Address review comments.

compiler/rustc_serialize/src/json.rs

@@ -2296,7 +2296,6 @@ impl crate::Decoder for Decoder {
         for c in s.iter_mut() {
             *c = self.read_u8();
         }
-        ()
     }
 
     fn read_enum<T, F>(&mut self, f: F) -> T

compiler/rustc_serialize/src/opaque.rs

@@ -676,11 +676,10 @@ impl<'a> serialize::Decoder for Decoder<'a> {
     }
 
     #[inline]
-    fn read_raw_bytes_into(&mut self, s: &mut [u8]) -> () {
+    fn read_raw_bytes_into(&mut self, s: &mut [u8]) {
         let start = self.position;
         self.position += s.len();
         s.copy_from_slice(&self.data[start..self.position]);
-        ()
     }
 }