diff --git a/src/librustc/front/assign_node_ids.rs b/src/librustc/front/assign_node_ids.rs
index 3d0b32fcee0..74e8e0de1d8 100644
--- a/src/librustc/front/assign_node_ids.rs
+++ b/src/librustc/front/assign_node_ids.rs
@@ -18,14 +18,14 @@ struct NodeIdAssigner {
 }
 
 impl ast_fold for NodeIdAssigner {
-    fn new_id(&self, old_id: ast::NodeId) -> ast::NodeId {
+    fn new_id(&mut self, old_id: ast::NodeId) -> ast::NodeId {
         assert_eq!(old_id, ast::DUMMY_NODE_ID);
         self.sess.next_node_id()
     }
 }
 
 pub fn assign_node_ids(sess: Session, crate: ast::Crate) -> ast::Crate {
-    let fold = NodeIdAssigner {
+    let mut fold = NodeIdAssigner {
         sess: sess,
     };
     fold.fold_crate(crate)
diff --git a/src/librustc/front/config.rs b/src/librustc/front/config.rs
index 6694bee582f..58e7963ca90 100644
--- a/src/librustc/front/config.rs
+++ b/src/librustc/front/config.rs
@@ -24,17 +24,17 @@ pub fn strip_unconfigured_items(crate: ast::Crate) -> ast::Crate {
 }
 
 impl<'a> fold::ast_fold for Context<'a> {
-    fn fold_mod(&self, module: &ast::_mod) -> ast::_mod {
+    fn fold_mod(&mut self, module: &ast::_mod) -> ast::_mod {
         fold_mod(self, module)
     }
-    fn fold_block(&self, block: ast::P<ast::Block>) -> ast::P<ast::Block> {
+    fn fold_block(&mut self, block: ast::P<ast::Block>) -> ast::P<ast::Block> {
         fold_block(self, block)
     }
-    fn fold_foreign_mod(&self, foreign_module: &ast::foreign_mod)
+    fn fold_foreign_mod(&mut self, foreign_module: &ast::foreign_mod)
                         -> ast::foreign_mod {
         fold_foreign_mod(self, foreign_module)
     }
-    fn fold_item_underscore(&self, item: &ast::item_) -> ast::item_ {
+    fn fold_item_underscore(&mut self, item: &ast::item_) -> ast::item_ {
         fold_item_underscore(self, item)
     }
 }
@@ -42,7 +42,7 @@ impl<'a> fold::ast_fold for Context<'a> {
 pub fn strip_items(crate: ast::Crate,
                    in_cfg: |attrs: &[ast::Attribute]| -> bool)
                    -> ast::Crate {
-    let ctxt = Context {
+    let mut ctxt = Context {
         in_cfg: in_cfg,
     };
     ctxt.fold_crate(crate)
@@ -57,7 +57,7 @@ fn filter_view_item<'r>(cx: &Context, view_item: &'r ast::view_item)
     }
 }
 
-fn fold_mod(cx: &Context, m: &ast::_mod) -> ast::_mod {
+fn fold_mod(cx: &mut Context, m: &ast::_mod) -> ast::_mod {
     let filtered_items = m.items.iter()
             .filter(|&a| item_in_cfg(cx, *a))
             .flat_map(|&x| cx.fold_item(x).move_iter())
@@ -80,7 +80,7 @@ fn filter_foreign_item(cx: &Context, item: @ast::foreign_item)
     }
 }
 
-fn fold_foreign_mod(cx: &Context, nm: &ast::foreign_mod) -> ast::foreign_mod {
+fn fold_foreign_mod(cx: &mut Context, nm: &ast::foreign_mod) -> ast::foreign_mod {
     let filtered_items = nm.items
                            .iter()
                            .filter_map(|a| filter_foreign_item(cx, *a))
@@ -95,7 +95,7 @@ fn fold_foreign_mod(cx: &Context, nm: &ast::foreign_mod) -> ast::foreign_mod {
     }
 }
 
-fn fold_item_underscore(cx: &Context, item: &ast::item_) -> ast::item_ {
+fn fold_item_underscore(cx: &mut Context, item: &ast::item_) -> ast::item_ {
     let item = match *item {
         ast::item_impl(ref a, ref b, c, ref methods) => {
             let methods = methods.iter().filter(|m| method_in_cfg(cx, **m))
@@ -129,7 +129,7 @@ fn retain_stmt(cx: &Context, stmt: @ast::Stmt) -> bool {
     }
 }
 
-fn fold_block(cx: &Context, b: ast::P<ast::Block>) -> ast::P<ast::Block> {
+fn fold_block(cx: &mut Context, b: ast::P<ast::Block>) -> ast::P<ast::Block> {
     let resulting_stmts = b.stmts.iter()
             .filter(|&a| retain_stmt(cx, *a))
             .flat_map(|&stmt| cx.fold_stmt(stmt).move_iter())
diff --git a/src/librustc/front/std_inject.rs b/src/librustc/front/std_inject.rs
index 03f9a3459a3..13bf867e06b 100644
--- a/src/librustc/front/std_inject.rs
+++ b/src/librustc/front/std_inject.rs
@@ -56,7 +56,7 @@ struct StandardLibraryInjector {
 }
 
 impl fold::ast_fold for StandardLibraryInjector {
-    fn fold_crate(&self, crate: ast::Crate) -> ast::Crate {
+    fn fold_crate(&mut self, crate: ast::Crate) -> ast::Crate {
         let version = STD_VERSION.to_managed();
         let vers_item = attr::mk_name_value_item_str(@"vers", version);
         let mut vis = ~[ast::view_item {
@@ -108,7 +108,7 @@ impl fold::ast_fold for StandardLibraryInjector {
         }
     }
 
-    fn fold_item(&self, item: @ast::item) -> SmallVector<@ast::item> {
+    fn fold_item(&mut self, item: @ast::item) -> SmallVector<@ast::item> {
         if !no_prelude(item.attrs) {
             // only recur if there wasn't `#[no_implicit_prelude];`
             // on this item, i.e. this means that the prelude is not
@@ -119,7 +119,7 @@ impl fold::ast_fold for StandardLibraryInjector {
         }
     }
 
