v.g.vassilev created this revision.
v.g.vassilev added reviewers: rsmith, rjmccall, lhames, teemperor, aprantl, 
sgraenitz, hfinkel.
Herald added subscribers: dexonsmith, kbarton, nemanjai.
v.g.vassilev requested review of this revision.

https://reviews.llvm.org/D96033 contained a discussion regarding efficient 
modeling of error recovery. @rjmccall has outlined the key ideas:

Conceptually, we can split the translation unit into a sequence of partial 
translation units (PTUs). Every declaration will be associated with a unique 
PTU that owns it.

      

The first key insight here is that the owning PTU isn't always the "active" 
(most recent) PTU, and it isn't always the PTU that the declaration "comes 
from". A new declaration (that isn't a redeclaration or specialization of 
anything) does belong to the active PTU. A template specialization, however, 
belongs to the most recent PTU of all the declarations in its signature - 
mostly that means that it can be pulled into a more recent PTU by its template 
arguments.

The second key insight is that processing a PTU might extend an earlier PTU. 
Rolling back the later PTU shouldn't throw that extension away. For example, if 
the second PTU defines a template, and the third PTU requires that template to 
be instantiated at float, that template specialization is still part of the 
second PTU. Similarly, if the fifth PTU uses an inline function belonging to 
the fourth, that definition still belongs to the fourth. When we go to emit 
code in a new PTU, we map each declaration we have to emit back to its owning 
PTU and emit it in a new module for just the extensions to that PTU. We keep 
track of all the modules we've emitted for a PTU so that we can unload them all 
if we decide to roll it back.

      

Most declarations/definitions will only refer to entities from the same or 
earlier PTUs. However, it is possible (primarily by defining a 
previously-declared entity, but also through templates or ADL) for an entity 
that belongs to one PTU to refer to something from a later PTU. We will have to 
keep track of this and prevent unwinding to later PTU when we recognize it.

Fortunately, this should be very rare; and crucially, we don't have to do the 
bookkeeping for this if we've only got one PTU, e.g. in normal compilation. 
Otherwise, PTUs after the first just need to record enough metadata to be able 
to revert any changes they've made to declarations belonging to earlier PTUs, 
e.g. to redeclaration chains or template specialization lists.

It should even eventually be possible for PTUs to provide their own slab 
allocators which can be thrown away as part of rolling back the PTU. We can 
maintain a notion of the active allocator and allocate things like Stmt/Expr 
nodes in it, temporarily changing it to the appropriate PTU whenever we go to 
do something like instantiate a function template. More care will be required 
when allocating declarations and types, though.

We would want the PTU to be efficiently recoverable from a Decl; I'm not sure 
how best to do that. An easy option that would cover most declarations would be 
to make multiple TranslationUnitDecls and parent the declarations 
appropriately, but I don't think that's good enough for things like member 
function templates, since an instantiation of that would still be parented by 
its original class. Maybe we can work this into the DC chain somehow, like how 
lexical DCs are.

      

This patch teaches clang-repl how to recover from errors by disconnecting the 
most recent PTU and update the primary PTU lookup tables. For instance:

  ./clang-repl
  clang-repl> int i = 12; error;
  In file included from <<< inputs >>>:1:
  input_line_0:1:13: error: C++ requires a type specifier for all declarations
  int i = 12; error;
                   ^
  error: Parsing failed.
  clang-repl> int i = 13; extern "C" int printf(const char*,...);
  clang-repl> auto r1 = printf("i=%d\n", i);
  i=13
  clang-repl> quit


