Disallow {foo} in search engine URLs.

This causes an KeyError otherwise when trying to use str.format to insert the
search term.

qutebrowser/config/configtypes.py

@@ -1110,8 +1110,15 @@ class SearchEngineUrl(BaseType):
                 return
             else:
                 raise configexc.ValidationError(value, "may not be empty!")
+
         if '{}' not in value:
             raise configexc.ValidationError(value, "must contain \"{}\"")
+        try:
+            value.format("")
+        except KeyError:
+            raise configexc.ValidationError(
+                value, "may not contain {...} (use {{ and }} for literal {/})")
+
         url = QUrl(value.replace('{}', 'foobar'))
         if not url.isValid():
             raise configexc.ValidationError(value, "invalid url, {}".format(

tests/config/test_configtypes.py

@@ -1881,6 +1881,11 @@ class TestSearchEngineUrl:
         with pytest.raises(configexc.ValidationError):
             self.t.validate(':{}')
 
+    def test_validate_format_string(self):
+        """Test validate with a {foo} format string."""
+        with pytest.raises(configexc.ValidationError):
+            self.t.validate('foo{bar}baz{}')
+
     def test_transform_empty(self):
         """Test transform with an empty value."""
         assert self.t.transform('') is None