-    fn fold_mod(&self, module: &ast::_mod) -> ast::_mod {
+    fn fold_mod(&mut self, module: &ast::_mod) -> ast::_mod {
         let prelude_path = ast::Path {
             span: dummy_sp(),
             global: false,
@@ -158,7 +158,7 @@ impl fold::ast_fold for StandardLibraryInjector {
 }
 
 fn inject_libstd_ref(sess: Session, crate: ast::Crate) -> ast::Crate {
-    let fold = StandardLibraryInjector {
+    let mut fold = StandardLibraryInjector {
         sess: sess,
     };
     fold.fold_crate(crate)
diff --git a/src/librustc/front/test.rs b/src/librustc/front/test.rs
index ed899561bc9..764935d5305 100644
--- a/src/librustc/front/test.rs
+++ b/src/librustc/front/test.rs
@@ -67,7 +67,7 @@ struct TestHarnessGenerator {
 }
 
 impl fold::ast_fold for TestHarnessGenerator {
-    fn fold_crate(&self, c: ast::Crate) -> ast::Crate {
+    fn fold_crate(&mut self, c: ast::Crate) -> ast::Crate {
         let folded = fold::noop_fold_crate(c, self);
 
         // Add a special __test module to the crate that will contain code
@@ -78,7 +78,7 @@ impl fold::ast_fold for TestHarnessGenerator {
         }
     }
 
-    fn fold_item(&self, i: @ast::item) -> SmallVector<@ast::item> {
+    fn fold_item(&mut self, i: @ast::item) -> SmallVector<@ast::item> {
         {
             let mut path = self.cx.path.borrow_mut();
             path.get().push(i.ident);
@@ -122,7 +122,7 @@ impl fold::ast_fold for TestHarnessGenerator {
         res
     }
 
-    fn fold_mod(&self, m: &ast::_mod) -> ast::_mod {
+    fn fold_mod(&mut self, m: &ast::_mod) -> ast::_mod {
         // Remove any #[main] from the AST so it doesn't clash with
         // the one we're going to add. Only if compiling an executable.
 
@@ -172,7 +172,7 @@ fn generate_test_harness(sess: session::Session, crate: ast::Crate)
         }
     });
 
-    let fold = TestHarnessGenerator {
+    let mut fold = TestHarnessGenerator {
         cx: cx
     };
     let res = fold.fold_crate(crate);
diff --git a/src/librustc/middle/astencode.rs b/src/librustc/middle/astencode.rs
index 3560a930237..90825289ac3 100644
--- a/src/librustc/middle/astencode.rs
+++ b/src/librustc/middle/astencode.rs
@@ -301,7 +301,7 @@ struct NestedItemsDropper {
 }
 
 impl fold::ast_fold for NestedItemsDropper {
-    fn fold_block(&self, blk: ast::P<ast::Block>) -> ast::P<ast::Block> {
+    fn fold_block(&mut self, blk: ast::P<ast::Block>) -> ast::P<ast::Block> {
         let stmts_sans_items = blk.stmts.iter().filter_map(|stmt| {
             match stmt.node {
                 ast::StmtExpr(_, _) | ast::StmtSemi(_, _) |
@@ -340,7 +340,7 @@ impl fold::ast_fold for NestedItemsDropper {
 // nested items, as otherwise it would get confused when translating
 // inlined items.
 fn simplify_ast(ii: &ast::inlined_item) -> ast::inlined_item {
-    let fld = NestedItemsDropper {
+    let mut fld = NestedItemsDropper {
         contents: (),
     };
 
@@ -365,17 +365,17 @@ struct AstRenumberer {
 }
 
 impl fold::ast_fold for AstRenumberer {
-    fn new_id(&self, id: ast::NodeId) -> ast::NodeId {
+    fn new_id(&mut self, id: ast::NodeId) -> ast::NodeId {
         self.xcx.tr_id(id)
     }
-    fn new_span(&self, span: Span) -> Span {
+    fn new_span(&mut self, span: Span) -> Span {
         self.xcx.tr_span(span)
     }
 }
 
 fn renumber_ast(xcx: @ExtendedDecodeContext, ii: ast::inlined_item)
     -> ast::inlined_item {
-    let fld = AstRenumberer {
+    let mut fld = AstRenumberer {
         xcx: xcx,
     };
     match ii {
diff --git a/src/librustpkg/util.rs b/src/librustpkg/util.rs
index b8d01f56e4d..17b46b883b3 100644
--- a/src/librustpkg/util.rs
+++ b/src/librustpkg/util.rs
@@ -80,7 +80,7 @@ struct ReadyCtx {
     fns: ~[ListenerFn]
 }
 
-fn fold_mod(_ctx: @mut ReadyCtx, m: &ast::_mod, fold: &CrateSetup)
+fn fold_mod(_ctx: @mut ReadyCtx, m: &ast::_mod, fold: &mut CrateSetup)
             -> ast::_mod {
     fn strip_main(item: @ast::item) -> @ast::item {
         @ast::item {
@@ -101,7 +101,7 @@ fn fold_mod(_ctx: @mut ReadyCtx, m: &ast::_mod, fold: &CrateSetup)
     }, fold)
 }
 
-fn fold_item(ctx: @mut ReadyCtx, item: @ast::item, fold: &CrateSetup)
+fn fold_item(ctx: @mut ReadyCtx, item: @ast::item, fold: &mut CrateSetup)
              -> SmallVector<@ast::item> {
     ctx.path.push(item.ident);
 
@@ -145,10 +145,10 @@ struct CrateSetup {
 }
 
 impl fold::ast_fold for CrateSetup {
-    fn fold_item(&self, item: @ast::item) -> SmallVector<@ast::item> {
+    fn fold_item(&mut self, item: @ast::item) -> SmallVector<@ast::item> {
         fold_item(self.ctx, item, self)
     }
-    fn fold_mod(&self, module: &ast::_mod) -> ast::_mod {
+    fn fold_mod(&mut self, module: &ast::_mod) -> ast::_mod {
         fold_mod(self.ctx, module, self)
     }
 }
@@ -162,7 +162,7 @@ pub fn ready_crate(sess: session::Session,
         path: ~[],
         fns: ~[]
     };
-    let fold = CrateSetup {
+    let mut fold = CrateSetup {
         ctx: ctx,
     };
     fold.fold_crate(crate)
diff --git a/src/libsyntax/ext/base.rs b/src/libsyntax/ext/base.rs
index 37a1ae6884d..842f2bfdffc 100644
--- a/src/libsyntax/ext/base.rs
+++ b/src/libsyntax/ext/base.rs
@@ -324,11 +324,11 @@ impl ExtCtxt {
         loop {
             match e.node {
                 ast::ExprMac(..) => {
-                    let expander = expand::MacroExpander {
+                    let mut expander = expand::MacroExpander {
                         extsbox: @mut syntax_expander_table(),
                         cx: self,
                     };
-                    e = expand::expand_expr(e, &expander);
+                    e = expand::expand_expr(e, &mut expander);
                 }
                 _ => return e
             }
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 007c06938c6..f5c3c1792c9 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -908,7 +908,7 @@ struct Duplicator<'a> {
 }
 