Repository:
  rC Clang

https://reviews.llvm.org/D104918

Files:
  clang/include/clang/AST/ASTContext.h
  clang/include/clang/AST/Decl.h
  clang/include/clang/AST/Redeclarable.h
  clang/include/clang/Basic/LangOptions.h
  clang/include/clang/Interpreter/Interpreter.h
  clang/include/clang/Interpreter/PartialTranslationUnit.h
  clang/include/clang/Interpreter/Transaction.h
  clang/include/clang/Lex/Preprocessor.h
  clang/include/clang/Sema/Sema.h
  clang/lib/AST/ASTContext.cpp
  clang/lib/AST/Decl.cpp
  clang/lib/AST/DeclBase.cpp
  clang/lib/Frontend/ASTUnit.cpp
  clang/lib/Frontend/CompilerInstance.cpp
  clang/lib/Interpreter/IncrementalParser.cpp
  clang/lib/Interpreter/IncrementalParser.h
  clang/lib/Interpreter/Interpreter.cpp
  clang/lib/Sema/Sema.cpp
  clang/lib/Serialization/ASTReader.cpp
  clang/tools/clang-import-test/clang-import-test.cpp
  clang/unittests/AST/ASTVectorTest.cpp
  clang/unittests/Interpreter/IncrementalProcessingTest.cpp
  clang/unittests/Interpreter/InterpreterTest.cpp
  clang/unittests/Lex/PPCallbacksTest.cpp

Index: clang/unittests/Lex/PPCallbacksTest.cpp
===================================================================
--- clang/unittests/Lex/PPCallbacksTest.cpp
+++ clang/unittests/Lex/PPCallbacksTest.cpp
@@ -279,7 +279,7 @@
     // according to LangOptions, so we init Parser to register opencl
     // pragma handlers
     ASTContext Context(OpenCLLangOpts, SourceMgr, PP.getIdentifierTable(),
-                       PP.getSelectorTable(), PP.getBuiltinInfo());
+                       PP.getSelectorTable(), PP.getBuiltinInfo(), PP.TUKind);
     Context.InitBuiltinTypes(*Target);
 
     ASTConsumer Consumer;
Index: clang/unittests/Interpreter/InterpreterTest.cpp
===================================================================
--- clang/unittests/Interpreter/InterpreterTest.cpp
+++ clang/unittests/Interpreter/InterpreterTest.cpp
@@ -37,17 +37,24 @@
   return cantFail(clang::Interpreter::create(std::move(CI)));
 }
 
+static size_t DeclsSize(TranslationUnitDecl *PTUDecl) {
+  return std::distance(PTUDecl->decls().begin(), PTUDecl->decls().end());
+}
+
 TEST(InterpreterTest, Sanity) {
   std::unique_ptr<Interpreter> Interp = createInterpreter();
-  Transaction &R1(cantFail(Interp->Parse("void g(); void g() {}")));
-  EXPECT_EQ(2U, R1.Decls.size());
 
-  Transaction &R2(cantFail(Interp->Parse("int i;")));
-  EXPECT_EQ(1U, R2.Decls.size());
+  using PTU = PartialTranslationUnit;
+
+  PTU &R1(cantFail(Interp->Parse("void g(); void g() {}")));
+  EXPECT_EQ(2U, DeclsSize(R1.TUPart));
+
+  PTU &R2(cantFail(Interp->Parse("int i;")));
+  EXPECT_EQ(1U, DeclsSize(R2.TUPart));
 }
 
-static std::string DeclToString(DeclGroupRef DGR) {
-  return llvm::cast<NamedDecl>(DGR.getSingleDecl())->getQualifiedNameAsString();
+static std::string DeclToString(Decl *D) {
+  return llvm::cast<NamedDecl>(D)->getQualifiedNameAsString();
 }
 
 TEST(InterpreterTest, IncrementalInputTopLevelDecls) {
@@ -55,16 +62,16 @@
   auto R1OrErr = Interp->Parse("int var1 = 42; int f() { return var1; }");
   // gtest doesn't expand into explicit bool conversions.
   EXPECT_TRUE(!!R1OrErr);
-  auto R1 = R1OrErr->Decls;
-  EXPECT_EQ(2U, R1.size());
-  EXPECT_EQ("var1", DeclToString(R1[0]));
-  EXPECT_EQ("f", DeclToString(R1[1]));
+  auto R1 = R1OrErr->TUPart->decls();
+  EXPECT_EQ(2U, DeclsSize(R1OrErr->TUPart));
+  EXPECT_EQ("var1", DeclToString(*R1.begin()));
+  EXPECT_EQ("f", DeclToString(*(++R1.begin())));
 
