diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index d419ce6f188..7562930b655 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -13,6 +13,7 @@ use core::prelude::*;
 use ast::*;
 use ast;
 use codemap::{span, spanned};
+use parse::token;
 use opt_vec::OptVec;
 
 use core::vec;
@@ -904,3 +905,74 @@ impl AstFoldExtensions for @ast_fold {
 pub fn make_fold(afp: ast_fold_fns) -> @ast_fold {
     afp as @ast_fold
 }
+
+#[cfg(test)]
+mod test {
+    use ast;
+    use util::parser_testing::{string_to_crate, matches_codepattern};
+    use parse::token;
+    use print::pprust;
+    use super::*;
+
+    // taken from expand
+    // given a function from idents to idents, produce
+    // an ast_fold that applies that function:
+    pub fn fun_to_ident_folder(f: @fn(ast::ident)->ast::ident) -> @ast_fold{
+        let afp = default_ast_fold();
+        let f_pre = @AstFoldFns{
+            fold_ident : |id, _| f(id),
+            .. *afp
+        };
+        make_fold(f_pre)
+    }
+
+    // this version doesn't care about getting comments or docstrings in.
+    fn fake_print_crate(s: @pprust::ps, crate: ast::crate) {
+        pprust::print_mod(s, &crate.node.module, crate.node.attrs);
+    }
+
+    // change every identifier to "zz"
+    pub fn to_zz() -> @fn(ast::ident)->ast::ident {
+        let zz_id = token::str_to_ident("zz");
+        |id| {zz_id}
+    }
+
+    // maybe add to expand.rs...
+    macro_rules! assert_pred (
+        ($pred:expr, $predname:expr, $a:expr , $b:expr) => (
+            {
+                let pred_val = $pred;
+                let a_val = $a;
+                let b_val = $b;
+                if !(pred_val(a_val,b_val)) {
+                    fail!("expected args satisfying %s, got %? and %?",
+                          $predname, a_val, b_val);
+                }
+            }
+        )
+    )
+
+    // make sure idents get transformed everywhere
+    #[test] fn ident_transformation () {
+        let zz_fold = fun_to_ident_folder(to_zz());
+        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,
+                                    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 = fun_to_ident_folder(to_zz());
+        let ast = string_to_crate(@~"macro_rules! a {(b $c:expr $(d $e:token)f+
+=> (g $(d $d $e)+))} ");
+        assert_pred!(matches_codepattern,
+                     "matches_codepattern",
+                     pprust::to_str(zz_fold.fold_crate(ast),fake_print_crate,
+                                    token::get_ident_interner()),
+                     ~"zz!zz((zz$zz:zz$(zz $zz:zz)zz+=>(zz$(zz$zz$zz)+)))");
+    }
+
+}
diff --git a/src/libsyntax/util/parser_testing.rs b/src/libsyntax/util/parser_testing.rs
index c5528069926..76055ca7914 100644
--- a/src/libsyntax/util/parser_testing.rs
+++ b/src/libsyntax/util/parser_testing.rs
@@ -69,3 +69,81 @@ pub fn string_to_pat(source_str : @~str) -> @ast::pat {
 pub fn strs_to_idents(ids: ~[&str]) -> ~[ast::ident] {
     ids.map(|u| token::str_to_ident(*u))
 }
+
+// does the given string match the pattern? whitespace in the first string
+// may be deleted or replaced with other whitespace to match the pattern.
+// this function is unicode-ignorant; fortunately, the careful design of
+// UTF-8 mitigates this ignorance.  In particular, this function only collapses
+// sequences of \n, \r, ' ', and \t, but it should otherwise tolerate unicode
+// chars. Unsurprisingly, it doesn't do NKF-normalization(?).
+pub fn matches_codepattern(a : &str, b : &str) -> bool {
+    let mut idx_a = 0;
+    let mut idx_b = 0;
+    loop {
+        if (idx_a == a.len() && idx_b == b.len()) {
+            return true;
+        }
+        else if (idx_a == a.len()) {return false;}
+        else if (idx_b == b.len()) {
+            // maybe the stuff left in a is all ws?
+            if (is_whitespace(a.char_at(idx_a))) {
+                return (scan_for_non_ws_or_end(a,idx_a) == a.len());
+            } else {
+                return false;
+            }
+        }
+        // ws in both given and pattern:
+        else if (is_whitespace(a.char_at(idx_a))
+           && is_whitespace(b.char_at(idx_b))) {
+            idx_a = scan_for_non_ws_or_end(a,idx_a);
+            idx_b = scan_for_non_ws_or_end(b,idx_b);
+        }
+        // ws in given only:
+        else if (is_whitespace(a.char_at(idx_a))) {
+            idx_a = scan_for_non_ws_or_end(a,idx_a);
+        }
+        // *don't* silently eat ws in expected only.
+        else if (a.char_at(idx_a) == b.char_at(idx_b)) {
+            idx_a += 1;
+            idx_b += 1;
+        }
+        else {
+            return false;
+        }
+    }
+}
+
+// given a string and an index, return the first uint >= idx
+// that is a non-ws-char or is outside of the legal range of
+// the string.
+fn scan_for_non_ws_or_end(a : &str, idx: uint) -> uint {
+    let mut i = idx;
+    let len = a.len();
+    while ((i < len) && (is_whitespace(a.char_at(i)))) {
+        i += 1;
+    }
+    i
+}
+
+// copied from lexer.
+pub fn is_whitespace(c: char) -> bool {
+    return c == ' ' || c == '\t' || c == '\r' || c == '\n';
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test] fn eqmodws() {
+        assert_eq!(matches_codepattern("",""),true);
+        assert_eq!(matches_codepattern("","a"),false);
+        assert_eq!(matches_codepattern("a",""),false);
+        assert_eq!(matches_codepattern("a","a"),true);
+        assert_eq!(matches_codepattern("a b","a   \n\t\r  b"),true);
+        assert_eq!(matches_codepattern("a b ","a   \n\t\r  b"),true);
+        assert_eq!(matches_codepattern("a b","a   \n\t\r  b "),false);
+        assert_eq!(matches_codepattern("a   b","a b"),true);
+        assert_eq!(matches_codepattern("ab","a b"),false);
+        assert_eq!(matches_codepattern("a   b","ab"),true);
+    }
+}