 impl<'a> ast_fold for Duplicator<'a> {
-    fn new_id(&self, _: NodeId) -> NodeId {
+    fn new_id(&mut self, _: NodeId) -> NodeId {
         ast::DUMMY_NODE_ID
     }
 }
@@ -925,7 +925,7 @@ pub trait Duplicate {
 
 impl Duplicate for @ast::Expr {
     fn duplicate(&self, cx: &ExtCtxt) -> @ast::Expr {
-        let folder = Duplicator {
+        let mut folder = Duplicator {
             cx: cx,
         };
         folder.fold_expr(*self)
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 31c44b7e574..e20161ea2d0 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -31,7 +31,7 @@ use util::small_vector::SmallVector;
 
 use std::vec;
 
-pub fn expand_expr(e: @ast::Expr, fld: &MacroExpander) -> @ast::Expr {
+pub fn expand_expr(e: @ast::Expr, fld: &mut MacroExpander) -> @ast::Expr {
     match e.node {
         // expr_mac should really be expr_ext or something; it's the
         // entry-point for all syntax extensions.
@@ -210,7 +210,7 @@ pub fn expand_expr(e: @ast::Expr, fld: &MacroExpander) -> @ast::Expr {
 //
 // NB: there is some redundancy between this and expand_item, below, and
 // they might benefit from some amount of semantic and language-UI merger.
-pub fn expand_mod_items(module_: &ast::_mod, fld: &MacroExpander) -> ast::_mod {
+pub fn expand_mod_items(module_: &ast::_mod, fld: &mut MacroExpander) -> ast::_mod {
     // Fold the contents first:
     let module_ = noop_fold_mod(module_, fld);
 
@@ -263,7 +263,8 @@ macro_rules! with_exts_frame (
 static special_block_name : &'static str = " block";
 
 // When we enter a module, record it, for the sake of `module!`
-pub fn expand_item(it: @ast::item, fld: &MacroExpander) -> SmallVector<@ast::item> {
+pub fn expand_item(it: @ast::item, fld: &mut MacroExpander)
+                   -> SmallVector<@ast::item> {
     match it.node {
         ast::item_mac(..) => expand_item_mac(it, fld),
         ast::item_mod(_) | ast::item_foreign_mod(_) => {
@@ -286,7 +287,7 @@ pub fn contains_macro_escape(attrs: &[ast::Attribute]) -> bool {
 
 // Support for item-position macro invocations, exactly the same
 // logic as for expression-position macro invocations.
-pub fn expand_item_mac(it: @ast::item, fld: &MacroExpander)
+pub fn expand_item_mac(it: @ast::item, fld: &mut MacroExpander)
                        -> SmallVector<@ast::item> {
     let (pth, tts, ctxt) = match it.node {
         item_mac(codemap::Spanned {
@@ -394,7 +395,7 @@ fn insert_macro(exts: SyntaxEnv, name: ast::Name, transformer: @Transformer) {
 }
 
 // expand a stmt
-pub fn expand_stmt(s: &Stmt, fld: &MacroExpander) -> SmallVector<@Stmt> {
+pub fn expand_stmt(s: &Stmt, fld: &mut MacroExpander) -> SmallVector<@Stmt> {
     // why the copying here and not in expand_expr?
     // looks like classic changed-in-only-one-place
     let (pth, tts, semi, ctxt) = match s.node {
@@ -487,7 +488,7 @@ pub fn expand_stmt(s: &Stmt, fld: &MacroExpander) -> SmallVector<@Stmt> {
 
 // expand a non-macro stmt. this is essentially the fallthrough for
 // expand_stmt, above.
-fn expand_non_macro_stmt(s: &Stmt, fld: &MacroExpander)
+fn expand_non_macro_stmt(s: &Stmt, fld: &mut MacroExpander)
                          -> SmallVector<@Stmt> {
     // is it a let?
     match s.node {
@@ -521,7 +522,7 @@ fn expand_non_macro_stmt(s: &Stmt, fld: &MacroExpander)
                 let new_name = fresh_name(ident);
                 new_pending_renames.push((*ident,new_name));
             }
-            let rename_fld = renames_to_fold(new_pending_renames);
+            let mut rename_fld = renames_to_fold(new_pending_renames);
             // rewrite the pattern using the new names (the old ones
             // have already been applied):
             let rewritten_pat = rename_fld.fold_pat(expanded_pat);
@@ -609,17 +610,17 @@ pub fn new_name_finder(idents: ~[ast::Ident]) -> NewNameFinderContext {
 }
 
 // expand a block. pushes a new exts_frame, then calls expand_block_elts
-pub fn expand_block(blk: &Block, fld: &MacroExpander) -> P<Block> {
+pub fn expand_block(blk: &Block, fld: &mut MacroExpander) -> P<Block> {
     // see note below about treatment of exts table
     with_exts_frame!(fld.extsbox,false,
                      expand_block_elts(blk, fld))
 }
 
 // expand the elements of a block.
-pub fn expand_block_elts(b: &Block, fld: &MacroExpander) -> P<Block> {
+pub fn expand_block_elts(b: &Block, fld: &mut MacroExpander) -> P<Block> {
     let block_info = get_block_info(*fld.extsbox);
     let pending_renames = block_info.pending_renames;
-    let rename_fld = renames_to_fold(pending_renames);
+    let mut rename_fld = renames_to_fold(pending_renames);
     let new_view_items = b.view_items.map(|x| fld.fold_view_item(x));
     let new_stmts = b.stmts.iter()
             .map(|x| rename_fld.fold_stmt(*x)
@@ -651,7 +652,7 @@ struct IdentRenamer {
 }
 
 impl ast_fold for IdentRenamer {
-    fn fold_ident(&self, id: ast::Ident) -> ast::Ident {
+    fn fold_ident(&mut self, id: ast::Ident) -> ast::Ident {
         let new_ctxt = self.renames.iter().fold(id.ctxt, |ctxt, &(from, to)| {
             new_rename(from, to, ctxt)
         });
@@ -664,10 +665,10 @@ impl ast_fold for IdentRenamer {
 