   auto R2OrErr = Interp->Parse("int var2 = f();");
   EXPECT_TRUE(!!R2OrErr);
-  auto R2 = R2OrErr->Decls;
-  EXPECT_EQ(1U, R2.size());
-  EXPECT_EQ("var2", DeclToString(R2[0]));
+  auto R2 = R2OrErr->TUPart->decls();
+  EXPECT_EQ(1U, DeclsSize(R2OrErr->TUPart));
+  EXPECT_EQ("var2", DeclToString(*R2.begin()));
 }
 
 TEST(InterpreterTest, Errors) {
@@ -83,9 +90,8 @@
               HasSubstr("error: unknown type name 'intentional_error'"));
   EXPECT_EQ("Parsing failed.", llvm::toString(std::move(Err)));
 
-#ifdef GTEST_HAS_DEATH_TEST
-  EXPECT_DEATH((void)Interp->Parse("int var1 = 42;"), "");
-#endif
+  auto RecoverErr = Interp->Parse("int var1 = 42;");
+  EXPECT_TRUE(!!RecoverErr);
 }
 
 // Here we test whether the user can mix declarations and statements. The
@@ -106,8 +112,8 @@
   // gtest doesn't expand into explicit bool conversions.
   EXPECT_TRUE(!!R1OrErr);
 
-  auto R1 = R1OrErr->Decls;
-  EXPECT_EQ(2U, R1.size());
+  auto R1 = R1OrErr->TUPart;
+  EXPECT_EQ(2U, DeclsSize(R1));
 
   // FIXME: Add support for wrapping and running statements.
   auto R2OrErr = Interp->Parse("var1++; printf(\"var1 value %d\\n\", var1);");
Index: clang/unittests/Interpreter/IncrementalProcessingTest.cpp
===================================================================
--- clang/unittests/Interpreter/IncrementalProcessingTest.cpp
+++ clang/unittests/Interpreter/IncrementalProcessingTest.cpp
@@ -55,23 +55,23 @@
   auto CI = llvm::cantFail(IncrementalCompilerBuilder::create(ClangArgv));
   auto Interp = llvm::cantFail(Interpreter::create(std::move(CI)));
 
-  std::array<clang::Transaction *, 2> Transactions;
+  std::array<clang::PartialTranslationUnit *, 2> PTUs;
 
-  Transactions[0] = &llvm::cantFail(Interp->Parse(TestProgram1));
-  ASSERT_TRUE(Transactions[0]->TheModule);
-  ASSERT_TRUE(Transactions[0]->TheModule->getFunction("funcForProg1"));
+  PTUs[0] = &llvm::cantFail(Interp->Parse(TestProgram1));
+  ASSERT_TRUE(PTUs[0]->TheModule);
+  ASSERT_TRUE(PTUs[0]->TheModule->getFunction("funcForProg1"));
 
-  Transactions[1] = &llvm::cantFail(Interp->Parse(TestProgram2));
-  ASSERT_TRUE(Transactions[1]->TheModule);
-  ASSERT_TRUE(Transactions[1]->TheModule->getFunction("funcForProg2"));
+  PTUs[1] = &llvm::cantFail(Interp->Parse(TestProgram2));
+  ASSERT_TRUE(PTUs[1]->TheModule);
+  ASSERT_TRUE(PTUs[1]->TheModule->getFunction("funcForProg2"));
   // First code should not end up in second module:
-  ASSERT_FALSE(Transactions[1]->TheModule->getFunction("funcForProg1"));
+  ASSERT_FALSE(PTUs[1]->TheModule->getFunction("funcForProg1"));
 
   // Make sure global inits exist and are unique:
-  const Function *GlobalInit1 = getGlobalInit(Transactions[0]->TheModule.get());
+  const Function *GlobalInit1 = getGlobalInit(PTUs[0]->TheModule.get());
   ASSERT_TRUE(GlobalInit1);
 
