Rollup merge of #25710 - Manishearth:no_decorator_clone, r=sfackler

fixes #25683

I have a very nonscientific measurement of the data via valgrind/massif [here](https://gist.github.com/Manishearth/4c47f15f6835cb3957c4)

I measured the memory usage for both --pretty=expanded and -Z no-trans

It *seems* like there's a 20-25MB decrease during expansion on stage2 librustc; but I'm not quite sure.

r? @eddyb

(have not yet run tests, but it compiles fine, might want to wait before giving r+)

cc @nrc @huon

src/test/auxiliary/custom_derive_plugin.rs

@@ -37,7 +37,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
 fn expand(cx: &mut ExtCtxt,
           span: Span,
           mitem: &ast::MetaItem,
-          item: Annotatable,
+          item: &Annotatable,
           push: &mut FnMut(Annotatable)) {
     let trait_def = TraitDef {
         span: span,
@@ -71,5 +71,5 @@ fn expand(cx: &mut ExtCtxt,
         ],
     };
 
-    trait_def.expand(cx, mitem, &item, push)
+    trait_def.expand(cx, mitem, item, push)
 }

src/test/auxiliary/custom_derive_plugin_attr.rs

@@ -39,7 +39,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
 fn expand(cx: &mut ExtCtxt,
           span: Span,
           mitem: &ast::MetaItem,
-          item: Annotatable,
+          item: &Annotatable,
           push: &mut FnMut(Annotatable)) {
     let trait_def = TraitDef {
         span: span,
@@ -62,7 +62,7 @@ fn expand(cx: &mut ExtCtxt,
         ],
     };
 
-    trait_def.expand(cx, mitem, &item, push)
+    trait_def.expand(cx, mitem, item, push)
 }
 
 // Mostly copied from syntax::ext::deriving::hash

src/test/auxiliary/macro_crate_test.rs

@@ -108,7 +108,7 @@ fn expand_into_foo_multi(cx: &mut ExtCtxt,
 fn expand_duplicate(cx: &mut ExtCtxt,
                     sp: Span,
                     mi: &MetaItem,
-                    it: Annotatable,
+                    it: &Annotatable,
                     push: &mut FnMut(Annotatable))
 {
     let copy_name = match mi.node {