Implement #pragma clang final extension

This patch adds a new preprocessor extension ``#pragma clang final``
which enables warning on undefinition and re-definition of macros.

The intent of this warning is to extend beyond ``-Wmacro-redefined`` to
warn against any and all alterations to macros that are marked `final`.

This warning is part of the ``-Wpedantic-macros`` diagnostics group.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D108567

clang/docs/LanguageExtensions.rst

@@ -3944,6 +3944,24 @@ will expansion of the macro within the main source file. For example:
 
 This warning is controlled by ``-Wpedantic-macros``.
 
+Final Macros
+============
+
+Clang supports the pragma ``#pragma clang final``, which can be used to
+mark macros as final, meaning they cannot be undef'd or re-defined. For example:
+
+.. code-block:: c
+   #define FINAL_MACRO 1
+   #pragma clang final(FINAL_MACRO)
+
+   #define FINAL_MACRO // warning: FINAL_MACRO is marked final and should not be redefined
+   #undef FINAL_MACRO  // warning: FINAL_MACRO is marked final and should not be undefined
+
+This is useful for enforcing system-provided macros that should not be altered
+in user headers or code. This is controlled by ``-Wpedantic-macros``. Final 
+macros will always warn on redefinition, including situations with identical
+bodies and in system headers.
+
 Extended Integer Types
 ======================
 

clang/include/clang/Basic/DiagnosticGroups.td

@@ -648,6 +648,7 @@ def KeywordAsMacro : DiagGroup<"keyword-macro">;
 def ReservedIdAsMacro : DiagGroup<"reserved-macro-identifier">;
 def ReservedIdAsMacroAlias : DiagGroup<"reserved-id-macro", [ReservedIdAsMacro]>;
 def RestrictExpansionMacro : DiagGroup<"restrict-expansion">;
+def FinalMacro : DiagGroup<"final-macro">;
 
 // Just silence warnings about -Wstrict-aliasing for now.
 def : DiagGroup<"strict-aliasing=0">;
@@ -1322,4 +1323,5 @@ def PedanticMacros : DiagGroup<"pedantic-macros",
                     [DeprecatedPragma,
                      MacroRedefined,
                      BuiltinMacroRedefined,
-                     RestrictExpansionMacro]>;
+                     RestrictExpansionMacro,
+                     FinalMacro]>;

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -551,7 +551,13 @@ def warn_pragma_restrict_expansion_macro_use :
 
 // - Note for macro annotations.
 def note_pp_macro_annotation :
-  Note<"macro marked '%select{deprecated|restrict_expansion}0' here">;
+  Note<"macro marked '%select{deprecated|restrict_expansion|final}0' here">;
+
+// - #pragma clang final(...)
+def warn_pragma_final_macro :
+  ExtWarn<"macro %0 has been marked as final and should not be "
+          "%select{undefined|redefined}1">,
+  InGroup<FinalMacro>;
 
 // - #pragma execution_character_set(...)
 def warn_pragma_exec_charset_expected :

clang/include/clang/Basic/IdentifierTable.h

@@ -127,7 +127,10 @@ class alignas(IdentifierInfoAlignment) IdentifierInfo {
   // True if this macro is unsafe in headers.
   unsigned IsRestrictExpansion : 1;
 
-  // 23 bits left in a 64-bit word.
+  // True if this macro is final.
+  unsigned IsFinal : 1;
+
+  // 22 bits left in a 64-bit word.
 
   // Managed by the language front-end.
   void *FETokenInfo = nullptr;
@@ -141,7 +144,7 @@ class alignas(IdentifierInfoAlignment) IdentifierInfo {
         NeedsHandleIdentifier(false), IsFromAST(false), ChangedAfterLoad(false),
         FEChangedAfterLoad(false), RevertedTokenID(false), OutOfDate(false),
         IsModulesImport(false), IsMangledOpenMPVariantName(false),
-        IsDeprecatedMacro(false), IsRestrictExpansion(false) {}
+        IsDeprecatedMacro(false), IsRestrictExpansion(false), IsFinal(false) {}
 