-  const Function *GlobalInit2 = getGlobalInit(Transactions[1]->TheModule.get());
+  const Function *GlobalInit2 = getGlobalInit(PTUs[1]->TheModule.get());
   ASSERT_TRUE(GlobalInit2);
 
   ASSERT_FALSE(GlobalInit1->getName() == GlobalInit2->getName());
Index: clang/unittests/AST/ASTVectorTest.cpp
===================================================================
--- clang/unittests/AST/ASTVectorTest.cpp
+++ clang/unittests/AST/ASTVectorTest.cpp
@@ -29,7 +29,7 @@
       : FileMgr(FileMgrOpts), DiagID(new DiagnosticIDs()),
         Diags(DiagID, new DiagnosticOptions, new IgnoringDiagConsumer()),
         SourceMgr(Diags, FileMgr), Idents(LangOpts, nullptr),
-        Ctxt(LangOpts, SourceMgr, Idents, Sels, Builtins) {}
+        Ctxt(LangOpts, SourceMgr, Idents, Sels, Builtins, TU_Complete) {}
 
   FileSystemOptions FileMgrOpts;
   FileManager FileMgr;
Index: clang/tools/clang-import-test/clang-import-test.cpp
===================================================================
--- clang/tools/clang-import-test/clang-import-test.cpp
+++ clang/tools/clang-import-test/clang-import-test.cpp
@@ -218,9 +218,10 @@
 
 std::unique_ptr<ASTContext>
 BuildASTContext(CompilerInstance &CI, SelectorTable &ST, Builtin::Context &BC) {
+  auto &PP = CI.getPreprocessor();
   auto AST = std::make_unique<ASTContext>(
       CI.getLangOpts(), CI.getSourceManager(),
-      CI.getPreprocessor().getIdentifierTable(), ST, BC);
+      PP.getIdentifierTable(), ST, BC, PP.TUKind);
   AST->InitBuiltinTypes(CI.getTarget());
   return AST;
 }
Index: clang/lib/Serialization/ASTReader.cpp
===================================================================
--- clang/lib/Serialization/ASTReader.cpp
+++ clang/lib/Serialization/ASTReader.cpp
@@ -7174,6 +7174,11 @@
     return;
   }
 
+  if (!D->getDeclContext()) {
+    assert(isa<TranslationUnitDecl>(D) && "Not a TU?");
+    return;
+  }
+
   const DeclContext *DC = D->getDeclContext()->getRedeclContext();
 
   // If this is a named declaration, complete it by looking it up
Index: clang/lib/Sema/Sema.cpp
===================================================================
--- clang/lib/Sema/Sema.cpp
+++ clang/lib/Sema/Sema.cpp
@@ -183,6 +183,7 @@
       DisableTypoCorrection(false), TyposCorrected(0), AnalysisWarnings(*this),
       ThreadSafetyDeclCache(nullptr), VarDataSharingAttributesStack(nullptr),
       CurScope(nullptr), Ident_super(nullptr), Ident___float128(nullptr) {
+  assert(pp.TUKind == TUKind);
   TUScope = nullptr;
   isConstantEvaluatedOverride = false;
 
Index: clang/lib/Interpreter/Interpreter.cpp
===================================================================
--- clang/lib/Interpreter/Interpreter.cpp
+++ clang/lib/Interpreter/Interpreter.cpp
@@ -198,11 +198,12 @@
   return IncrParser->getCI();
 }
 
-llvm::Expected<Transaction &> Interpreter::Parse(llvm::StringRef Code) {
+llvm::Expected<PartialTranslationUnit &>
+Interpreter::Parse(llvm::StringRef Code) {
   return IncrParser->Parse(Code);
 }
 
