Add pragma clang scope [push|pop]

clang/docs/LanguageExtensions.rst

@@ -5738,6 +5738,40 @@ 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.
 
+Macro Scope
+===========
+
+Clang supports the pragma ``#pragma clang scope`` which is provided with an
+argument ``push`` or ``pop`` to denote entering and leaving macro scopes. On
+entering a macro scope all macro definitions and undefinitions are recorded so
+that they can be reverted on leaving the scope.
+
+.. code-block:: c
+
+   #define NUM_DOGGOS 2
+
+   #pragma clang scope push
+   #define NUM_DOGGOS 3
+   #pragma clang scope pop // NUM_DOGGOS is restored to 2
+
+   #pragma clang scope push
+   #undef NUM_DOGGOS
+   #pragma clang scope pop // NUM_DOGGOS is restored to 2
+
+   #undef NUM_DOGGOS
+   #pragma clang scope push
+   #define NUM_DOGGOS 1
+   #pragma clang scope pop // NUM_DOGGOS is restored to undefined
+
+A macro scope can be used to wrap header includes to isolate headers from
+leaking macros to the outer source file.
+
+.. code-block:: c
+
+   #pragma clang scope push
+   #include <SomeSystemHeader.h>
+   #pragma clang scope pop // None of the defines from the included header persist.
+
 Line Control
 ============
 

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -693,8 +693,8 @@ def warn_pragma_diagnostic_invalid :
    ExtWarn<"pragma diagnostic expected 'error', 'warning', 'ignored', 'fatal',"
             " 'push', or 'pop'">,
    InGroup<UnknownPragmas>;
-def warn_pragma_diagnostic_cannot_pop :
-   ExtWarn<"pragma diagnostic pop could not pop, no matching push">,
+def warn_pragma_cannot_pop :
+   ExtWarn<"pragma %select{diagnostic|scope}0 pop could not pop, no matching push">,
    InGroup<UnknownPragmas>;
 def warn_pragma_diagnostic_invalid_option :
    ExtWarn<"pragma diagnostic expected option name (e.g. \"-Wundef\")">,

clang/include/clang/Lex/Preprocessor.h

@@ -1059,6 +1059,10 @@ class Preprocessor {
   /// Warning information for macro annotations.
   llvm::DenseMap<const IdentifierInfo *, MacroAnnotations> AnnotationInfos;
 
+  using MacroScopeVec = llvm::SmallVector<std::pair<IdentifierInfo *, MacroDirective *> >;
+  MacroScopeVec *CurScope = nullptr;
+  llvm::SmallVector<MacroScopeVec> MacroScopeStack;
+
   /// A "freelist" of MacroArg objects that can be
   /// reused for quick allocation.
   MacroArgs *MacroArgCache = nullptr;
@@ -2896,6 +2900,9 @@ class Preprocessor {
     AnnotationInfos[II].FinalAnnotationLoc = AnnotationLoc;
   }
 
+  void pushMacroScope();
+  void popMacroScope(SourceLocation Loc);
+
   const MacroAnnotations &getMacroAnnotations(const IdentifierInfo *II) const {
     return AnnotationInfos.find(II)->second;
   }