[feaLib] Round-trip useExtension keyword

Lib/fontTools/feaLib/ast.py

@@ -293,7 +293,10 @@ class FeatureBlock(Block):
         builder.end_feature()
 
     def asFea(self, indent=""):
-        res = indent + "feature %s {\n" % self.name.strip()
+        res = indent + "feature %s " % self.name.strip()
+        if self.use_extension:
+            res += "useExtension "
+        res += "{\n"
         res += Block.asFea(self, indent=indent)
         res += indent + "} %s;\n" % self.name.strip()
         return res
@@ -329,7 +332,10 @@ class LookupBlock(Block):
         builder.end_lookup_block()
 
     def asFea(self, indent=""):
-        res = "lookup {} {{\n".format(self.name)
+        res = "lookup {} ".format(self.name)
+        if self.use_extension:
+            res += "useExtension "
+        res += "{\n"
         res += Block.asFea(self, indent=indent)
         res += "{}}} {};\n".format(indent, self.name)
         return res

Tests/feaLib/parser_test.py

@@ -215,6 +215,8 @@ class ParserTest(unittest.TestCase):
         [liga] = self.parse("feature liga useExtension {} liga;").statements
         self.assertEqual(liga.name, "liga")
         self.assertTrue(liga.use_extension)
+        self.assertEqual(liga.asFea(),
+                         "feature liga useExtension {\n    \n} liga;\n")
 
     def test_feature_comment(self):
         [liga] = self.parse("feature liga { # Comment\n } liga;").statements
@@ -608,6 +610,8 @@ class ParserTest(unittest.TestCase):
         [lookup] = self.parse("lookup Foo useExtension {} Foo;").statements
         self.assertEqual(lookup.name, "Foo")
         self.assertTrue(lookup.use_extension)
+        self.assertEqual(lookup.asFea(),
+                         "lookup Foo useExtension {\n    \n} Foo;\n")
 
     def test_lookup_block_name_mismatch(self):
         self.assertRaisesRegex(