Support parsing and testing unimplemented expressions (#957)

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

executable_semantics/syntax/parse_test_matchers_internal.h

@@ -0,0 +1,59 @@
+// 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_INTERNAL_H_
+#define EXECUTABLE_SEMANTICS_SYNTAX_PARSE_TEST_MATCHERS_INTERNAL_H_
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <ostream>
+#include <variant>
+
+#include "executable_semantics/syntax/parse.h"
+
+namespace Carbon {
+namespace TestingInternal {
+
+// Implementation of ParsedAs(). See there for detailed documentation.
+class ParsedAsMatcher {
+ public:
+  using is_gtest_matcher = void;
+
+  explicit ParsedAsMatcher(::testing::Matcher<AST> ast_matcher)
+      : ast_matcher_(std::move(ast_matcher)) {}
+
+  void DescribeTo(std::ostream* out) const {
+    DescribeToImpl(out, /*negated=*/false);
+  }
+
+  void DescribeNegationTo(std::ostream* out) const {
+    DescribeToImpl(out, /*negated=*/true);
+  }
+
+  auto MatchAndExplain(const std::variant<AST, SyntaxErrorCode>& result,
+                       ::testing::MatchResultListener* listener) const -> bool {
+    if (std::holds_alternative<SyntaxErrorCode>(result)) {
+      *listener << "holds error code " << std::get<SyntaxErrorCode>(result);
+      return false;
+    } else {
+      *listener << "is a successful parse whose ";
+      return ast_matcher_.MatchAndExplain(std::get<AST>(result), listener);
+    }
+  }
+
+ private:
+  void DescribeToImpl(std::ostream* out, bool negated) const {
+    *out << "is " << (negated ? "not " : "")
+         << "a successful parse result whose ";
+    ast_matcher_.DescribeTo(out);
+  }
+
+  ::testing::Matcher<AST> ast_matcher_;
+};
+
+}  // namespace TestingInternal
+}  // namespace Carbon
+
+#endif  // EXECUTABLE_SEMANTICS_SYNTAX_PARSE_TEST_MATCHERS_INTERNAL_H_