[clang-format] Allow specifying the language for .h files (#128122)

Closes #128119

Author: owenca

clang/docs/ClangFormatStyleOptions.rst

@@ -4782,7 +4782,13 @@ the configuration (without a prefix: ``Auto``).
 .. _Language:
 
 **Language** (``LanguageKind``) :versionbadge:`clang-format 3.5` :ref:`¶ <Language>`
-  Language, this format style is targeted at.
+  The language that this format style targets.
+
+  .. note::
+
+   You can also specify the language (``Cpp`` or ``ObjC``) for ``.h`` files
+   by adding a ``// clang-format Language:`` line before the first
+   non-comment (and non-empty) line, e.g. ``// clang-format Language: Cpp``.
 
   Possible values:
 

clang/docs/ReleaseNotes.rst

@@ -271,6 +271,9 @@ clang-format
 - Adds ``BreakBeforeTemplateCloser`` option.
 - Adds ``BinPackLongBracedList`` option to override bin packing options in
   long (20 item or more) braced list initializer lists.
+- Allow specifying the language (C++ or Objective-C) for a ``.h`` file by adding
+  a special comment (e.g. ``// clang-format Language: ObjC``) near the top of
+  the file.
 
 libclang
 --------

clang/include/clang/Format/Format.h

@@ -3353,7 +3353,12 @@ struct FormatStyle {
   }
   bool isTableGen() const { return Language == LK_TableGen; }
 
-  /// Language, this format style is targeted at.
+  /// The language that this format style targets.
+  /// \note
+  ///  You can also specify the language (``Cpp`` or ``ObjC``) for ``.h`` files
+  ///  by adding a ``// clang-format Language:`` line before the first
+  ///  non-comment (and non-empty) line, e.g. ``// clang-format Language: Cpp``.
+  /// \endnote
   /// \version 3.5
   LanguageKind Language;
 

clang/lib/Format/Format.cpp

@@ -4021,6 +4021,33 @@ static FormatStyle::LanguageKind getLanguageByFileName(StringRef FileName) {
   return FormatStyle::LK_Cpp;
 }
 
+static FormatStyle::LanguageKind getLanguageByComment(const Environment &Env) {
+  const auto ID = Env.getFileID();
+  const auto &SourceMgr = Env.getSourceManager();
+
+  LangOptions LangOpts;
+  LangOpts.CPlusPlus = 1;
+  LangOpts.LineComment = 1;
+
+  Lexer Lex(ID, SourceMgr.getBufferOrFake(ID), SourceMgr, LangOpts);
+  Lex.SetCommentRetentionState(true);
+
+  for (Token Tok; !Lex.LexFromRawLexer(Tok) && Tok.is(tok::comment);) {
+    auto Text = StringRef(SourceMgr.getCharacterData(Tok.getLocation()),
+                          Tok.getLength());
+    if (!Text.consume_front("// clang-format Language:"))
+      continue;
+
+    Text = Text.trim();
+    if (Text == "Cpp")
+      return FormatStyle::LK_Cpp;
+    if (Text == "ObjC")
+      return FormatStyle::LK_ObjC;
+  }
+
+  return FormatStyle::LK_None;
+}
+
 FormatStyle::LanguageKind guessLanguage(StringRef FileName, StringRef Code) {
   const auto GuessedLanguage = getLanguageByFileName(FileName);
   if (GuessedLanguage == FormatStyle::LK_Cpp) {
@@ -4030,6 +4057,10 @@ FormatStyle::LanguageKind guessLanguage(StringRef FileName, StringRef Code) {
     if (!Code.empty() && (Extension.empty() || Extension == ".h")) {
       auto NonEmptyFileName = FileName.empty() ? "guess.h" : FileName;
       Environment Env(Code, NonEmptyFileName, /*Ranges=*/{});
+      if (const auto Language = getLanguageByComment(Env);
+          Language != FormatStyle::LK_None) {
+        return Language;
+      }
       ObjCHeaderStyleGuesser Guesser(Env, getLLVMStyle());
       Guesser.process();
       if (Guesser.isObjC())

clang/unittests/Format/FormatTest.cpp

@@ -25136,6 +25136,15 @@ TEST_F(FormatTest, GuessLanguageWithChildLines) {
       guessLanguage("foo.h", "#define FOO ({ foo(); ({ NSString *s; }) })"));
 }
 
+TEST_F(FormatTest, GetLanguageByComment) {
+  EXPECT_EQ(FormatStyle::LK_Cpp,
+            guessLanguage("foo.h", "// clang-format Language: Cpp\n"
+                                   "int DoStuff(CGRect rect);"));
+  EXPECT_EQ(FormatStyle::LK_ObjC,
+            guessLanguage("foo.h", "// clang-format Language: ObjC\n"
+                                   "int i;"));
+}
+
 TEST_F(FormatTest, TypenameMacros) {
   std::vector<std::string> TypenameMacros = {"STACK_OF", "LIST", "TAILQ_ENTRY"};