Use AST to start inserting fn (#530)

migrate_cpp/cpp_refactoring/BUILD

@@ -0,0 +1,16 @@
+# 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
+
+load("@rules_cc//cc:defs.bzl", "cc_binary")
+
+package(default_visibility = ["//visibility:public"])
+
+cc_binary(
+    name = "cpp_refactoring",
+    srcs = ["main.cpp"],
+    deps = [
+        "@llvm-project//clang:ast_matchers",
+        "@llvm-project//clang:tooling",
+    ],
+)

migrate_cpp/cpp_refactoring/main.cpp

@@ -0,0 +1,96 @@
+// 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 "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/Tooling/CommonOptionsParser.h"
+#include "clang/Tooling/Refactoring.h"
+
+namespace cam = ::clang::ast_matchers;
+namespace ct = ::clang::tooling;
+
+namespace carbon {
+
+class Matcher : public cam::MatchFinder::MatchCallback {
+ public:
+  explicit Matcher(std::map<std::string, ct::Replacements>* in_replacements)
+      : replacements(in_replacements) {}
+
+  virtual ~Matcher() {}
+
+  void AddReplacement(const clang::SourceManager& sm,
+                      clang::CharSourceRange range,
+                      llvm::StringRef replacement_text) {
+    if (!range.isValid()) {
+      llvm::errs() << "Invalid range: " << range.getAsRange().printToString(sm)
+                   << "\n";
+      return;
+    }
+    if (sm.getDecomposedLoc(range.getBegin()).first !=
+        sm.getDecomposedLoc(range.getEnd()).first) {
+      llvm::errs() << "Range spans macro expansions: "
+                   << range.getAsRange().printToString(sm) << "\n";
+      return;
+    }
+    if (sm.getFileID(range.getBegin()) != sm.getFileID(range.getEnd())) {
+      llvm::errs() << "Range spans files: "
+                   << range.getAsRange().printToString(sm) << "\n";
+      return;
+    }
+
+    auto rep =
+        ct::Replacement(sm, sm.getExpansionRange(range), replacement_text);
+    auto err = (*replacements)[std::string(rep.getFilePath())].add(rep);
+    if (err) {
+      llvm::report_fatal_error("Error with replacement `" + rep.toString() +
+                               "`: " + llvm::toString(std::move(err)) + "\n");
+    }
+  }
+
+ private:
+  std::map<std::string, ct::Replacements>* replacements;
+};
+
+class FnInserter : public Matcher {
+ public:
+  explicit FnInserter(std::map<std::string, ct::Replacements>* in_replacements,
+                      cam::MatchFinder* finder)
+      : Matcher(in_replacements) {
+    finder->addMatcher(cam::functionDecl(cam::isExpansionInMainFile(),
+                                         cam::hasTrailingReturn())
+                           .bind(Label),
+                       this);
+  }
+
+  void run(const cam::MatchFinder::MatchResult& result) override {
+    // The matched 'if' statement was bound to 'ifStmt'.
+    const auto* decl = result.Nodes.getNodeAs<clang::FunctionDecl>(Label);
+    if (!decl) {
+      llvm::report_fatal_error(std::string("getNodeAs failed for ") + Label);
+    }
+    auto begin = decl->getBeginLoc();
+    // Replace the first token in the range, `auto`.
+    auto range = clang::CharSourceRange::getTokenRange(begin, begin);
+    AddReplacement(*(result.SourceManager), range, "fn");
+  }
+
+ private:
+  static constexpr char Label[] = "FnInserter";
+};
+
+}  // namespace carbon
+
+auto main(int argc, const char** argv) -> int {
+  llvm::cl::OptionCategory category("C++ refactoring options");
+  clang::tooling::CommonOptionsParser op(argc, argv, category);
+  clang::tooling::RefactoringTool tool(op.getCompilations(),
+                                       op.getSourcePathList());
+
+  // Set up AST matcher callbacks.
+  cam::MatchFinder finder;
+  carbon::FnInserter fn_inserter(&tool.getReplacements(), &finder);
+
+  return tool.runAndSave(
+      clang::tooling::newFrontendActionFactory(&finder).get());
+}