 public:
   IdentifierInfo(const IdentifierInfo &) = delete;
@@ -189,10 +192,14 @@ public:
       NeedsHandleIdentifier = true;
       HadMacro = true;
     } else {
+      // If this is a final macro, make the deprecation and header unsafe bits
+      // stick around after the undefinition so they apply to any redefinitions.
+      if (!IsFinal) {
         // Because calling the setters of these calls recomputes, just set them
         // manually to avoid recomputing a bunch of times.
         IsDeprecatedMacro = false;
         IsRestrictExpansion = false;
+      }
       RecomputeNeedsHandleIdentifier();
     }
   }
@@ -227,6 +234,10 @@ public:
       RecomputeNeedsHandleIdentifier();
   }
 
+  bool isFinal() const { return IsFinal; }
+
+  void setIsFinal(bool Val) { IsFinal = Val; }
+
   /// If this is a source-language token (e.g. 'for'), this API
   /// can be used to cause the lexer to map identifiers to source-language
   /// tokens.

clang/include/clang/Lex/Preprocessor.h

@@ -793,12 +793,35 @@ private:
   /// annotation pragma for use producing diagnostics and notes.
   using MsgLocationPair = std::pair<std::string, SourceLocation>;
 
-  /// Deprecation messages for macros provided in #pragma clang deprecated.
+  struct MacroAnnotationInfo {
-  llvm::DenseMap<const IdentifierInfo *, std::string> MacroDeprecationMsgs;
+    SourceLocation Location;
+    std::string Message;
+  };
 
-  /// Usage warning for macros marked by #pragma clang restrict_expansion.
+  struct MacroAnnotations {
-  llvm::DenseMap<const IdentifierInfo *, MsgLocationPair>
+    llvm::Optional<MacroAnnotationInfo> DeprecationInfo;
-      RestrictExpansionMacroMsgs;
+    llvm::Optional<MacroAnnotationInfo> RestrictExpansionInfo;
+    llvm::Optional<SourceLocation> FinalAnnotationLoc;
+
+    static MacroAnnotations makeDeprecation(SourceLocation Loc,
+                                            std::string Msg) {
+      return MacroAnnotations{MacroAnnotationInfo{Loc, std::move(Msg)},
+                              llvm::None, llvm::None};
+    }
+
+    static MacroAnnotations makeRestrictExpansion(SourceLocation Loc,
+                                                  std::string Msg) {
+      return MacroAnnotations{
+          llvm::None, MacroAnnotationInfo{Loc, std::move(Msg)}, llvm::None};
+    }
+
+    static MacroAnnotations makeFinal(SourceLocation Loc) {
+      return MacroAnnotations{llvm::None, llvm::None, Loc};
+    }
+  };
+
+  /// Warning information for macro annotations.
+  llvm::DenseMap<const IdentifierInfo *, MacroAnnotations> AnnotationInfos;
 
   /// A "freelist" of MacroArg objects that can be
   /// reused for quick allocation.
@@ -2402,39 +2425,56 @@ public:
   /// warnings.
   void markMacroAsUsed(MacroInfo *MI);
 
-  void addMacroDeprecationMsg(const IdentifierInfo *II, std::string Msg) {
+  void addMacroDeprecationMsg(const IdentifierInfo *II, std::string Msg,
-    MacroDeprecationMsgs.insert(std::make_pair(II, Msg));
+                              SourceLocation AnnotationLoc) {
-  }
+    auto Annotations = AnnotationInfos.find(II);
-
+    if (Annotations == AnnotationInfos.end())
-  llvm::Optional<std::string> getMacroDeprecationMsg(const IdentifierInfo *II) {
+      AnnotationInfos.insert(std::make_pair(
-    auto MsgEntry = MacroDeprecationMsgs.find(II);
+          II,
-    if (MsgEntry == MacroDeprecationMsgs.end())
+          MacroAnnotations::makeDeprecation(AnnotationLoc, std::move(Msg))));
-      return llvm::None;
+    else
-    return MsgEntry->second;
+      Annotations->second.DeprecationInfo =
+          MacroAnnotationInfo{AnnotationLoc, std::move(Msg)};
   }
 