 // given a mutable list of renames, return a tree-folder that applies those
 // renames.
-pub fn renames_to_fold(renames: @mut ~[(ast::Ident,ast::Name)]) -> @ast_fold {
-    @IdentRenamer {
+pub fn renames_to_fold(renames: @mut ~[(ast::Ident,ast::Name)]) -> IdentRenamer {
+    IdentRenamer {
         renames: renames,
-    } as @ast_fold
+    }
 }
 
 pub fn new_span(cx: &ExtCtxt, sp: Span) -> Span {
@@ -897,7 +898,7 @@ struct Injector {
 }
 
 impl ast_fold for Injector {
-    fn fold_mod(&self, module: &ast::_mod) -> ast::_mod {
+    fn fold_mod(&mut self, module: &ast::_mod) -> ast::_mod {
         // Just inject the standard macros at the start of the first module
         // in the crate: that is, at the start of the crate file itself.
         let items = vec::append(~[ self.sm ], module.items);
@@ -923,9 +924,9 @@ pub fn inject_std_macros(parse_sess: @mut parse::ParseSess,
         None => fail!("expected core macros to parse correctly")
     };
 
-    let injector = @Injector {
+    let mut injector = Injector {
         sm: sm,
-    } as @ast_fold;
+    };
     injector.fold_crate(c)
 }
 
@@ -935,27 +936,27 @@ pub struct MacroExpander<'a> {
 }
 
 impl<'a> ast_fold for MacroExpander<'a> {
-    fn fold_expr(&self, expr: @ast::Expr) -> @ast::Expr {
+    fn fold_expr(&mut self, expr: @ast::Expr) -> @ast::Expr {
         expand_expr(expr, self)
     }
 
-    fn fold_mod(&self, module: &ast::_mod) -> ast::_mod {
+    fn fold_mod(&mut self, module: &ast::_mod) -> ast::_mod {
         expand_mod_items(module, self)
     }
 
-    fn fold_item(&self, item: @ast::item) -> SmallVector<@ast::item> {
+    fn fold_item(&mut self, item: @ast::item) -> SmallVector<@ast::item> {
         expand_item(item, self)
     }
 
-    fn fold_stmt(&self, stmt: &ast::Stmt) -> SmallVector<@ast::Stmt> {
+    fn fold_stmt(&mut self, stmt: &ast::Stmt) -> SmallVector<@ast::Stmt> {
         expand_stmt(stmt, self)
     }
 
-    fn fold_block(&self, block: P<Block>) -> P<Block> {
+    fn fold_block(&mut self, block: P<Block>) -> P<Block> {
         expand_block(block, self)
     }
 
-    fn new_span(&self, span: Span) -> Span {
+    fn new_span(&mut self, span: Span) -> Span {
         new_span(self.cx, span)
     }
 }
@@ -970,7 +971,7 @@ pub fn expand_crate(parse_sess: @mut parse::ParseSess,
     // every method/element of AstFoldFns in fold.rs.
     let extsbox = syntax_expander_table();
     let cx = ExtCtxt::new(parse_sess, cfg.clone());
-    let expander = MacroExpander {
+    let mut expander = MacroExpander {
         extsbox: @mut extsbox,
         cx: &cx,
     };
@@ -1046,7 +1047,7 @@ pub struct ContextWrapper {
 }
 
 impl ast_fold for ContextWrapper {
-    fn fold_ident(&self, id: ast::Ident) -> ast::Ident {
+    fn fold_ident(&mut self, id: ast::Ident) -> ast::Ident {
         let ast::Ident {
             name,
             ctxt
@@ -1056,7 +1057,7 @@ impl ast_fold for ContextWrapper {
             ctxt: self.context_function.f(ctxt),
         }
     }
-    fn fold_mac(&self, m: &ast::mac) -> ast::mac {
+    fn fold_mac(&mut self, m: &ast::mac) -> ast::mac {
         let macro = match m.node {
             mac_invoc_tt(ref path, ref tts, ctxt) => {
                 mac_invoc_tt(self.fold_path(path),
@@ -1073,24 +1074,24 @@ impl ast_fold for ContextWrapper {
 
 // given a function from ctxts to ctxts, produce
 // an ast_fold that applies that function to all ctxts:
-pub fn fun_to_ctxt_folder<T : 'static + CtxtFn>(cf: @T) -> @ContextWrapper {
-    @ContextWrapper {
+pub fn fun_to_ctxt_folder<T : 'static + CtxtFn>(cf: @T) -> ContextWrapper {
+    ContextWrapper {
         context_function: cf as @CtxtFn,
     }
 }
 
 // just a convenience:
-pub fn new_mark_folder(m: Mrk) -> @ContextWrapper {
+pub fn new_mark_folder(m: Mrk) -> ContextWrapper {
     fun_to_ctxt_folder(@Marker{mark:m})
 }
 
-pub fn new_rename_folder(from: ast::Ident, to: ast::Name) -> @ContextWrapper {
+pub fn new_rename_folder(from: ast::Ident, to: ast::Name) -> ContextWrapper {
     fun_to_ctxt_folder(@Renamer{from:from,to:to})
 }
 
 // apply a given mark to the given token trees. Used prior to expansion of a macro.
 fn mark_tts(tts : &[token_tree], m : Mrk) -> ~[token_tree] {
-    fold_tts(tts,new_mark_folder(m))
+    fold_tts(tts, &mut new_mark_folder(m))
 }
 
