diff --git a/mercurial/parsers.c b/mercurial/parsers.c
--- a/mercurial/parsers.c
+++ b/mercurial/parsers.c
@@ -1508,6 +1508,7 @@ static char parsers_doc[] = "Efficient c
 
 PyObject *encodedir(PyObject *self, PyObject *args);
 PyObject *pathencode(PyObject *self, PyObject *args);
+PyObject *lowerencode(PyObject *self, PyObject *args);
 
 static PyMethodDef methods[] = {
 	{"pack_dirstate", pack_dirstate, METH_VARARGS, "pack a dirstate\n"},
@@ -1516,6 +1517,7 @@ static PyMethodDef methods[] = {
 	{"parse_index2", parse_index2, METH_VARARGS, "parse a revlog index\n"},
 	{"encodedir", encodedir, METH_VARARGS, "encodedir a path\n"},
 	{"pathencode", pathencode, METH_VARARGS, "fncache-encode a path\n"},
+	{"lowerencode", lowerencode, METH_VARARGS, "lower-encode a path\n"},
 	{NULL, NULL}
 };
 
diff --git a/mercurial/pathencode.c b/mercurial/pathencode.c
--- a/mercurial/pathencode.c
+++ b/mercurial/pathencode.c
@@ -15,6 +15,7 @@
  * required.
  */
 
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include <assert.h>
 #include <ctype.h>
@@ -481,6 +482,47 @@ static Py_ssize_t basicencode(char *dest
 
 static const Py_ssize_t maxstorepathlen = 120;
 
+static Py_ssize_t _lowerencode(char *dest, size_t destsize,
+			       const char *src, Py_ssize_t len)
+{
+	static const uint32_t onebyte[8] = {
+		1, 0x2bfffbfb, 0xe8000001, 0x2fffffff
+	};
+
+	static const uint32_t lower[8] = { 0, 0, 0x7fffffe };
+
+	Py_ssize_t i, destlen = 0;
+
+	for (i = 0; i < len; i++) {
+		if (inset(onebyte, src[i]))
+			charcopy(dest, &destlen, destsize, src[i]);
+		else if (inset(lower, src[i]))
+			charcopy(dest, &destlen, destsize, src[i] + 32);
+		else
+			escape3(dest, &destlen, destsize, src[i]);
+	}
+
+	return destlen;
+}
+
+PyObject *lowerencode(PyObject *self, PyObject *args)
+{
+	char *path;
+	Py_ssize_t len, newlen;
+	PyObject *ret;
+
+	if (!PyArg_ParseTuple(args, "s#:lowerencode", &path, &len))
+		return NULL;
+
+	newlen = _lowerencode(NULL, 0, path, len);
+	ret = PyString_FromStringAndSize(NULL, newlen);
+	if (ret)
+		newlen = _lowerencode(PyString_AS_STRING(ret), newlen,
+				      path, len);
+
+	return ret;
+}
+
 /*
  * We currently implement only basic encoding.
  *
diff --git a/mercurial/store.py b/mercurial/store.py
--- a/mercurial/store.py
+++ b/mercurial/store.py
@@ -132,7 +132,7 @@ def _buildlowerencodefun():
         cmap[chr(x)] = chr(x).lower()
     return lambda s: "".join([cmap[c] for c in s])
 
-lowerencode = _buildlowerencodefun()
+lowerencode = getattr(parsers, 'lowerencode', None) or _buildlowerencodefun()
 
 # Windows reserved names: con, prn, aux, nul, com1..com9, lpt1..lpt9
 _winres3 = ('aux', 'con', 'prn', 'nul') # length 3