   void addRestrictExpansionMsg(const IdentifierInfo *II, std::string Msg,
                                SourceLocation AnnotationLoc) {
-    RestrictExpansionMacroMsgs.insert(
+    auto Annotations = AnnotationInfos.find(II);
-        std::make_pair(II, std::make_pair(std::move(Msg), AnnotationLoc)));
+    if (Annotations == AnnotationInfos.end())
+      AnnotationInfos.insert(
+          std::make_pair(II, MacroAnnotations::makeRestrictExpansion(
+                                 AnnotationLoc, std::move(Msg))));
+    else
+      Annotations->second.RestrictExpansionInfo =
+          MacroAnnotationInfo{AnnotationLoc, std::move(Msg)};
   }
 
-  MsgLocationPair getRestrictExpansionMsg(const IdentifierInfo *II) {
+  void addFinalLoc(const IdentifierInfo *II, SourceLocation AnnotationLoc) {
-    return RestrictExpansionMacroMsgs.find(II)->second;
+    auto Annotations = AnnotationInfos.find(II);
+    if (Annotations == AnnotationInfos.end())
+      AnnotationInfos.insert(
+          std::make_pair(II, MacroAnnotations::makeFinal(AnnotationLoc)));
+    else
+      Annotations->second.FinalAnnotationLoc = AnnotationLoc;
   }
 
-  void emitMacroExpansionWarnings(const Token &Identifier) {
+  const MacroAnnotations &getMacroAnnotations(const IdentifierInfo *II) const {
+    return AnnotationInfos.find(II)->second;
+  }
+
+  void emitMacroExpansionWarnings(const Token &Identifier) const {
     if (Identifier.getIdentifierInfo()->isDeprecatedMacro())
       emitMacroDeprecationWarning(Identifier);
 
     if (Identifier.getIdentifierInfo()->isRestrictExpansion() &&
         !SourceMgr.isInMainFile(Identifier.getLocation()))
-      emitMacroUnsafeHeaderWarning(Identifier);
+      emitRestrictExpansionWarning(Identifier);
   }
 
 private:
-  void emitMacroDeprecationWarning(const Token &Identifier);
+  void emitMacroDeprecationWarning(const Token &Identifier) const;
-  void emitMacroUnsafeHeaderWarning(const Token &Identifier);
+  void emitRestrictExpansionWarning(const Token &Identifier) const;
+  void emitFinalMacroWarning(const Token &Identifier, bool IsUndef) const;
 
   Optional<unsigned>
   getSkippedRangeForExcludedConditionalBlock(SourceLocation HashLoc);

clang/lib/Lex/PPDirectives.cpp

@@ -2865,6 +2865,12 @@ void Preprocessor::HandleDefineDirective(
   if (MacroNameTok.is(tok::eod))
     return;
 
+  IdentifierInfo *II = MacroNameTok.getIdentifierInfo();
+  // Issue a final pragma warning if we're defining a macro that was has been
+  // undefined and is being redefined.
+  if (!II->hasMacroDefinition() && II->hadMacroDefinition() && II->isFinal())
+    emitFinalMacroWarning(MacroNameTok, /*IsUndef=*/false);
+
   // If we are supposed to keep comments in #defines, reenable comment saving
   // mode.
   if (CurLexer) CurLexer->SetCommentRetentionState(KeepMacroComments);
@@ -2907,6 +2913,12 @@ void Preprocessor::HandleDefineDirective(
   // Finally, if this identifier already had a macro defined for it, verify that
   // the macro bodies are identical, and issue diagnostics if they are not.
   if (const MacroInfo *OtherMI=getMacroInfo(MacroNameTok.getIdentifierInfo())) {
+    // Final macros are hard-mode: they always warn. Even if the bodies are
+    // identical. Even if they are in system headers. Even if they are things we
+    // would silently allow in the past.
+    if (MacroNameTok.getIdentifierInfo()->isFinal())
+      emitFinalMacroWarning(MacroNameTok, /*IsUndef=*/false);
+
     // In Objective-C, ignore attempts to directly redefine the builtin
     // definitions of the ownership qualifiers.  It's still possible to
     // #undef them.
@@ -2936,6 +2948,7 @@ void Preprocessor::HandleDefineDirective(
     // then don't bother calling MacroInfo::isIdenticalTo.
     if (!getDiagnostics().getSuppressSystemWarnings() ||
         !SourceMgr.isInSystemHeader(DefineTok.getLocation())) {
+
       if (!OtherMI->isUsed() && OtherMI->isWarnIfUnused())
         Diag(OtherMI->getDefinitionLoc(), diag::pp_macro_not_used);
 
