Make -frewrite-includes put an endif at the end of the included text (#67613)

The #if now has a conditional expression, so a user can add 
`-D__CLANG_REWRITTEN_SYSTEM_INCLUDES` to include the system headers
instead of using the expanded content, or 
`-D__CLANG_REWRITTEN_INCLUDES` to include all headers.
Also added the filename to the comments it emits, to help identify where
included text ends, making it easier to identify and remove the content of
individual headers.

Author: pogo59

clang/docs/ReleaseNotes.rst

@@ -168,6 +168,9 @@ Modified Compiler Flags
 
 * ``-Woverriding-t-option`` is renamed to ``-Woverriding-option``.
 * ``-Winterrupt-service-routine`` is renamed to ``-Wexcessive-regsave`` as a generalization
+* ``-frewrite-includes`` now guards the original #include directives with
+  ``__CLANG_REWRITTEN_INCLUDES``, and ``__CLANG_REWRITTEN_SYSTEM_INCLUDES`` as
+  appropriate.
 
 Removed Compiler Flags
 -------------------------

clang/lib/Frontend/Rewrite/InclusionRewriter.cpp

@@ -90,8 +90,10 @@ class InclusionRewriter : public PPCallbacks {
                          bool EnsureNewline);
   void CommentOutDirective(Lexer &DirectivesLex, const Token &StartToken,
                            const MemoryBufferRef &FromFile, StringRef EOL,
-                           unsigned &NextToWrite, int &Lines);
+                           unsigned &NextToWrite, int &Lines,
+                           const IncludedFile *Inc = nullptr);
   const IncludedFile *FindIncludeAtLocation(SourceLocation Loc) const;
+  StringRef getIncludedFileName(const IncludedFile *Inc) const;
   const Module *FindModuleAtLocation(SourceLocation Loc) const;
   const Module *FindEnteredModule(SourceLocation Loc) const;
   bool IsIfAtLocationTrue(SourceLocation Loc) const;
@@ -311,6 +313,17 @@ void InclusionRewriter::OutputContentUpTo(const MemoryBufferRef &FromFile,
   WriteFrom = WriteTo;
 }
 
+StringRef
+InclusionRewriter::getIncludedFileName(const IncludedFile *Inc) const {
+  if (Inc) {
+    auto B = SM.getBufferOrNone(Inc->Id);
+    assert(B && "Attempting to process invalid inclusion");
+    if (B)
+      return llvm::sys::path::filename(B->getBufferIdentifier());
+  }
+  return StringRef();
+}
+
 /// Print characters from \p FromFile starting at \p NextToWrite up until the
 /// inclusion directive at \p StartToken, then print out the inclusion
 /// inclusion directive disabled by a #if directive, updating \p NextToWrite
@@ -320,7 +333,8 @@ void InclusionRewriter::CommentOutDirective(Lexer &DirectiveLex,
                                             const Token &StartToken,
                                             const MemoryBufferRef &FromFile,
                                             StringRef LocalEOL,
-                                            unsigned &NextToWrite, int &Line) {
+                                            unsigned &NextToWrite, int &Line,
+                                            const IncludedFile *Inc) {
   OutputContentUpTo(FromFile, NextToWrite,
                     SM.getFileOffset(StartToken.getLocation()), LocalEOL, Line,
                     false);
@@ -332,12 +346,21 @@ void InclusionRewriter::CommentOutDirective(Lexer &DirectiveLex,
     // OutputContentUpTo() would not output anything anyway.
     return;
   }
-  OS << "#if 0 /* expanded by -frewrite-includes */" << MainEOL;
+  if (Inc) {
+    OS << "#if defined(__CLANG_REWRITTEN_INCLUDES) ";
+    if (isSystem(Inc->FileType))
+      OS << "|| defined(__CLANG_REWRITTEN_SYSTEM_INCLUDES) ";
+    OS << "/* " << getIncludedFileName(Inc);
+  } else {
+    OS << "#if 0 /*";
+  }
+  OS << " expanded by -frewrite-includes */" << MainEOL;
   OutputContentUpTo(FromFile, NextToWrite,
                     SM.getFileOffset(DirectiveToken.getLocation()) +
                         DirectiveToken.getLength(),
                     LocalEOL, Line, true);
-  OS << "#endif /* expanded by -frewrite-includes */" << MainEOL;
+  OS << (Inc ? "#else /* " : "#endif /*") << getIncludedFileName(Inc)
+     << " expanded by -frewrite-includes */" << MainEOL;
 }
 
 /// Find the next identifier in the pragma directive specified by \p RawToken.
@@ -400,15 +423,16 @@ void InclusionRewriter::Process(FileID FileId,
           case tok::pp_include:
           case tok::pp_include_next:
           case tok::pp_import: {
-            CommentOutDirective(RawLex, HashToken, FromFile, LocalEOL, NextToWrite,
-              Line);
+            SourceLocation Loc = HashToken.getLocation();
+            const IncludedFile *Inc = FindIncludeAtLocation(Loc);
+            CommentOutDirective(RawLex, HashToken, FromFile, LocalEOL,
+                                NextToWrite, Line, Inc);
             if (FileId != PP.getPredefinesFileID())
               WriteLineInfo(FileName, Line - 1, FileType, "");
             StringRef LineInfoExtra;
-            SourceLocation Loc = HashToken.getLocation();
             if (const Module *Mod = FindModuleAtLocation(Loc))
               WriteImplicitModuleImport(Mod);
-            else if (const IncludedFile *Inc = FindIncludeAtLocation(Loc)) {
+            else if (Inc) {
               const Module *Mod = FindEnteredModule(Loc);
               if (Mod)
                 OS << "#pragma clang module begin "
@@ -420,6 +444,8 @@ void InclusionRewriter::Process(FileID FileId,
               if (Mod)
                 OS << "#pragma clang module end /*"
                    << Mod->getFullModuleName(true) << "*/\n";
+              OS << "#endif /* " << getIncludedFileName(Inc)
+                 << " expanded by -frewrite-includes */" << LocalEOL;
 
               // Add line marker to indicate we're returning from an included
               // file.

clang/test/Frontend/rewrite-includes-cli-include.c

@@ -3,6 +3,7 @@ main_file_line
 // CHECK: {{^}}# 1 "<built-in>"{{$}}
 // CHECK-NEXT: {{^}}# 1 "{{.*[/\\]Inputs(/|\\\\)}}rewrite-includes2.h" 1{{$}}
 // CHECK-NEXT: {{^}}int included_line2;{{$}}
+// CHECK-NEXT: {{^}}#endif /* rewrite-includes2.h expanded by -frewrite-includes */{{$}}
 // CHECK-NEXT: {{^}}# 1 "<built-in>" 2{{$}}
 // CHECK-NEXT: {{^}}# 1 "{{.*}}rewrite-includes-cli-include.c"{{$}}
 // CHECK-NEXT: FileCheck