Extract a test helper to its own library. (#2828)

This is a convenient test helper for anything that can use injected
streams. Extract this to where it can be used by other tests and add
some basic tests, mostly documenting how it works.

common/BUILD

@@ -146,6 +146,26 @@ cc_test(
     ],
 )
 
+cc_library(
+    name = "test_raw_ostream",
+    testonly = 1,
+    hdrs = ["test_raw_ostream.h"],
+    deps = [
+        ":ostream",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_test(
+    name = "test_raw_ostream_test",
+    srcs = ["test_raw_ostream_test.cpp"],
+    deps = [
+        ":test_raw_ostream",
+        "//common:gtest_main",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
 cc_library(
     name = "vlog",
     srcs = ["vlog_internal.h"],

common/test_raw_ostream.h

@@ -0,0 +1,41 @@
+// 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 CARBON_COMMON_TEST_RAW_OSTREAM_H_
+#define CARBON_COMMON_TEST_RAW_OSTREAM_H_
+
+#include <gtest/gtest.h>
+
+#include <string>
+
+#include "common/ostream.h"
+
+namespace Carbon::Testing {
+
+// A raw_ostream that makes it easy to repeatedly check streamed output.
+class TestRawOstream : public llvm::raw_string_ostream {
+ public:
+  explicit TestRawOstream() : llvm::raw_string_ostream(buffer_) {}
+
+  ~TestRawOstream() override {
+    if (!buffer_.empty()) {
+      ADD_FAILURE() << "Unchecked output:\n" << buffer_;
+    }
+  }
+
+  // Flushes the stream and returns the contents so far, clearing the stream
+  // back to empty.
+  auto TakeStr() -> std::string {
+    std::string result = std::move(buffer_);
+    buffer_.clear();
+    return result;
+  }
+
+ private:
+  std::string buffer_;
+};
+
+}  // namespace Carbon::Testing
+
+#endif  // CARBON_COMMON_TEST_RAW_OSTREAM_H_

common/test_raw_ostream_test.cpp

@@ -0,0 +1,63 @@
+// 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
+
+#include "common/test_raw_ostream.h"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+namespace Carbon::Testing {
+namespace {
+
+using ::testing::HasSubstr;
+using ::testing::StrEq;
+
+TEST(TestRawOstreamTest, Basics) {
+  TestRawOstream os;
+
+  os << "test 1";
+  EXPECT_THAT(os.TakeStr(), StrEq("test 1"));
+
+  os << "test"
+     << " "
+     << "2";
+  EXPECT_THAT(os.TakeStr(), StrEq("test 2"));
+
+  os << "test";
+  os << " ";
+  os << "3";
+  EXPECT_THAT(os.TakeStr(), StrEq("test 3"));
+}
+
+TEST(TestRawOstreamTest, MultipleStreams) {
+  TestRawOstream os1;
+  TestRawOstream os2;
+
+  os1 << "test ";
+  os2 << "test stream 2";
+  os1 << "stream 1";
+  EXPECT_THAT(os1.TakeStr(), StrEq("test stream 1"));
+  EXPECT_THAT(os2.TakeStr(), StrEq("test stream 2"));
+}
+
+TEST(TestRawOstreamTest, MultipleLines) {
+  TestRawOstream os;
+
+  os << "test line 1\n";
+  os << "test line 2\n";
+  os << "test line 3\n";
+  EXPECT_THAT(os.TakeStr(), StrEq("test line 1\ntest line 2\ntest line 3\n"));
+}
+
+TEST(TestRawOstreamTest, Substring) {
+  TestRawOstream os;
+
+  os << "test line 1\n";
+  os << "test line 2\n";
+  os << "test line 3\n";
+  EXPECT_THAT(os.TakeStr(), HasSubstr("test line 2"));
+}
+
+}  // namespace
+}  // namespace Carbon::Testing

toolchain/driver/BUILD

@@ -33,6 +33,7 @@ cc_test(
     deps = [
         ":driver",
         "//common:gtest_main",
+        "//common:test_raw_ostream",
         "//toolchain/common:yaml_test_helpers",
         "//toolchain/diagnostics:diagnostic_emitter",
         "//toolchain/lexer:tokenized_buffer_test_helpers",

toolchain/driver/driver_test.cpp

@@ -7,6 +7,7 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
+#include "common/test_raw_ostream.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/SourceMgr.h"
@@ -20,40 +21,9 @@ using ::testing::ElementsAre;
 using ::testing::HasSubstr;
 using ::testing::StrEq;
 
