Don't pass in a vector to Encoder::new.

It's not necessary.

compiler/rustc_serialize/src/opaque.rs

@@ -18,8 +18,8 @@ pub struct Encoder {
 }
 
 impl Encoder {
-    pub fn new(data: Vec<u8>) -> Encoder {
-        Encoder { data }
+    pub fn new() -> Encoder {
+        Encoder { data: vec![] }
     }
 
     pub fn into_inner(self) -> Vec<u8> {

compiler/rustc_serialize/tests/opaque.rs

@@ -31,7 +31,7 @@ struct Struct {
 fn check_round_trip<T: Encodable<Encoder> + for<'a> Decodable<Decoder<'a>> + PartialEq + Debug>(
     values: Vec<T>,
 ) {
-    let mut encoder = Encoder::new(Vec::new());
+    let mut encoder = Encoder::new();
 
     for value in &values {
         Encodable::encode(value, &mut encoder).unwrap();