[feaLib] Raise exception when GSUB statement doesn't match a rule. (#1876)

* [feaLib] Raise exception when substitute statement doesnt match any of the rules. Add tests that trigger said exception.

Lib/fontTools/feaLib/parser.py

@@ -826,8 +826,14 @@ class Parser(object):
                 'is not supported',
                 location)
 
+        # If there are remaining glyphs to parse, this is an invalid GSUB statement
+        if len(new) != 0:
+            raise FeatureLibError(
+                'Invalid substitution statement',
+                location
+            )
+
         # GSUB lookup type 6: Chaining contextual substitution.
-        assert len(new) == 0, new
         rule = self.ast.ChainContextSubstStatement(
             old_prefix, old, old_suffix, lookups, location=location)
         return rule

Tests/feaLib/data/GSUB_error.fea

@@ -0,0 +1,13 @@
+# Trigger a parser error in the function parse_substitute_ in order to improve the error message.
+# Note that this is not a valid substitution, this test is made to trigger an error.
+
+languagesystem latn dflt;
+
+@base = [a e];
+@accents = [acute grave];
+
+feature abvs {
+    lookup lookup1 {
+        sub @base @accents by @base;
+    } lookup1;
+} abvs;

Tests/feaLib/parser_test.py

@@ -1503,6 +1503,13 @@ class ParserTest(unittest.TestCase):
             'Expected "by", "from" or explicit lookup references',
             self.parse, "feature liga {substitute f f i;} liga;")
     
+    def test_substitute_invalid_statement(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            "Invalid substitution statement",
+            Parser(self.getpath("GSUB_error.fea"), GLYPHNAMES).parse
+        )
+
     def test_subtable(self):
         doc = self.parse("feature test {subtable;} test;")
         s = doc.statements[0].statements[0]