@@ -3013,6 +3026,9 @@ void Preprocessor::HandleUndefDirective() {
   auto MD = getMacroDefinition(II);
   UndefMacroDirective *Undef = nullptr;
 
+  if (II->isFinal())
+    emitFinalMacroWarning(MacroNameTok, /*IsUndef=*/true);
+
   // If the macro is not defined, this is a noop undef.
   if (const MacroInfo *MI = MD.getMacroInfo()) {
     if (!MI->isUsed() && MI->isWarnIfUnused())

clang/lib/Lex/Pragma.cpp

@@ -1990,7 +1990,7 @@ static IdentifierInfo *HandleMacroAnnotationPragma(Preprocessor &PP, Token &Tok,
   IdentifierInfo *II = Tok.getIdentifierInfo();
 
   if (!II->hasMacroDefinition()) {
-    PP.Diag(Tok, diag::err_pp_visibility_non_macro) << II->getName();
+    PP.Diag(Tok, diag::err_pp_visibility_non_macro) << II;
     return nullptr;
   }
 
@@ -2025,8 +2025,8 @@ struct PragmaDeprecatedHandler : public PragmaHandler {
     if (IdentifierInfo *II = HandleMacroAnnotationPragma(
             PP, Tok, "#pragma clang deprecated", MessageString)) {
       II->setIsDeprecatedMacro(true);
-      if (!MessageString.empty())
+      PP.addMacroDeprecationMsg(II, std::move(MessageString),
-        PP.addMacroDeprecationMsg(II, std::move(MessageString));
+                                Tok.getLocation());
     }
   }
 };
@@ -2053,6 +2053,47 @@ struct PragmaRestrictExpansionHandler : public PragmaHandler {
   }
 };
 
+/// "\#pragma clang final(...)"
+///
+/// The syntax is
+/// \code
+///   #pragma clang final(MACRO_NAME)
+/// \endcode
+struct PragmaFinalHandler : public PragmaHandler {
+  PragmaFinalHandler() : PragmaHandler("final") {}
+
+  void HandlePragma(Preprocessor &PP, PragmaIntroducer Introducer,
+                    Token &Tok) override {
+    std::string Macro;
+
+    PP.Lex(Tok);
+    if (Tok.isNot(tok::l_paren)) {
+      PP.Diag(Tok, diag::err_expected) << "(";
+      return;
+    }
+
+    PP.LexUnexpandedToken(Tok);
+    if (!Tok.is(tok::identifier)) {
+      PP.Diag(Tok, diag::err_expected) << tok::identifier;
+      return;
+    }
+    IdentifierInfo *II = Tok.getIdentifierInfo();
+
+    if (!II->hasMacroDefinition()) {
+      PP.Diag(Tok, diag::err_pp_visibility_non_macro) << II;
+      return;
+    }
+
+    PP.Lex(Tok);
+    if (Tok.isNot(tok::r_paren)) {
+      PP.Diag(Tok, diag::err_expected) << ")";
+      return;
+    }
+    II->setIsFinal(true);
+    PP.addFinalLoc(II, Tok.getLocation());
+  }
+};
+
 } // namespace
 
 /// RegisterBuiltinPragmas - Install the standard preprocessor pragmas:
@@ -2084,6 +2125,7 @@ void Preprocessor::RegisterBuiltinPragmas() {
   AddPragmaHandler("clang", new PragmaAssumeNonNullHandler());
   AddPragmaHandler("clang", new PragmaDeprecatedHandler());
   AddPragmaHandler("clang", new PragmaRestrictExpansionHandler());
+  AddPragmaHandler("clang", new PragmaFinalHandler());
 
