gh-1436: Fix incorrectly rejecting trailing comma after **rest (#1438)

* Add a unittest to expose the issue reported in gh-1436

* Fix issue reported in gh-1436

* formatting

* Add roundtrip tests for trailing comma after `**rest`

Author: itamaro

libcst/_nodes/statement.py

@@ -3245,8 +3245,6 @@ class MatchMapping(MatchPattern):
     rpar: Sequence[RightParen] = ()
 
     def _validate(self) -> None:
-        if isinstance(self.trailing_comma, Comma) and self.rest is not None:
-            raise CSTValidationError("Cannot have a trailing comma without **rest")
         super(MatchMapping, self)._validate()
 
     def _visit_and_replace_children(self, visitor: CSTVisitorT) -> "MatchMapping":

libcst/_nodes/tests/test_match.py

@@ -226,6 +226,22 @@ class MatchTest(CSTNodeTest):
                             ),
                             body=cst.SimpleStatementSuite((cst.Pass(),)),
                         ),
+                        cst.MatchCase(  # rest with trailing comma - valid syntax (issue #1436)
+                            pattern=cst.MatchMapping(
+                                [
+                                    cst.MatchMappingElement(
+                                        key=cst.SimpleString('"a"'),
+                                        pattern=cst.MatchValue(cst.Integer("1")),
+                                        comma=cst.Comma(
+                                            whitespace_after=cst.SimpleWhitespace(" ")
+                                        ),
+                                    ),
+                                ],
+                                rest=cst.Name("keys"),
+                                trailing_comma=cst.Comma(),
+                            ),
+                            body=cst.SimpleStatementSuite((cst.Pass(),)),
+                        ),
                     ],
                 ),
                 "code": (
@@ -234,6 +250,7 @@ class MatchTest(CSTNodeTest):
                     + '    case {"a": None,"b": None}: pass\n'
                     + '    case {"a": None,}: pass\n'
                     + "    case {**rest}: pass\n"
+                    + '    case {"a": 1, **keys,}: pass\n'
                 ),
                 "parser": parser,
             },

native/libcst/tests/fixtures/malicious_match.py

@@ -39,4 +39,6 @@
     case 1, 2: pass
     case ( Foo  (   )    ) : pass
     case (lol)  if (  True , )  :pass
+    case {"a": 1, **keys,} : pass
+    case {**rest,} : pass