Fix fallout in libsyntax from RFC 599. Clarity and efficiency seems to be...

Fix fallout in libsyntax from RFC 599. Clarity and efficiency seems to be mostly improved, to my eye.
Nonetheless, as this commit demonstrates, the previous commits was a [breaking-change].

In practice, breakage is focused on functions of this form:

```rust
fn foo(..., object: Box<FnMut()>)
````

where `FnMut()` could be any trait object type. The older scheme defaulted objects in argument
position so that they were bounded by a fresh lifetime:

```rust
fn foo<'a>(..., object: Box<FnMut()+'a>)
```

This meant that the object could contain borrowed data. The newer
scheme defaults to a lifetime bound of `'static`:

```rust
fn foo(..., object: Box<FnMut()+'static>)
```

This means that the object cannot contain borrowed data. In some cases, the best fix
is to stop using `Box`:

```rust
fn foo(..., object: &mut FnMut())
```

but another option is to write an explicit annotation for the `'a`
lifetime that used to be implicit.  Both fixes are demonstrated in
this commit.

src/libsyntax/ext/base.rs

@@ -35,18 +35,18 @@ fn expand(&self,
               sp: Span,
               meta_item: &ast::MetaItem,
               item: &ast::Item,
-              push: Box<FnMut(P<ast::Item>)>);
+              push: &mut FnMut(P<ast::Item>));
 }
 
 impl<F> ItemDecorator for F
-    where F : Fn(&mut ExtCtxt, Span, &ast::MetaItem, &ast::Item, Box<FnMut(P<ast::Item>)>)
+    where F : Fn(&mut ExtCtxt, Span, &ast::MetaItem, &ast::Item, &mut FnMut(P<ast::Item>))
 {
     fn expand(&self,
               ecx: &mut ExtCtxt,
               sp: Span,
               meta_item: &ast::MetaItem,
               item: &ast::Item,
-              push: Box<FnMut(P<ast::Item>)>) {
+              push: &mut FnMut(P<ast::Item>)) {
         (*self)(ecx, sp, meta_item, item, push)
     }
 }

src/libsyntax/ext/deriving/mod.rs

@@ -72,7 +72,7 @@ pub fn expand_deprecated_deriving(cx: &mut ExtCtxt,
                                   span: Span,
                                   _: &MetaItem,
                                   _: &Item,
-                                  _: Box<FnMut(P<Item>)>) {
+                                  _: &mut FnMut(P<Item>)) {
     cx.span_err(span, "`deriving` has been renamed to `derive`");
 }
 
@@ -80,7 +80,7 @@ pub fn expand_meta_derive(cx: &mut ExtCtxt,
                           _span: Span,
                           mitem: &MetaItem,
                           item: &Item,
-                          mut push: Box<FnMut(P<Item>)>) {
+                          push: &mut FnMut(P<Item>)) {
     match mitem.node {
         MetaNameValue(_, ref l) => {
             cx.span_err(l.span, "unexpected value in `derive`");

src/libsyntax/ext/expand.rs

@@ -363,7 +363,7 @@ fn expand_mac_invoc<T, F, G>(mac: ast::Mac, span: codemap::Span,
                              mark_thunk: G,
                              fld: &mut MacroExpander)
                              -> Option<T> where
-    F: FnOnce(Box<MacResult>) -> Option<T>,
+    F: for<'a> FnOnce(Box<MacResult+'a>) -> Option<T>,
     G: FnOnce(T, Mrk) -> T,
 {
     match mac.node {
@@ -1102,9 +1102,10 @@ fn expand_annotatable(a: Annotatable,
                     // but that double-mut-borrows fld
                     let mut items: SmallVector<P<ast::Item>> = SmallVector::zero();
                     dec.expand(fld.cx, attr.span, &*attr.node.value, &**it,
-                               box |item| items.push(item));
-                    decorator_items.extend(items.into_iter()
-                        .flat_map(|item| expand_item(item, fld).into_iter()));
+                               &mut |item| items.push(item));
+                    decorator_items.extend(
+                        items.into_iter()
+                             .flat_map(|item| expand_item(item, fld).into_iter()));
 
                     fld.cx.bt_pop();
                 }