-llvm::Error Interpreter::Execute(Transaction &T) {
+llvm::Error Interpreter::Execute(PartialTranslationUnit &T) {
   assert(T.TheModule);
   if (!IncrExecutor) {
     const llvm::Triple &Triple =
Index: clang/lib/Interpreter/IncrementalParser.h
===================================================================
--- clang/lib/Interpreter/IncrementalParser.h
+++ clang/lib/Interpreter/IncrementalParser.h
@@ -13,7 +13,7 @@
 #ifndef LLVM_CLANG_LIB_INTERPRETER_INCREMENTALPARSER_H
 #define LLVM_CLANG_LIB_INTERPRETER_INCREMENTALPARSER_H
 
-#include "clang/Interpreter/Transaction.h"
+#include "clang/Interpreter/PartialTranslationUnit.h"
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/StringRef.h"
@@ -55,7 +55,7 @@
 
   /// List containing every information about every incrementally parsed piece
   /// of code.
-  std::list<Transaction> Transactions;
+  std::list<PartialTranslationUnit> PTUs;
 
 public:
   IncrementalParser(std::unique_ptr<CompilerInstance> Instance,
@@ -65,12 +65,12 @@
   const CompilerInstance *getCI() const { return CI.get(); }
 
   /// Parses incremental input by creating an in-memory file.
-  ///\returns a \c Transaction which holds information about the \c Decls and
-  /// \c llvm::Module corresponding to the input.
-  llvm::Expected<Transaction &> Parse(llvm::StringRef Input);
+  ///\returns a \c PartialTranslationUnit which holds information about the
+  /// \c TranslationUnitDecl and \c llvm::Module corresponding to the input.
+  llvm::Expected<PartialTranslationUnit &> Parse(llvm::StringRef Input);
 
 private:
-  llvm::Expected<Transaction &> ParseOrWrapTopLevelDecl();
+  llvm::Expected<PartialTranslationUnit &> ParseOrWrapTopLevelDecl();
 };
 } // end namespace clang
 
Index: clang/lib/Interpreter/IncrementalParser.cpp
===================================================================
--- clang/lib/Interpreter/IncrementalParser.cpp
+++ clang/lib/Interpreter/IncrementalParser.cpp
@@ -12,6 +12,7 @@
 
 #include "IncrementalParser.h"
 
+#include "clang/AST/DeclContextInternals.h"
 #include "clang/CodeGen/BackendUtil.h"
 #include "clang/CodeGen/CodeGenAction.h"
 #include "clang/CodeGen/ModuleBuilder.h"
@@ -75,6 +76,7 @@
           return Act;
         }()) {}
   FrontendAction *getWrapped() const { return WrappedAction.get(); }
+  TranslationUnitKind getTranslationUnitKind() override { return TU_Partial; }
   void ExecuteAction() override {
     CompilerInstance &CI = getCompilerInstance();
     assert(CI.hasPreprocessor() && "No PP!");
@@ -130,26 +132,32 @@
 
 IncrementalParser::~IncrementalParser() { Act->FinalizeAction(); }
 
-llvm::Expected<Transaction &> IncrementalParser::ParseOrWrapTopLevelDecl() {
-  DiagnosticsEngine &Diags = getCI()->getDiagnostics();
-
-  if (Diags.hasErrorOccurred())
-    llvm::report_fatal_error("Previous input had errors, "
-                             "recovery not yet supported",
-                             /*GenCrashDiag=*/false);
-
+llvm::Expected<PartialTranslationUnit &>
+IncrementalParser::ParseOrWrapTopLevelDecl() {
   // Recover resources if we crash before exiting this method.
   Sema &S = CI->getSema();
   llvm::CrashRecoveryContextCleanupRegistrar<Sema> CleanupSema(&S);
   Sema::GlobalEagerInstantiationScope GlobalInstantiations(S, /*Enabled=*/true);
   Sema::LocalEagerInstantiationScope LocalInstantiations(S);
 
