Make the 'library' lines in tests use a substitution. (#4278)

This opens the door for replacing all `library ...` lines in toolchain
test files with `library "[[@TEST_NAME]]";`. That's technically more
typing in a lot of cases, but OTOH means we can just do some copy-paste
boilerplate and stop carefully writing library names.

Also cleans up the test setup, because it's getting messy. I'm trying to
make it easier to see the divisions of tests and the output associated
with them. StringSwitch looked like a way to do this, with a few edits
to make it work nicely.

testing/file_test/file_test_base.cpp

@@ -443,7 +443,7 @@ struct SplitState {
   auto has_splits() const -> bool { return file_index > 0; }
 
   auto add_content(llvm::StringRef line) -> void {
-    content.append(line);
+    content.append(line.str());
     content.append("\n");
   }
 
@@ -462,13 +462,63 @@ struct SplitState {
   int file_index = 0;
 };
 
+// Replaces the content keywords.
+//
+// TEST_NAME is the only content keyword at present, but we do validate that
+// other names are reserved.
+static auto ReplaceContentKeywords(llvm::StringRef filename,
+                                   std::string* content) -> ErrorOr<Success> {
+  static constexpr llvm::StringLiteral Prefix = "[[@";
+
+  auto keyword_pos = content->find(Prefix);
+  // Return early if not finding anything.
+  if (keyword_pos == std::string::npos) {
+    return Success();
+  }
+
+  // Construct the test name by getting the base name without the extension,
+  // then removing any "fail_" or "todo_" prefixes.
+  llvm::StringRef test_name = filename;
+  if (auto last_slash = test_name.rfind("/");
+      last_slash != llvm::StringRef::npos) {
+    test_name = test_name.substr(last_slash + 1);
+  }
+  if (auto ext_dot = test_name.find("."); ext_dot != llvm::StringRef::npos) {
+    test_name = test_name.substr(0, ext_dot);
+  }
+  // Note this also handles `fail_todo_` and `todo_fail_`.
+  test_name.consume_front("todo_");
+  test_name.consume_front("fail_");
+  test_name.consume_front("todo_");
+
+  while (keyword_pos != std::string::npos) {
+    static constexpr llvm::StringLiteral TestName = "[[@TEST_NAME]]";
+    auto keyword = llvm::StringRef(*content).substr(keyword_pos);
+    if (keyword.starts_with(TestName)) {
+      content->replace(keyword_pos, TestName.size(), test_name);
+      keyword_pos += test_name.size();
+    } else if (keyword.starts_with("[[@LINE")) {
+      // Just move past the prefix to find the next one.
+      keyword_pos += Prefix.size();
+    } else {
+      return ErrorBuilder()
+             << "Unexpected use of `[[@` at `" << keyword.substr(0, 5) << "`";
+    }
+    keyword_pos = content->find(Prefix, keyword_pos);
+  }
+  return Success();
+}
+
 // Adds a file. Used for both split and unsplit test files.
 static auto AddTestFile(llvm::StringRef filename, std::string* content,
                         llvm::SmallVector<FileTestBase::TestFile>* test_files)
-    -> void {
+    -> ErrorOr<Success> {
+  CARBON_RETURN_IF_ERROR(ReplaceContentKeywords(filename, content));
+
   test_files->push_back(
       {.filename = filename.str(), .content = std::move(*content)});
   content->clear();
+  return Success();
 }
 
 // Process file split ("---") lines when found. Returns true if the line is
@@ -500,7 +550,8 @@ static auto TryConsumeSplit(
 
   // On a file split, add the previous file, then start a new one.
   if (split->has_splits()) {
-    AddTestFile(split->filename, &split->content, test_files);
+    CARBON_RETURN_IF_ERROR(
+        AddTestFile(split->filename, &split->content, test_files));
   } else {
     split->content.clear();
     if (split->found_code_pre_split) {
@@ -646,14 +697,14 @@ static auto TransformExpectation(int line_index, llvm::StringRef in)
 // Once all content is processed, do any remaining split processing.
 static auto FinishSplit(llvm::StringRef test_name, SplitState* split,
                         llvm::SmallVector<FileTestBase::TestFile>* test_files)
-    -> void {
+    -> ErrorOr<Success> {
   if (split->has_splits()) {
-    AddTestFile(split->filename, &split->content, test_files);
+    return AddTestFile(split->filename, &split->content, test_files);
   } else {
     // If no file splitting happened, use the main file as the test file.
     // There will always be a `/` unless tests are in the repo root.
-    AddTestFile(test_name.drop_front(test_name.rfind("/") + 1), &split->content,
-                test_files);
+    return AddTestFile(test_name.drop_front(test_name.rfind("/") + 1),
+                       &split->content, test_files);
   }
 }
 
@@ -826,7 +877,7 @@ auto FileTestBase::ProcessTestFile(TestContext& context) -> ErrorOr<Success> {
   }
 
   context.has_splits = split.has_splits();
-  FinishSplit(test_name_, &split, &context.test_files);
+  CARBON_RETURN_IF_ERROR(FinishSplit(test_name_, &split, &context.test_files));
 
   // Assume there is always a suffix `\n` in output.
   if (!context.expected_stdout.empty()) {