 // apply a given mark to the given expr. Used following the expansion of a macro.
@@ -1295,7 +1296,7 @@ mod test {
         let ident_str = @"x";
         let tts = string_to_tts(ident_str);
         let fm = fresh_mark();
-        let marked_once = fold::fold_tts(tts,new_mark_folder(fm));
+        let marked_once = fold::fold_tts(tts,&mut new_mark_folder(fm));
         assert_eq!(marked_once.len(),1);
         let marked_once_ctxt =
             match marked_once[0] {
@@ -1317,7 +1318,7 @@ mod test {
         let item_ast = string_to_crate(@"fn f() -> int { a }");
         let a_name = intern("a");
         let a2_name = gensym("a2");
-        let renamer = new_rename_folder(ast::Ident{name:a_name,ctxt:EMPTY_CTXT},
+        let mut renamer = new_rename_folder(ast::Ident{name:a_name,ctxt:EMPTY_CTXT},
                                         a2_name);
         let renamed_ast = renamer.fold_crate(item_ast.clone());
         let mut path_finder = new_path_finder(~[]);
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index a7faeee494e..9edde7c9c87 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -17,15 +17,15 @@ use util::small_vector::SmallVector;
 
 // We may eventually want to be able to fold over type parameters, too.
 pub trait ast_fold {
-    fn fold_crate(&self, c: Crate) -> Crate {
+    fn fold_crate(&mut self, c: Crate) -> Crate {
         noop_fold_crate(c, self)
     }
 
-    fn fold_meta_items(&self, meta_items: &[@MetaItem]) -> ~[@MetaItem] {
+    fn fold_meta_items(&mut self, meta_items: &[@MetaItem]) -> ~[@MetaItem] {
         meta_items.map(|x| fold_meta_item_(*x, self))
     }
 
-    fn fold_view_paths(&self, view_paths: &[@view_path]) -> ~[@view_path] {
+    fn fold_view_paths(&mut self, view_paths: &[@view_path]) -> ~[@view_path] {
         view_paths.map(|view_path| {
             let inner_view_path = match view_path.node {
                 view_path_simple(ref ident, ref path, node_id) => {
@@ -62,7 +62,7 @@ pub trait ast_fold {
         })
     }
 
-    fn fold_view_item(&self, vi: &view_item) -> view_item {
+    fn fold_view_item(&mut self, vi: &view_item) -> view_item {
         let inner_view_item = match vi.node {
             view_item_extern_mod(ref ident,
                                  string,
@@ -85,7 +85,7 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_foreign_item(&self, ni: @foreign_item) -> @foreign_item {
+    fn fold_foreign_item(&mut self, ni: @foreign_item) -> @foreign_item {
         let fold_attribute = |x| fold_attribute_(x, self);
 
         @ast::foreign_item {
@@ -114,11 +114,11 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_item(&self, i: @item) -> SmallVector<@item> {
+    fn fold_item(&mut self, i: @item) -> SmallVector<@item> {
         noop_fold_item(i, self)
     }
 
-    fn fold_struct_field(&self, sf: &struct_field) -> struct_field {
+    fn fold_struct_field(&mut self, sf: &struct_field) -> struct_field {
         let fold_attribute = |x| fold_attribute_(x, self);
 
         Spanned {
@@ -132,15 +132,15 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_item_underscore(&self, i: &item_) -> item_ {
+    fn fold_item_underscore(&mut self, i: &item_) -> item_ {
         noop_fold_item_underscore(i, self)
     }
 
-    fn fold_type_method(&self, m: &TypeMethod) -> TypeMethod {
+    fn fold_type_method(&mut self, m: &TypeMethod) -> TypeMethod {
         noop_fold_type_method(m, self)
     }
 
-    fn fold_method(&self, m: @method) -> @method {
+    fn fold_method(&mut self, m: @method) -> @method {
         @ast::method {
             ident: self.fold_ident(m.ident),
             attrs: m.attrs.map(|a| fold_attribute_(*a, self)),
@@ -156,15 +156,15 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_block(&self, b: P<Block>) -> P<Block> {
+    fn fold_block(&mut self, b: P<Block>) -> P<Block> {
         noop_fold_block(b, self)
     }
 
-    fn fold_stmt(&self, s: &Stmt) -> SmallVector<@Stmt> {
+    fn fold_stmt(&mut self, s: &Stmt) -> SmallVector<@Stmt> {
         noop_fold_stmt(s, self)
     }
 
-    fn fold_arm(&self, a: &Arm) -> Arm {
+    fn fold_arm(&mut self, a: &Arm) -> Arm {
         Arm {
             pats: a.pats.map(|x| self.fold_pat(*x)),
             guard: a.guard.map(|x| self.fold_expr(x)),
@@ -172,7 +172,7 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_pat(&self, p: @Pat) -> @Pat {
+    fn fold_pat(&mut self, p: @Pat) -> @Pat {
         let node = match p.node {
             PatWild => PatWild,
             PatWildMulti => PatWildMulti,
@@ -217,7 +217,7 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_decl(&self, d: @Decl) -> SmallVector<@Decl> {
+    fn fold_decl(&mut self, d: @Decl) -> SmallVector<@Decl> {
         let node = match d.node {
             DeclLocal(ref l) => SmallVector::one(DeclLocal(self.fold_local(*l))),
             DeclItem(it) => {
@@ -233,11 +233,11 @@ pub trait ast_fold {
         }).collect()
     }
 
-    fn fold_expr(&self, e: @Expr) -> @Expr {
+    fn fold_expr(&mut self, e: @Expr) -> @Expr {
         noop_fold_expr(e, self)
     }
 
-    fn fold_ty(&self, t: P<Ty>) -> P<Ty> {
+    fn fold_ty(&mut self, t: P<Ty>) -> P<Ty> {
         let node = match t.node {
             ty_nil | ty_bot | ty_infer => t.node.clone(),
             ty_box(ref mt) => ty_box(fold_mt(mt, self)),
@@ -284,11 +284,11 @@ pub trait ast_fold {
         })
     }
 
-    fn fold_mod(&self, m: &_mod) -> _mod {
+    fn fold_mod(&mut self, m: &_mod) -> _mod {
         noop_fold_mod(m, self)
     }
 
-    fn fold_foreign_mod(&self, nm: &foreign_mod) -> foreign_mod {
+    fn fold_foreign_mod(&mut self, nm: &foreign_mod) -> foreign_mod {
         ast::foreign_mod {
             abis: nm.abis,
             view_items: nm.view_items
@@ -302,7 +302,7 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_variant(&self, v: &variant) -> P<variant> {
+    fn fold_variant(&mut self, v: &variant) -> P<variant> {
         let kind;
         match v.node.kind {
             tuple_variant_kind(ref variant_args) => {
@@ -339,11 +339,11 @@ pub trait ast_fold {
         })
     }
 
