diff --git a/contrib/hgfixes/fix_bytes.py b/contrib/hgfixes/fix_bytes.py
new file mode 100644
--- /dev/null
+++ b/contrib/hgfixes/fix_bytes.py
@@ -0,0 +1,96 @@
+"""Fixer that changes plain strings to bytes strings."""
+
+import re
+
+from lib2to3 import fixer_base
+from lib2to3.pgen2 import token
+from lib2to3.fixer_util import Name
+from lib2to3.pygram import python_symbols as syms
+
+_re = re.compile(r'[rR]?[\'\"]')
+
+# XXX: Implementing a blacklist in 2to3 turned out to be more troublesome than
+# blacklisting some modules inside the fixers. So, this is what I came with.
+
+blacklist = ['mercurial/demandimport.py',
+             'mercurial/i18n.py',
+            ]
+
+def isdocstring(node):
+    def isclassorfunction(ancestor):
+        symbols = (syms.funcdef, syms.classdef)
+        # if the current node is a child of a function definition, a class
+        # definition or a file, then it is a docstring
+        if ancestor.type == syms.simple_stmt:
+            try:
+                while True:
+                    if ancestor.type in symbols:
+                        return True
+                    ancestor = ancestor.parent
+            except AttributeError:
+                return False
+        return False
+
+    def ismodule(ancestor):
+        # Our child is a docstring if we are a simple statement, and our
+        # ancestor is file_input. In other words, our child is a lone string in
+        # the source file.
+        try:
+            if (ancestor.type == syms.simple_stmt and
+                ancestor.parent.type == syms.file_input):
+                    return True
+        except AttributeError:
+            return False
+
+    def isdocassignment(ancestor):
+        # Assigning to __doc__, definitely a string
+        try:
+            while True:
+                if (ancestor.type == syms.expr_stmt and
+                    Name('__doc__') in ancestor.children):
+                        return True
+                ancestor = ancestor.parent
+        except AttributeError:
+            return False
+
+    if ismodule(node.parent) or \
+       isdocassignment(node.parent) or \
+       isclassorfunction(node.parent):
+        return True
+    return False
+
+def shouldtransform(node):
+    specialnames = ['__main__']
+
+    if node.value in specialnames:
+        return False
+
+    ggparent = node.parent.parent.parent
+    sggparent = str(ggparent)
+
+    if 'getattr' in sggparent or \
+       'hasattr' in sggparent or \
+       'setattr' in sggparent or \
+       'encode' in sggparent or \
+       'decode' in sggparent:
+           return False
+
+    return True
+
+class FixBytes(fixer_base.BaseFix):
+
+    PATTERN = 'STRING'
+
+    def transform(self, node, results):
+        if self.filename in blacklist:
+            return
+        if node.type == token.STRING:
+            if _re.match(node.value):
+                if isdocstring(node):
+                    return
+                if not shouldtransform(node):
+                    return
+                new = node.clone()
+                new.value = 'b' + new.value
+                return new
+