+  PTUs.emplace_back(PartialTranslationUnit());
+  PartialTranslationUnit &LastPTU = PTUs.back();
+  // Add a new PTU.
+  ASTContext &C = S.getASTContext();
+  C.addTranslationUnitDecl();
+  LastPTU.TUPart = C.getTranslationUnitDecl();
+
   // Skip previous eof due to last incremental input.
-  if (P->getCurToken().is(tok::eof))
+  if (P->getCurToken().is(tok::eof)) {
     P->ConsumeToken();
-
-  Transactions.emplace_back(Transaction());
-  Transaction &LastTransaction = Transactions.back();
+    // FIXME: Clang does not call ExitScope on finalizing the regular TU, we
+    // might want to do that around HandleEndOfTranslationUnit.
+    P->ExitScope();
+    S.CurContext = nullptr;
+    // Start a new PTU.
+    P->EnterScope(Scope::DeclScope);
+    S.ActOnTranslationUnitScope(P->getCurScope());
+  }
 
   Parser::DeclGroupPtrTy ADecl;
   for (bool AtEOF = P->ParseFirstTopLevelDecl(ADecl); !AtEOF;
@@ -161,26 +169,51 @@
       return llvm::make_error<llvm::StringError>("Parsing failed. "
                                                  "The consumer rejected a decl",
                                                  std::error_code());
-    LastTransaction.Decls.push_back(ADecl.get());
   }
 
+  DiagnosticsEngine &Diags = getCI()->getDiagnostics();
+  if (Diags.hasErrorOccurred()) {
+    TranslationUnitDecl *MostRecentTU = C.getTranslationUnitDecl();
+    TranslationUnitDecl *PreviousTU = MostRecentTU->getPreviousDecl();
+    assert(PreviousTU);
+    TranslationUnitDecl *FirstTU = MostRecentTU->getFirstDecl();
+    assert(FirstTU);
+    FirstTU->RedeclLink.setLatest(PreviousTU);
+    C.TUDecl = PreviousTU;
+    S.TUScope->setEntity(PreviousTU);
+
+    // Clean up the lookup table
+    if (StoredDeclsMap *Map = PreviousTU->getLookupPtr()) {
+      for (auto I = Map->begin(); I != Map->end(); ++I) {
+        StoredDeclsList &List = I->second;
+        DeclContextLookupResult R = List.getLookupResult();
+        for (NamedDecl *D : R)
+          if (D->getTranslationUnitDecl() == MostRecentTU)
+            List.remove(D);
+        if (List.isNull())
+          Map->erase(I);
+      }
+    }
+
+    // FIXME: Do not reset the pragma handlers.
+    Diags.Reset();
+    return llvm::make_error<llvm::StringError>("Parsing failed.",
+                                               std::error_code());
+  }
+
+
   // Process any TopLevelDecls generated by #pragma weak.
   for (Decl *D : S.WeakTopLevelDecls()) {
     DeclGroupRef DGR(D);
-    LastTransaction.Decls.push_back(DGR);
     Consumer->HandleTopLevelDecl(DGR);
   }
 
   LocalInstantiations.perform();
   GlobalInstantiations.perform();
 
-  Consumer->HandleTranslationUnit(S.getASTContext());
-
-  if (Diags.hasErrorOccurred())
-    return llvm::make_error<llvm::StringError>("Parsing failed.",
-                                               std::error_code());
+  Consumer->HandleTranslationUnit(C);
 
-  return LastTransaction;
+  return LastPTU;
 }
 
 static CodeGenerator *getCodeGen(FrontendAction *Act) {
@@ -191,7 +224,8 @@
   return static_cast<CodeGenAction *>(WrappedAct)->getCodeGenerator();
 }
 
-llvm::Expected<Transaction &> IncrementalParser::Parse(llvm::StringRef input) {
+llvm::Expected<PartialTranslationUnit &>
+IncrementalParser::Parse(llvm::StringRef input) {
   Preprocessor &PP = CI->getPreprocessor();
   assert(PP.isIncrementalProcessingEnabled() && "Not in incremental mode!?");
 
@@ -224,8 +258,8 @@
                                                "Cannot enter source file.",
                                                std::error_code());
 