-    fn fold_ident(&self, i: Ident) -> Ident {
+    fn fold_ident(&mut self, i: Ident) -> Ident {
         i
     }
 
-    fn fold_path(&self, p: &Path) -> Path {
+    fn fold_path(&mut self, p: &Path) -> Path {
         ast::Path {
             span: self.new_span(p.span),
             global: p.global,
@@ -355,7 +355,7 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_local(&self, l: @Local) -> @Local {
+    fn fold_local(&mut self, l: @Local) -> @Local {
         @Local {
             ty: self.fold_ty(l.ty),
             pat: self.fold_pat(l.pat),
@@ -365,7 +365,7 @@ pub trait ast_fold {
         }
     }
 
-    fn fold_mac(&self, macro: &mac) -> mac {
+    fn fold_mac(&mut self, macro: &mac) -> mac {
         Spanned {
             node: match macro.node {
                 mac_invoc_tt(ref p, ref tts, ctxt) => {
@@ -382,22 +382,22 @@ pub trait ast_fold {
         es.map(|x| f(*x))
     }
 
-    fn new_id(&self, i: NodeId) -> NodeId {
+    fn new_id(&mut self, i: NodeId) -> NodeId {
         i
     }
 
-    fn new_span(&self, sp: Span) -> Span {
+    fn new_span(&mut self, sp: Span) -> Span {
         sp
     }
 
-    fn fold_explicit_self(&self, es: &explicit_self) -> explicit_self {
+    fn fold_explicit_self(&mut self, es: &explicit_self) -> explicit_self {
         Spanned {
             span: self.new_span(es.span),
             node: self.fold_explicit_self_(&es.node)
         }
     }
 
-    fn fold_explicit_self_(&self, es: &explicit_self_) -> explicit_self_ {
+    fn fold_explicit_self_(&mut self, es: &explicit_self_) -> explicit_self_ {
         match *es {
             sty_static | sty_value(_) | sty_uniq(_) | sty_box(_) => {
                 *es
@@ -412,7 +412,7 @@ pub trait ast_fold {
 /* some little folds that probably aren't useful to have in ast_fold itself*/
 
 //used in noop_fold_item and noop_fold_crate and noop_fold_crate_directive
-fn fold_meta_item_<T:ast_fold>(mi: @MetaItem, fld: &T) -> @MetaItem {
+fn fold_meta_item_<T:ast_fold>(mi: @MetaItem, fld: &mut T) -> @MetaItem {
     @Spanned {
         node:
             match mi.node {
@@ -430,7 +430,7 @@ fn fold_meta_item_<T:ast_fold>(mi: @MetaItem, fld: &T) -> @MetaItem {
 }
 
 //used in noop_fold_item and noop_fold_crate
-fn fold_attribute_<T:ast_fold>(at: Attribute, fld: &T) -> Attribute {
+fn fold_attribute_<T:ast_fold>(at: Attribute, fld: &mut T) -> Attribute {
     Spanned {
         span: fld.new_span(at.span),
         node: ast::Attribute_ {
@@ -442,7 +442,7 @@ fn fold_attribute_<T:ast_fold>(at: Attribute, fld: &T) -> Attribute {
 }
 
 //used in noop_fold_foreign_item and noop_fold_fn_decl
-fn fold_arg_<T:ast_fold>(a: &arg, fld: &T) -> arg {
+fn fold_arg_<T:ast_fold>(a: &arg, fld: &mut T) -> arg {
     ast::arg {
         ty: fld.fold_ty(a.ty),
         pat: fld.fold_pat(a.pat),
@@ -452,7 +452,7 @@ fn fold_arg_<T:ast_fold>(a: &arg, fld: &T) -> arg {
 
 // build a new vector of tts by appling the ast_fold's fold_ident to
 // all of the identifiers in the token trees.
-pub fn fold_tts<T:ast_fold>(tts: &[token_tree], fld: &T) -> ~[token_tree] {
+pub fn fold_tts<T:ast_fold>(tts: &[token_tree], fld: &mut T) -> ~[token_tree] {
     tts.map(|tt| {
         match *tt {
             tt_tok(span, ref tok) =>
@@ -470,7 +470,7 @@ pub fn fold_tts<T:ast_fold>(tts: &[token_tree], fld: &T) -> ~[token_tree] {
 }
 
 // apply ident folder if it's an ident, otherwise leave it alone
-fn maybe_fold_ident<T:ast_fold>(t: &token::Token, fld: &T) -> token::Token {
+fn maybe_fold_ident<T:ast_fold>(t: &token::Token, fld: &mut T) -> token::Token {
     match *t {
         token::IDENT(id, followed_by_colons) => {
             token::IDENT(fld.fold_ident(id), followed_by_colons)
@@ -479,7 +479,7 @@ fn maybe_fold_ident<T:ast_fold>(t: &token::Token, fld: &T) -> token::Token {
     }
 }
 
-pub fn fold_fn_decl<T:ast_fold>(decl: &ast::fn_decl, fld: &T)
+pub fn fold_fn_decl<T:ast_fold>(decl: &ast::fn_decl, fld: &mut T)
                                 -> P<fn_decl> {
     P(fn_decl {
         inputs: decl.inputs.map(|x| fold_arg_(x, fld)), // bad copy
@@ -489,7 +489,7 @@ pub fn fold_fn_decl<T:ast_fold>(decl: &ast::fn_decl, fld: &T)
     })
 }
 
-fn fold_ty_param_bound<T:ast_fold>(tpb: &TyParamBound, fld: &T)
+fn fold_ty_param_bound<T:ast_fold>(tpb: &TyParamBound, fld: &mut T)
                                    -> TyParamBound {
     match *tpb {
         TraitTyParamBound(ref ty) => TraitTyParamBound(fold_trait_ref(ty, fld)),
@@ -497,7 +497,7 @@ fn fold_ty_param_bound<T:ast_fold>(tpb: &TyParamBound, fld: &T)
     }
 }
 
-pub fn fold_ty_param<T:ast_fold>(tp: &TyParam, fld: &T) -> TyParam {
+pub fn fold_ty_param<T:ast_fold>(tp: &TyParam, fld: &mut T) -> TyParam {
     TyParam {
         ident: tp.ident,
         id: fld.new_id(tp.id),
@@ -505,12 +505,12 @@ pub fn fold_ty_param<T:ast_fold>(tp: &TyParam, fld: &T) -> TyParam {
     }
 }
 