-/// A raw_ostream that makes it easy to repeatedly check streamed output.
-class RawTestOstream : public llvm::raw_ostream {
- public:
-  ~RawTestOstream() override {
-    flush();
-    if (!buffer_.empty()) {
-      ADD_FAILURE() << "Unchecked output:\n" << buffer_;
-    }
-  }
-
-  /// Flushes the stream and returns the contents so far, clearing the stream
-  /// back to empty.
-  auto TakeStr() -> std::string {
-    flush();
-    std::string result = std::move(buffer_);
-    buffer_.clear();
-    return result;
-  }
-
- private:
-  void write_impl(const char* ptr, size_t size) override {
-    buffer_.append(ptr, ptr + size);
-  }
-
-  [[nodiscard]] auto current_pos() const -> uint64_t override {
-    return buffer_.size();
-  }
-
-  std::string buffer_;
-};
-
 TEST(DriverTest, FullCommandErrors) {
-  RawTestOstream test_output_stream;
-  RawTestOstream test_error_stream;
+  TestRawOstream test_output_stream;
+  TestRawOstream test_error_stream;
   Driver driver = Driver(test_output_stream, test_error_stream);
 
   EXPECT_FALSE(driver.RunFullCommand({}));
@@ -67,8 +37,8 @@ TEST(DriverTest, FullCommandErrors) {
 }
 
 TEST(DriverTest, Help) {
-  RawTestOstream test_output_stream;
-  RawTestOstream test_error_stream;
+  TestRawOstream test_output_stream;
+  TestRawOstream test_error_stream;
   Driver driver = Driver(test_output_stream, test_error_stream);
 
   EXPECT_TRUE(driver.RunHelpSubcommand(ConsoleDiagnosticConsumer(), {}));
@@ -87,8 +57,8 @@ TEST(DriverTest, Help) {
 }
 
 TEST(DriverTest, HelpErrors) {
-  RawTestOstream test_output_stream;
-  RawTestOstream test_error_stream;
+  TestRawOstream test_output_stream;
+  TestRawOstream test_error_stream;
   Driver driver = Driver(test_output_stream, test_error_stream);
 
   EXPECT_FALSE(driver.RunHelpSubcommand(ConsoleDiagnosticConsumer(), {"foo"}));
@@ -122,8 +92,8 @@ auto CreateTestFile(llvm::StringRef text) -> std::string {
 }
 
 TEST(DriverTest, DumpTokens) {
-  RawTestOstream test_output_stream;
-  RawTestOstream test_error_stream;
+  TestRawOstream test_output_stream;
+  TestRawOstream test_error_stream;
   Driver driver = Driver(test_output_stream, test_error_stream);
 
   auto test_file_path = CreateTestFile("Hello World");
@@ -164,8 +134,8 @@ TEST(DriverTest, DumpTokens) {
 }
 
 TEST(DriverTest, DumpErrors) {
-  RawTestOstream test_output_stream;
-  RawTestOstream test_error_stream;
+  TestRawOstream test_output_stream;
+  TestRawOstream test_error_stream;
   Driver driver = Driver(test_output_stream, test_error_stream);
 
   EXPECT_FALSE(driver.RunDumpSubcommand(ConsoleDiagnosticConsumer(), {"foo"}));
@@ -189,8 +159,8 @@ TEST(DriverTest, DumpErrors) {
 }
 
 TEST(DriverTest, DumpParseTree) {
-  RawTestOstream test_output_stream;
-  RawTestOstream test_error_stream;
+  TestRawOstream test_output_stream;
+  TestRawOstream test_error_stream;
   Driver driver = Driver(test_output_stream, test_error_stream);
 
   auto test_file_path = CreateTestFile("var v: Int = 42;");