src/libstd/io/buffered.rs
index ab9a837713630..5f3b88d520578 100644
--- a/src/libstd/io/buffered.rs
+++ b/src/libstd/io/buffered.rs
@@ -87,7 +87,7 @@ impl<R: Reader> BufferedReader<R> {
 }
 
 impl<R: Reader> Buffer for BufferedReader<R> {
-    fn fill<'a>(&'a mut self) -> IoResult<&'a [u8]> {
+    fn fill_buf<'a>(&'a mut self) -> IoResult<&'a [u8]> {
         if self.pos == self.cap {
             self.cap = try!(self.inner.read(self.buf));
             self.pos = 0;
@@ -104,7 +104,7 @@ impl<R: Reader> Buffer for BufferedReader<R> {
 impl<R: Reader> Reader for BufferedReader<R> {
     fn read(&mut self, buf: &mut [u8]) -> IoResult<uint> {
         let nread = {
-            let available = try!(self.fill());
+            let available = try!(self.fill_buf());
             let nread = cmp::min(available.len(), buf.len());
             slice::bytes::copy_memory(buf, available.slice_to(nread));
             nread
@@ -336,7 +336,7 @@ impl<S: Stream> BufferedStream<S> {
 }
 
 impl<S: Stream> Buffer for BufferedStream<S> {
-    fn fill<'a>(&'a mut self) -> IoResult<&'a [u8]> { self.inner.fill() }
+    fn fill_buf<'a>(&'a mut self) -> IoResult<&'a [u8]> { self.inner.fill_buf() }
     fn consume(&mut self, amt: uint) { self.inner.consume(amt) }
 }
 
diff --git a/src/libstd/io/mem.rs b/src/libstd/io/mem.rs
index d0c4ef308b317..7ae717cfccf97 100644
--- a/src/libstd/io/mem.rs
+++ b/src/libstd/io/mem.rs
@@ -190,7 +190,7 @@ impl Seek for MemReader {
 }
 
 impl Buffer for MemReader {
-    fn fill<'a>(&'a mut self) -> IoResult<&'a [u8]> {
+    fn fill_buf<'a>(&'a mut self) -> IoResult<&'a [u8]> {
         if self.pos < self.buf.len() {
             Ok(self.buf.slice_from(self.pos))
         } else {
@@ -322,7 +322,7 @@ impl<'a> Seek for BufReader<'a> {
 }
 
 impl<'a> Buffer for BufReader<'a> {
-    fn fill<'a>(&'a mut self) -> IoResult<&'a [u8]> {
+    fn fill_buf<'a>(&'a mut self) -> IoResult<&'a [u8]> {
         if self.pos < self.buf.len() {
             Ok(self.buf.slice_from(self.pos))
         } else {
@@ -555,4 +555,18 @@ mod test {
         let mut r = BufWriter::new(buf);
         assert!(r.seek(-1, SeekSet).is_err());
     }
+
+    #[test]
+    fn io_fill() {
+        let mut r = MemReader::new(~[1, 2, 3, 4, 5, 6, 7, 8]);
+        let mut buf = [0, ..3];
+        assert_eq!(r.fill(buf), Ok(()));
+        assert_eq!(buf.as_slice(), &[1, 2, 3]);
+        assert_eq!(r.fill(buf.mut_slice_to(0)), Ok(()));
+        assert_eq!(buf.as_slice(), &[1, 2, 3]);
+        assert_eq!(r.fill(buf), Ok(()));
+        assert_eq!(buf.as_slice(), &[4, 5, 6]);
+        assert!(r.fill(buf).is_err());
+        assert_eq!(buf.as_slice(), &[7, 8, 6]);
+    }
 }
diff --git a/src/libstd/io/mod.rs b/src/libstd/io/mod.rs
index 88ffc363e8875..6bd8f119ba29b 100644
--- a/src/libstd/io/mod.rs
+++ b/src/libstd/io/mod.rs
@@ -360,6 +360,23 @@ pub trait Reader {
         }
     }
 
+    /// Fills the provided slice with bytes from this reader
+    ///
+    /// This will continue to call `read` until the slice has been completely
+    /// filled with bytes.
+    ///
+    /// # Error
+    ///
+    /// If an error occurs at any point, that error is returned, and no further
+    /// bytes are read.
+    fn fill(&mut self, buf: &mut [u8]) -> IoResult<()> {
+        let mut read = 0;
+        while read < buf.len() {
+            read += try!(self.read(buf.mut_slice_from(read)));
+        }
+        Ok(())
+    }
+
     /// Reads exactly `len` bytes and appends them to a vector.
     ///
     /// May push fewer than the requested number of bytes on error
@@ -1045,7 +1062,7 @@ pub trait Buffer: Reader {
     /// This function will return an I/O error if the underlying reader was
     /// read, but returned an error. Note that it is not an error to return a
     /// 0-length buffer.
-    fn fill<'a>(&'a mut self) -> IoResult<&'a [u8]>;
+    fn fill_buf<'a>(&'a mut self) -> IoResult<&'a [u8]>;
 
     /// Tells this buffer that `amt` bytes have been consumed from the buffer,
     /// so they should no longer be returned in calls to `fill` or `read`.
@@ -1116,7 +1133,7 @@ pub trait Buffer: Reader {
         let mut used;
         loop {
             {
-                let available = match self.fill() {
+                let available = match self.fill_buf() {
                     Ok(n) => n,
                     Err(ref e) if res.len() > 0 && e.kind == EndOfFile => {
                         used = 0;