Add RawStringOstream for slightly simpler streaming to strings (#4817)

This adds a RawStringOstream. Versus TestRawOstream, which is
consolidated over to RawStringOstream, it uses a string for storage
instead of a vector, mainly to support move-to-string semantics. Versus
llvm::raw_string_ostream, it owns the string and supports pwrite (which
is needed for driver and its fd_ostream compatibility requirement).

This converts most uses of llvm::raw_string_ostream, leaving behind a
few in InstNamer that explicitly cannot own the string, such as:

```
     llvm::raw_string_ostream(name)
          << "_" << tree.tokens().GetColumnNumber(token);
```

I have this as its own library so that it can use CHECK.

Yes this doesn't save much code, but it's code we repeatedly write.

---------

Co-authored-by: Geoff Romer <gromer@google.com>

testing/base/BUILD

@@ -53,6 +53,7 @@ cc_library(
     deps = [
         "//common:check",
         "//common:map",
+        "//common:raw_string_ostream",
         "//common:set",
         "//toolchain/lex:token_kind",
         "@abseil-cpp//absl/random",
@@ -90,28 +91,6 @@ cc_binary(
     ],
 )
 
-cc_library(
-    name = "test_raw_ostream",
-    testonly = 1,
-    hdrs = ["test_raw_ostream.h"],
-    deps = [
-        "//common:ostream",
-        "@googletest//:gtest",
-        "@llvm-project//llvm:Support",
-    ],
-)
-
-cc_test(
-    name = "test_raw_ostream_test",
-    size = "small",
-    srcs = ["test_raw_ostream_test.cpp"],
-    deps = [
-        ":test_raw_ostream",
-        "//testing/base:gtest_main",
-        "@googletest//:gtest",
-    ],
-)
-
 cc_library(
     name = "global_exe_path",
     testonly = 1,

testing/base/source_gen.cpp

@@ -6,6 +6,7 @@
 
 #include <numeric>
 
+#include "common/raw_string_ostream.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/Sequence.h"
 #include "llvm/ADT/SmallVector.h"
@@ -311,8 +312,7 @@ static auto EstimateAvgClassDefLines(SourceGen::ClassParams params) -> double {
 auto SourceGen::GenAPIFileDenseDecls(int target_lines,
                                      const DenseDeclParams& params)
     -> std::string {
-  std::string source;
-  llvm::raw_string_ostream os(source);
+  RawStringOstream source;
 
   // Figure out how many classes fit in our target lines, each separated by a
   // blank line. We need to account the comment lines below to start the file.
@@ -327,27 +327,29 @@ auto SourceGen::GenAPIFileDenseDecls(int target_lines,
   int expected_lines =
       NumFileCommentLines + num_classes * (avg_class_lines + 1);
 
-  os << "// Generated " << (!IsCpp() ? "Carbon" : "C++") << " source file.\n";
-  os << llvm::formatv("// {0} target lines: {1} classes, {2} expected lines",
-                      target_lines, num_classes, expected_lines)
-     << "\n";
-  os << "//\n// Generating as an API file with dense declarations.\n";
+  source << "// Generated " << (!IsCpp() ? "Carbon" : "C++")
+         << " source file.\n";
+  source << llvm::formatv(
+                "// {0} target lines: {1} classes, {2} expected lines",
+                target_lines, num_classes, expected_lines)
+         << "\n";
+  source << "//\n// Generating as an API file with dense declarations.\n";
 
   // Carbon uses an implicitly imported prelude to get builtin types, but C++
   // requires header files so include those.
   if (IsCpp()) {
-    os << "\n";
+    source << "\n";
     // Header for specific integer types like `std::int64_t`.
-    os << "#include <cstdint>\n";
+    source << "#include <cstdint>\n";
     // Header for `std::pair`.
-    os << "#include <utility>\n";
+    source << "#include <utility>\n";
   }
 
   auto class_gen_state = ClassGenState(*this, num_classes, params.class_params,
                                        params.type_use_params);
   for ([[maybe_unused]] int _ : llvm::seq(num_classes)) {
-    os << "\n";
-    GenerateClassDef(params.class_params, class_gen_state, os);
+    source << "\n";
+    GenerateClassDef(params.class_params, class_gen_state, source);
   }
 
   // Make sure we consumed all the state.
@@ -358,7 +360,7 @@ auto SourceGen::GenAPIFileDenseDecls(int target_lines,
   CARBON_CHECK(class_gen_state.class_names().empty());
   CARBON_CHECK(class_gen_state.type_names().empty());
 
-  return source;
+  return source.TakeStr();
 }
 
 auto SourceGen::GetShuffledIdentifiers(int number, int min_length,

testing/base/test_raw_ostream.h

@@ -1,42 +0,0 @@
-// 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_TESTING_BASE_TEST_RAW_OSTREAM_H_
-#define CARBON_TESTING_BASE_TEST_RAW_OSTREAM_H_
-
-#include <gtest/gtest.h>
-
-#include <string>
-
-#include "common/ostream.h"
-#include "llvm/ADT/SmallString.h"
-
-namespace Carbon::Testing {
-
-// A raw_ostream that makes it easy to repeatedly check streamed output.
-class TestRawOstream : public llvm::raw_svector_ostream {
- public:
-  explicit TestRawOstream() : llvm::raw_svector_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 = buffer_.str().str();
-    buffer_.clear();
-    return result;
-  }
-
- private:
-  llvm::SmallString<16> buffer_;
-};
-
-}  // namespace Carbon::Testing
-
-#endif  // CARBON_TESTING_BASE_TEST_RAW_OSTREAM_H_

testing/base/test_raw_ostream_test.cpp

@@ -1,63 +0,0 @@
-// 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 "testing/base/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