managed-mut-sadness.rs

        let replace_id = @mut None;

        let rewriter = @fold::AstFoldFns {
            fold_expr: do fold::wrap |e, fld| {
                match fold_expr(e, fld, self.def_map, replace_id) {
                    Some(f) => {
                        debug!("Rewriting: wrote expr_struct");
                        f
                    }
                    None    => fold::noop_fold_expr(e, fld)
                }
            },
            new_id: |id| replace_id.get_or_default(id),
            .. *fold::default_ast_fold()
        };

mutable-sadness.rs

        let mut replace_id = None;

        let rewriter = @fold::AstFoldFns {
            fold_expr: do fold::wrap |e, fld| {
                match fold_expr(e, fld, self.def_map, &mut replace_id) {
                    Some(f) => {
                        debug!("Rewriting: wrote expr_struct");
                        f
                    }
                    None    => fold::noop_fold_expr(e, fld)
                }
            },
            new_id: |id| replace_id.get_or_default(id),
            .. *fold::default_ast_fold()
        };