   // #pragma clang module ...
   auto *ModuleHandler = new PragmaNamespace("module");

clang/lib/Lex/Preprocessor.cpp

@@ -1409,25 +1409,46 @@ bool Preprocessor::HandleComment(Token &result, SourceRange Comment) {
   return true;
 }
 
-void Preprocessor::emitMacroDeprecationWarning(const Token &Identifier) {
+void Preprocessor::emitMacroDeprecationWarning(const Token &Identifier) const {
-  auto DepMsg = getMacroDeprecationMsg(Identifier.getIdentifierInfo());
+  const MacroAnnotations &A =
-  if (!DepMsg)
+      getMacroAnnotations(Identifier.getIdentifierInfo());
+  assert(A.DeprecationInfo &&
+         "Macro deprecation warning without recorded annotation!");
+  const MacroAnnotationInfo &Info = *A.DeprecationInfo;
+  if (Info.Message.empty())
     Diag(Identifier, diag::warn_pragma_deprecated_macro_use)
         << Identifier.getIdentifierInfo() << 0;
   else
     Diag(Identifier, diag::warn_pragma_deprecated_macro_use)
-        << Identifier.getIdentifierInfo() << 1 << *DepMsg;
+        << Identifier.getIdentifierInfo() << 1 << Info.Message;
+  Diag(Info.Location, diag::note_pp_macro_annotation) << 0;
 }
 
-void Preprocessor::emitMacroUnsafeHeaderWarning(const Token &Identifier) {
+void Preprocessor::emitRestrictExpansionWarning(const Token &Identifier) const {
-  auto DepMsg = getRestrictExpansionMsg(Identifier.getIdentifierInfo());
+  const MacroAnnotations &A =
-  if (DepMsg.first.empty())
+      getMacroAnnotations(Identifier.getIdentifierInfo());
+  assert(A.RestrictExpansionInfo &&
+         "Macro restricted expansion warning without recorded annotation!");
+  const MacroAnnotationInfo &Info = *A.RestrictExpansionInfo;
+  if (Info.Message.empty())
     Diag(Identifier, diag::warn_pragma_restrict_expansion_macro_use)
         << Identifier.getIdentifierInfo() << 0;
   else
     Diag(Identifier, diag::warn_pragma_restrict_expansion_macro_use)
-        << Identifier.getIdentifierInfo() << 1 << DepMsg.first;
+        << Identifier.getIdentifierInfo() << 1 << Info.Message;
-  Diag(DepMsg.second, diag::note_pp_macro_annotation) << 1;
+  Diag(Info.Location, diag::note_pp_macro_annotation) << 1;
+}
+
+void Preprocessor::emitFinalMacroWarning(const Token &Identifier,
+                                         bool IsUndef) const {
+  const MacroAnnotations &A =
+      getMacroAnnotations(Identifier.getIdentifierInfo());
+  assert(A.FinalAnnotationLoc &&
+         "Final macro warning without recorded annotation!");
+
+  Diag(Identifier, diag::warn_pragma_final_macro)
+      << Identifier.getIdentifierInfo() << (IsUndef ? 0 : 1);
+  Diag(*A.FinalAnnotationLoc, diag::note_pp_macro_annotation) << 2;
 }
 
 ModuleLoader::~ModuleLoader() = default;

clang/test/Lexer/Inputs/final-macro.h

@@ -0,0 +1,4 @@
+// expected-warning@+2{{macro 'Foo' has been marked as deprecated}}
+// expected-warning@+1{{macro 'Foo' has been marked as unsafe for use in headers}}
+#if Foo
+#endif

clang/test/Lexer/Inputs/unsafe-macro.h

@@ -4,7 +4,7 @@
 // expected-error@+1{{expected identifier}}
 #pragma clang restrict_expansion(4
 
-// expected-error@+1{{no macro named foo}}
+// expected-error@+1{{no macro named 'foo'}}
 #pragma clang restrict_expansion(foo)
 
 

clang/test/Lexer/deprecate-macro.c

@@ -6,10 +6,11 @@
 // expected-error@+1{{expected identifier}}
 #pragma clang deprecated(4
 
