Change ItemModifier and ItemDecorator to traits

For convenience, the traits are implemented for the respective bare
functions. Change code from this:

```rust
ItemDecorator(some_function)
// or
ItemModifier(some_other_function)
```
to
```rust
ItemDecorator(box some_function)
// or
ItemModifier(box some_other_function)
```

[breaking-change]

src/libsyntax/ext/base.rs

@@ -39,11 +39,45 @@ pub struct MacroDef {
     pub ext: SyntaxExtension
 }
 
-pub type ItemDecorator =
-    fn(&mut ExtCtxt, Span, Gc<ast::MetaItem>, Gc<ast::Item>, |Gc<ast::Item>|);
+pub trait ItemDecorator {
+    fn expand(&self,
+              ecx: &mut ExtCtxt,
+              sp: Span,
+              meta_item: Gc<ast::MetaItem>,
+              item: Gc<ast::Item>,
+              push: |Gc<ast::Item>|);
+}
+
+impl ItemDecorator for fn(&mut ExtCtxt, Span, Gc<ast::MetaItem>, Gc<ast::Item>, |Gc<ast::Item>|) {
+    fn expand(&self,
+              ecx: &mut ExtCtxt,
+              sp: Span,
+              meta_item: Gc<ast::MetaItem>,
+              item: Gc<ast::Item>,
+              push: |Gc<ast::Item>|) {
+        (*self)(ecx, sp, meta_item, item, push)
+    }
+}
 
-pub type ItemModifier =
-    fn(&mut ExtCtxt, Span, Gc<ast::MetaItem>, Gc<ast::Item>) -> Gc<ast::Item>;
+pub trait ItemModifier {
+    fn expand(&self,
+              ecx: &mut ExtCtxt,
+              span: Span,
+              meta_item: Gc<ast::MetaItem>,
+              item: Gc<ast::Item>)
+              -> Gc<ast::Item>;
+}
+
+impl ItemModifier for fn(&mut ExtCtxt, Span, Gc<ast::MetaItem>, Gc<ast::Item>) -> Gc<ast::Item> {
+    fn expand(&self,
+              ecx: &mut ExtCtxt,
+              span: Span,
+              meta_item: Gc<ast::MetaItem>,
+              item: Gc<ast::Item>)
+              -> Gc<ast::Item> {
+        (*self)(ecx, span, meta_item, item)
+    }
+}
 
 pub struct BasicMacroExpander {
     pub expander: MacroExpanderFn,
@@ -281,11 +315,11 @@ pub enum SyntaxExtension {
     /// based upon it.
     ///
     /// `#[deriving(...)]` is an `ItemDecorator`.
-    ItemDecorator(ItemDecorator),
+    ItemDecorator(Box<ItemDecorator + 'static>),
 
     /// A syntax extension that is attached to an item and modifies it
     /// in-place.
-    ItemModifier(ItemModifier),
+    ItemModifier(Box<ItemModifier + 'static>),
 
     /// A normal, function-like syntax extension.
     ///
@@ -371,7 +405,7 @@ fn initial_syntax_expander_table() -> SyntaxEnv {
                             builtin_normal_expander(
                                     ext::log_syntax::expand_syntax_ext));
     syntax_expanders.insert(intern("deriving"),
-                            ItemDecorator(ext::deriving::expand_meta_deriving));
+                            ItemDecorator(box ext::deriving::expand_meta_deriving));
 
     // Quasi-quoting expanders
     syntax_expanders.insert(intern("quote_tokens"),

src/libsyntax/ext/expand.rs

@@ -251,7 +251,7 @@ fn expand_item(it: Gc<ast::Item>, fld: &mut MacroExpander)
 
         match fld.cx.syntax_env.find(&intern(mname.get())) {
             Some(rc) => match *rc {
-                ItemDecorator(dec_fn) => {
+                ItemDecorator(ref dec) => {
                     attr::mark_used(attr);
 
                     fld.cx.bt_push(ExpnInfo {
@@ -266,8 +266,7 @@ fn expand_item(it: Gc<ast::Item>, fld: &mut MacroExpander)
                     // we'd ideally decorator_items.push_all(expand_item(item, fld)),
                     // but that double-mut-borrows fld
                     let mut items: SmallVector<Gc<ast::Item>> = SmallVector::zero();
-                    dec_fn(fld.cx, attr.span, attr.node.value, it,
-                        |item| items.push(item));
+                    dec.expand(fld.cx, attr.span, attr.node.value, it, |item| items.push(item));
                     decorator_items.extend(items.move_iter()
                         .flat_map(|item| expand_item(item, fld).move_iter()));
 
@@ -328,7 +327,7 @@ fn expand_item_modifiers(mut it: Gc<ast::Item>, fld: &mut MacroExpander)
 
         match fld.cx.syntax_env.find(&intern(mname.get())) {
             Some(rc) => match *rc {
-                ItemModifier(dec_fn) => {
+                ItemModifier(ref mac) => {
                     attr::mark_used(attr);
                     fld.cx.bt_push(ExpnInfo {
                         call_site: attr.span,
@@ -338,7 +337,7 @@ fn expand_item_modifiers(mut it: Gc<ast::Item>, fld: &mut MacroExpander)
                             span: None,
                         }
                     });
-                    it = dec_fn(fld.cx, attr.span, attr.node.value, it);
+                    it = mac.expand(fld.cx, attr.span, attr.node.value, it);
                     fld.cx.bt_pop();
                 }
                 _ => unreachable!()

src/test/auxiliary/macro_crate_test.rs

@@ -36,7 +36,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_macro("identity", expand_identity);
     reg.register_syntax_extension(
         token::intern("into_foo"),
-        ItemModifier(expand_into_foo));
+        ItemModifier(box expand_into_foo));
 }
 
 fn expand_make_a_1(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree])