Validate config separately from migrations

It makes for cleaner code, and it makes sure the target of renamed options
actually exists.

qutebrowser/config/configfiles.py

@@ -166,6 +166,7 @@ class YamlConfig(QObject):
         self._dirty = False
 
         self._handle_migrations()
+        self._validate()
 
     def _handle_migrations(self):
         """Handle unknown/renamed keys."""
@@ -178,9 +179,11 @@ class YamlConfig(QObject):
             elif name in configdata.MIGRATIONS.deleted:
                 log.config.debug("Removing {}".format(name))
                 del self._values[name]
-            elif name in configdata.DATA:
-                pass
-            else:
+
+    def _validate(self):
+        """Make sure all settings exist."""
+        for name in self._values:
+            if name not in configdata.DATA:
                 desc = configexc.ConfigErrorDesc(
                     "While loading options",
                     "Unknown option {}".format(name))

tests/unit/config/test_configfiles.py

@@ -145,7 +145,8 @@ class TestYaml:
         autoconfig = config_tmpdir / 'autoconfig.yml'
         autoconfig.write_text('global:\n  old: value', encoding='utf-8')
 
-        monkeypatch.setattr(configdata.MIGRATIONS, 'renamed', {'old': 'new'})
+        monkeypatch.setattr(configdata.MIGRATIONS, 'renamed',
+                            {'old': 'tabs.show'})
 
         yaml.load()
         yaml._save()
@@ -154,6 +155,22 @@ class TestYaml:
         assert '  old:' not in lines
         assert '  new:' not in lines
 
+    def test_renamed_key_unknown_target(self, monkeypatch, yaml, config_tmpdir):
+        """A key marked as renamed with invalid name should raise an error."""
+        autoconfig = config_tmpdir / 'autoconfig.yml'
+        autoconfig.write_text('global:\n  old: value', encoding='utf-8')
+
+        monkeypatch.setattr(configdata.MIGRATIONS, 'renamed',
+                            {'old': 'new'})
+
+        with pytest.raises(configexc.ConfigFileErrors) as excinfo:
+            yaml.load()
+
+        assert len(excinfo.value.errors) == 1
+        error = excinfo.value.errors[0]
+        assert error.text == "While loading options"
+        assert str(error.exception) == "Unknown option new"
+
     @pytest.mark.parametrize('old_config', [
         None,
         'global:\n  colors.hints.fg: magenta',