-pub fn fold_ty_params<T:ast_fold>(tps: &OptVec<TyParam>, fld: &T)
+pub fn fold_ty_params<T:ast_fold>(tps: &OptVec<TyParam>, fld: &mut T)
                                   -> OptVec<TyParam> {
     tps.map(|tp| fold_ty_param(tp, fld))
 }
 
-pub fn fold_lifetime<T:ast_fold>(l: &Lifetime, fld: &T) -> Lifetime {
+pub fn fold_lifetime<T:ast_fold>(l: &Lifetime, fld: &mut T) -> Lifetime {
     Lifetime {
         id: fld.new_id(l.id),
         span: fld.new_span(l.span),
@@ -518,22 +518,22 @@ pub fn fold_lifetime<T:ast_fold>(l: &Lifetime, fld: &T) -> Lifetime {
     }
 }
 
-pub fn fold_lifetimes<T:ast_fold>(lts: &OptVec<Lifetime>, fld: &T)
+pub fn fold_lifetimes<T:ast_fold>(lts: &OptVec<Lifetime>, fld: &mut T)
                                   -> OptVec<Lifetime> {
     lts.map(|l| fold_lifetime(l, fld))
 }
 
-pub fn fold_opt_lifetime<T:ast_fold>(o_lt: &Option<Lifetime>, fld: &T)
+pub fn fold_opt_lifetime<T:ast_fold>(o_lt: &Option<Lifetime>, fld: &mut T)
                                      -> Option<Lifetime> {
     o_lt.as_ref().map(|lt| fold_lifetime(lt, fld))
 }
 
-pub fn fold_generics<T:ast_fold>(generics: &Generics, fld: &T) -> Generics {
+pub fn fold_generics<T:ast_fold>(generics: &Generics, fld: &mut T) -> Generics {
     Generics {ty_params: fold_ty_params(&generics.ty_params, fld),
               lifetimes: fold_lifetimes(&generics.lifetimes, fld)}
 }
 
-fn fold_struct_def<T:ast_fold>(struct_def: @ast::struct_def, fld: &T)
+fn fold_struct_def<T:ast_fold>(struct_def: @ast::struct_def, fld: &mut T)
                                -> @ast::struct_def {
     @ast::struct_def {
         fields: struct_def.fields.map(|f| fold_struct_field(f, fld)),
@@ -541,14 +541,14 @@ fn fold_struct_def<T:ast_fold>(struct_def: @ast::struct_def, fld: &T)
     }
 }
 
-fn fold_trait_ref<T:ast_fold>(p: &trait_ref, fld: &T) -> trait_ref {
+fn fold_trait_ref<T:ast_fold>(p: &trait_ref, fld: &mut T) -> trait_ref {
     ast::trait_ref {
         path: fld.fold_path(&p.path),
         ref_id: fld.new_id(p.ref_id),
     }
 }
 
-fn fold_struct_field<T:ast_fold>(f: &struct_field, fld: &T) -> struct_field {
+fn fold_struct_field<T:ast_fold>(f: &struct_field, fld: &mut T) -> struct_field {
     Spanned {
         node: ast::struct_field_ {
             kind: f.node.kind,
@@ -560,7 +560,7 @@ fn fold_struct_field<T:ast_fold>(f: &struct_field, fld: &T) -> struct_field {
     }
 }
 
-fn fold_field_<T:ast_fold>(field: Field, folder: &T) -> Field {
+fn fold_field_<T:ast_fold>(field: Field, folder: &mut T) -> Field {
     ast::Field {
         ident: respan(field.ident.span, folder.fold_ident(field.ident.node)),
         expr: folder.fold_expr(field.expr),
@@ -568,14 +568,14 @@ fn fold_field_<T:ast_fold>(field: Field, folder: &T) -> Field {
     }
 }
 
-fn fold_mt<T:ast_fold>(mt: &mt, folder: &T) -> mt {
+fn fold_mt<T:ast_fold>(mt: &mt, folder: &mut T) -> mt {
     mt {
         ty: folder.fold_ty(mt.ty),
         mutbl: mt.mutbl,
     }
 }
 
-fn fold_opt_bounds<T:ast_fold>(b: &Option<OptVec<TyParamBound>>, folder: &T)
+fn fold_opt_bounds<T:ast_fold>(b: &Option<OptVec<TyParamBound>>, folder: &mut T)
                                -> Option<OptVec<TyParamBound>> {
     b.as_ref().map(|bounds| {
         bounds.map(|bound| {
@@ -584,7 +584,7 @@ fn fold_opt_bounds<T:ast_fold>(b: &Option<OptVec<TyParamBound>>, folder: &T)
     })
 }
 
-fn fold_variant_arg_<T:ast_fold>(va: &variant_arg, folder: &T)
+fn fold_variant_arg_<T:ast_fold>(va: &variant_arg, folder: &mut T)
                                  -> variant_arg {
     ast::variant_arg {
         ty: folder.fold_ty(va.ty),
@@ -592,7 +592,7 @@ fn fold_variant_arg_<T:ast_fold>(va: &variant_arg, folder: &T)
     }
 }
 
-pub fn noop_fold_block<T:ast_fold>(b: P<Block>, folder: &T) -> P<Block> {
+pub fn noop_fold_block<T:ast_fold>(b: P<Block>, folder: &mut T) -> P<Block> {
     let view_items = b.view_items.map(|x| folder.fold_view_item(x));
     let stmts = b.stmts.iter().flat_map(|s| folder.fold_stmt(*s).move_iter()).collect();
     P(Block {
@@ -605,7 +605,7 @@ pub fn noop_fold_block<T:ast_fold>(b: P<Block>, folder: &T) -> P<Block> {
     })
 }
 
-pub fn noop_fold_item_underscore<T:ast_fold>(i: &item_, folder: &T) -> item_ {
+pub fn noop_fold_item_underscore<T:ast_fold>(i: &item_, folder: &mut T) -> item_ {
     match *i {
         item_static(t, m, e) => {
             item_static(folder.fold_ty(t), m, folder.fold_expr(e))
@@ -662,7 +662,7 @@ pub fn noop_fold_item_underscore<T:ast_fold>(i: &item_, folder: &T) -> item_ {
     }
 }
 