-// expected-error@+1{{no macro named foo}}
+// expected-error@+1{{no macro named 'foo'}}
 #pragma clang deprecated(foo)
 
 #define bar 1
+// expected-note@+1{{macro marked 'deprecated' here}} 
 #pragma clang deprecated(bar, "bar is deprecated use 1")
 
 // expected-warning@+1{{macro 'bar' has been marked as deprecated: bar is deprecated use 1}}
@@ -17,6 +18,14 @@
 #endif
 
 #define foo 1
+// expected-note@+8{{macro marked 'deprecated' here}} 
+// expected-note@+7{{macro marked 'deprecated' here}} 
+// expected-note@+6{{macro marked 'deprecated' here}} 
+// expected-note@+5{{macro marked 'deprecated' here}} 
+// expected-note@+4{{macro marked 'deprecated' here}} 
+// expected-note@+3{{macro marked 'deprecated' here}} 
+// expected-note@+2{{macro marked 'deprecated' here}} 
+// expected-note@+1{{macro marked 'deprecated' here}} 
 #pragma clang deprecated(foo)
 
 // expected-error@+1{{expected )}}
@@ -39,7 +48,7 @@
 #endif
 
 int main(int argc, char** argv) {
-  // expected-error@+1{{no macro named main}}
+// expected-error@+1{{no macro named 'main'}}
 #pragma clang deprecated(main)
 
   // expected-warning@+1{{macro 'foo' has been marked as deprecated}}

clang/test/Lexer/final-macro.c

@@ -0,0 +1,45 @@
+// RUN: %clang_cc1 -Wfinal-macro %s -fsyntax-only -verify
+
+// Test warning production
+#define Foo 1
+// expected-note@+1 4{{macro marked 'final' here}}
+#pragma clang final(Foo)
+
+// expected-warning@+2{{macro 'Foo' has been marked as final and should not be redefined}}
+// expected-note@+1{{previous definition is here}}
+#define Foo 1
+
+// expected-warning@+2{{macro 'Foo' has been marked as final and should not be redefined}}
+// expected-warning@+1{{'Foo' macro redefined}}
+#define Foo 2
+
+// expected-warning@+1{{redefining builtin macro}}
+#define __TIME__ 1
+
+// expected-warning@+1{{undefining builtin macro}}
+#undef __TIMESTAMP__
+
+// expected-warning@+1{{macro 'Foo' has been marked as final and should not be undefined}}
+#undef Foo
+// expected-warning@+1{{macro 'Foo' has been marked as final and should not be redefined}}
+#define Foo 3
+
+// Test parse errors
+// expected-error@+1{{expected (}}
+#pragma clang final
+
+// expected-error@+1{{expected )}}
+#pragma clang final(Foo
+
+// expected-error@+1{{no macro named 'Baz'}}
+#pragma clang final(Baz)
+
+// expected-error@+1{{expected identifier}}
+#pragma clang final(4)
+
+// expected-error@+1{{expected (}}
+#pragma clang final Baz
+
+// no diagnostics triggered by these pragmas.
+#pragma clang deprecated(Foo)
+#pragma clang restrict_expansion(Foo)

clang/test/Lexer/pedantic-macro-interplay.c

@@ -11,4 +11,17 @@
 // not-expected-warning@+1{{macro 'UNSAFE_MACRO_2' has been marked as deprecated: Don't use this!}}
 #pragma clang restrict_expansion(UNSAFE_MACRO_2, "Don't use this!")
 
-// expected-no-diagnostics
+
+#define Foo 1
+#pragma clang final(Foo)
+// expected-note@+2{{macro marked 'deprecated' here}}
+// expected-note@+1{{macro marked 'deprecated' here}}
+#pragma clang deprecated(Foo)
+// expected-note@+1{{macro marked 'restrict_expansion' here}}
+#pragma clang restrict_expansion(Foo)
+
+// Test that unsafe_header and deprecated markings stick around after the undef
+#include "Inputs/final-macro.h"
+
+// expected-warning@+1{{macro 'Foo' has been marked as deprecated}}
+const int X = Foo;