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 @@ pub trait ItemDecorator {
               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();
                 }