-pub fn noop_fold_type_method<T:ast_fold>(m: &TypeMethod, fld: &T)
+pub fn noop_fold_type_method<T:ast_fold>(m: &TypeMethod, fld: &mut T)
                                          -> TypeMethod {
     TypeMethod {
         ident: fld.fold_ident(m.ident),
@@ -676,7 +676,7 @@ pub fn noop_fold_type_method<T:ast_fold>(m: &TypeMethod, fld: &T)
     }
 }
 
-pub fn noop_fold_mod<T:ast_fold>(m: &_mod, folder: &T) -> _mod {
+pub fn noop_fold_mod<T:ast_fold>(m: &_mod, folder: &mut T) -> _mod {
     ast::_mod {
         view_items: m.view_items
                      .iter()
@@ -685,7 +685,7 @@ pub fn noop_fold_mod<T:ast_fold>(m: &_mod, folder: &T) -> _mod {
     }
 }
 
-pub fn noop_fold_crate<T:ast_fold>(c: Crate, folder: &T) -> Crate {
+pub fn noop_fold_crate<T:ast_fold>(c: Crate, folder: &mut T) -> Crate {
     let fold_meta_item = |x| fold_meta_item_(x, folder);
     let fold_attribute = |x| fold_attribute_(x, folder);
 
@@ -697,7 +697,7 @@ pub fn noop_fold_crate<T:ast_fold>(c: Crate, folder: &T) -> Crate {
     }
 }
 
-pub fn noop_fold_item<T:ast_fold>(i: @ast::item, folder: &T)
+pub fn noop_fold_item<T:ast_fold>(i: @ast::item, folder: &mut T)
                                   -> SmallVector<@ast::item> {
     let fold_attribute = |x| fold_attribute_(x, folder);
 
@@ -711,7 +711,7 @@ pub fn noop_fold_item<T:ast_fold>(i: @ast::item, folder: &T)
     })
 }
 
-pub fn noop_fold_expr<T:ast_fold>(e: @ast::Expr, folder: &T) -> @ast::Expr {
+pub fn noop_fold_expr<T:ast_fold>(e: @ast::Expr, folder: &mut T) -> @ast::Expr {
     let fold_field = |x| fold_field_(x, folder);
 
     let node = match e.node {
@@ -719,7 +719,7 @@ pub fn noop_fold_expr<T:ast_fold>(e: @ast::Expr, folder: &T) -> @ast::Expr {
             ExprVstore(folder.fold_expr(e), v)
         }
         ExprVec(ref exprs, mutt) => {
-            ExprVec(folder.map_exprs(|x| folder.fold_expr(x), *exprs), mutt)
+            ExprVec(exprs.map(|&x| folder.fold_expr(x)), mutt)
         }
         ExprRepeat(expr, count, mutt) => {
             ExprRepeat(folder.fold_expr(expr), folder.fold_expr(count), mutt)
@@ -727,7 +727,7 @@ pub fn noop_fold_expr<T:ast_fold>(e: @ast::Expr, folder: &T) -> @ast::Expr {
         ExprTup(ref elts) => ExprTup(elts.map(|x| folder.fold_expr(*x))),
         ExprCall(f, ref args, blk) => {
             ExprCall(folder.fold_expr(f),
-                     folder.map_exprs(|x| folder.fold_expr(x), *args),
+                     args.map(|&x| folder.fold_expr(x)),
                      blk)
         }
         ExprMethodCall(callee_id, f, i, ref tps, ref args, blk) => {
@@ -736,7 +736,7 @@ pub fn noop_fold_expr<T:ast_fold>(e: @ast::Expr, folder: &T) -> @ast::Expr {
                 folder.fold_expr(f),
                 folder.fold_ident(i),
                 tps.map(|&x| folder.fold_ty(x)),
-                folder.map_exprs(|x| folder.fold_expr(x), *args),
+                args.map(|&x| folder.fold_expr(x)),
                 blk
             )
         }
@@ -837,7 +837,7 @@ pub fn noop_fold_expr<T:ast_fold>(e: @ast::Expr, folder: &T) -> @ast::Expr {
     }
 }
 
-pub fn noop_fold_stmt<T:ast_fold>(s: &Stmt, folder: &T) -> SmallVector<@Stmt> {
+pub fn noop_fold_stmt<T:ast_fold>(s: &Stmt, folder: &mut T) -> SmallVector<@Stmt> {
     let nodes = match s.node {
         StmtDecl(d, nid) => {
             folder.fold_decl(d).move_iter()
@@ -876,7 +876,7 @@ mod test {
     struct ToZzIdentFolder;
 
     impl ast_fold for ToZzIdentFolder {
-        fn fold_ident(&self, _: ast::Ident) -> ast::Ident {
+        fn fold_ident(&mut self, _: ast::Ident) -> ast::Ident {
             token::str_to_ident("zz")
         }
     }
@@ -898,23 +898,23 @@ mod test {
 
     // make sure idents get transformed everywhere
     #[test] fn ident_transformation () {
-        let zz_fold = ToZzIdentFolder;
+        let mut zz_fold = ToZzIdentFolder;
         let ast = string_to_crate(@"#[a] mod b {fn c (d : e, f : g) {h!(i,j,k);l;m}}");
         assert_pred!(matches_codepattern,
                      "matches_codepattern",
-                     pprust::to_str(&zz_fold.fold_crate(ast),fake_print_crate,
+                     pprust::to_str(&mut zz_fold.fold_crate(ast),fake_print_crate,
                                     token::get_ident_interner()),
                      ~"#[a]mod zz{fn zz(zz:zz,zz:zz){zz!(zz,zz,zz);zz;zz}}");
     }
 
     // even inside macro defs....
     #[test] fn ident_transformation_in_defs () {
-        let zz_fold = ToZzIdentFolder;
+        let mut zz_fold = ToZzIdentFolder;
         let ast = string_to_crate(@"macro_rules! a {(b $c:expr $(d $e:token)f+
 => (g $(d $d $e)+))} ");
         assert_pred!(matches_codepattern,
                      "matches_codepattern",
-                     pprust::to_str(&zz_fold.fold_crate(ast),fake_print_crate,
+                     pprust::to_str(&mut zz_fold.fold_crate(ast),fake_print_crate,
                                     token::get_ident_interner()),
                      ~"zz!zz((zz$zz:zz$(zz $zz:zz)zz+=>(zz$(zz$zz$zz)+)))");
     }