Support parsing and testing unimplemented expressions (#957)

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/syntax/parse_test_matchers.h

@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef EXECUTABLE_SEMANTICS_SYNTAX_PARSE_TEST_MATCHERS_H_
+#define EXECUTABLE_SEMANTICS_SYNTAX_PARSE_TEST_MATCHERS_H_
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "executable_semantics/syntax/parse_test_matchers_internal.h"
+
+namespace Carbon {
+
+// Matches the return value of `Parse()` if it represents a successful parse
+// whose output matches the given matcher.
+inline auto ParsedAs(::testing::Matcher<AST> ast_matcher) {
+  return TestingInternal::ParsedAsMatcher(std::move(ast_matcher));
+}
+
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_SYNTAX_PARSE_TEST_MATCHERS_H_