Correct one case where the inference was detecting a looser result than the

explicit annotation, leading to "extra `mut` declaration" lint errors.

Author: nikomatsakis

src/libsyntax/ext/deriving/rand.rs

@@ -66,7 +66,7 @@ fn rand_substructure(cx: &mut ExtCtxt, trait_span: Span, substr: &Substructure)
         cx.ident_of("Rand"),
         cx.ident_of("rand")
     );
-    let mut rand_call = |&mut: cx: &mut ExtCtxt, span| {
+    let rand_call = |&: cx: &mut ExtCtxt, span| {
         cx.expr_call_global(span,
                             rand_ident.clone(),
                             vec!(rng.clone()))