Silence unused variable warning for EncoderWriter.

src/writer.rs

@@ -9,14 +9,14 @@ type EwResult = IoResult<()>;
 
 pub struct EncoderWriter<'a, W: 'a> {
     writer: &'a mut W,
-    size_limit: SizeLimit
+    _size_limit: SizeLimit
 }
 
 impl <'a, W: Writer> EncoderWriter<'a, W> {
     pub fn new(w: &'a mut W, size_limit: SizeLimit) -> EncoderWriter<'a, W> {
         EncoderWriter {
             writer: w,
-            size_limit: size_limit
+            _size_limit: size_limit
         }
     }
 }