-  auto ErrOrTransaction = ParseOrWrapTopLevelDecl();
-  if (auto Err = ErrOrTransaction.takeError())
+  auto ErrOrPTU = ParseOrWrapTopLevelDecl();
+  if (auto Err = ErrOrPTU.takeError())
     return std::move(Err);
 
   if (PP.getLangOpts().DelayedTemplateParsing) {
@@ -246,12 +280,12 @@
 
   if (CodeGenerator *CG = getCodeGen(Act.get())) {
     std::unique_ptr<llvm::Module> M(CG->ReleaseModule());
-    CG->StartModule("incr_module_" + std::to_string(Transactions.size()),
+    CG->StartModule("incr_module_" + std::to_string(PTUs.size()),
                     M->getContext());
 
-    ErrOrTransaction->TheModule = std::move(M);
+    ErrOrPTU->TheModule = std::move(M);
   }
 
-  return ErrOrTransaction;
+  return ErrOrPTU;
 }
 } // end namespace clang
Index: clang/lib/Frontend/CompilerInstance.cpp
===================================================================
--- clang/lib/Frontend/CompilerInstance.cpp
+++ clang/lib/Frontend/CompilerInstance.cpp
@@ -551,7 +551,7 @@
   Preprocessor &PP = getPreprocessor();
   auto *Context = new ASTContext(getLangOpts(), PP.getSourceManager(),
                                  PP.getIdentifierTable(), PP.getSelectorTable(),
-                                 PP.getBuiltinInfo());
+                                 PP.getBuiltinInfo(), PP.TUKind);
   Context->InitBuiltinTypes(getTarget(), getAuxTarget());
   setASTContext(Context);
 }
Index: clang/lib/Frontend/ASTUnit.cpp
===================================================================
--- clang/lib/Frontend/ASTUnit.cpp
+++ clang/lib/Frontend/ASTUnit.cpp
@@ -807,7 +807,8 @@
   if (ToLoad >= LoadASTOnly)
     AST->Ctx = new ASTContext(*AST->LangOpts, AST->getSourceManager(),
                               PP.getIdentifierTable(), PP.getSelectorTable(),
-                              PP.getBuiltinInfo());
+                              PP.getBuiltinInfo(),
+                              AST->getTranslationUnitKind());
 
   DisableValidationForModuleKind disableValid =
       DisableValidationForModuleKind::None;
Index: clang/lib/AST/DeclBase.cpp
===================================================================
--- clang/lib/AST/DeclBase.cpp
+++ clang/lib/AST/DeclBase.cpp
@@ -1218,7 +1218,6 @@
 
 DeclContext *DeclContext::getPrimaryContext() {
   switch (getDeclKind()) {
-  case Decl::TranslationUnit:
   case Decl::ExternCContext:
   case Decl::LinkageSpec:
   case Decl::Export:
@@ -1230,6 +1229,8 @@
     // There is only one DeclContext for these entities.
     return this;
 
+  case Decl::TranslationUnit:
+    return static_cast<TranslationUnitDecl *>(this)->getFirstDecl();
   case Decl::Namespace:
     // The original namespace is our primary context.
     return static_cast<NamespaceDecl *>(this)->getOriginalNamespace();
@@ -1284,21 +1285,26 @@
   }
 }
 
+template <typename T>
+void collectAllContextsImpl(T* Self, SmallVectorImpl<DeclContext *> &Contexts) {
+  for (T *D = Self->getMostRecentDecl(); D; D = D->getPreviousDecl())
+    Contexts.push_back(D);
+
+  std::reverse(Contexts.begin(), Contexts.end());
+}
+
 void
-DeclContext::collectAllContexts(SmallVectorImpl<DeclContext *> &Contexts){
+DeclContext::collectAllContexts(SmallVectorImpl<DeclContext *> &Contexts) {
   Contexts.clear();
 
-  if (getDeclKind() != Decl::Namespace) {
-    Contexts.push_back(this);
-    return;
-  }
-
-  auto *Self = static_cast<NamespaceDecl *>(this);
-  for (NamespaceDecl *N = Self->getMostRecentDecl(); N;
-       N = N->getPreviousDecl())
-    Contexts.push_back(N);
+  Decl::Kind Kind = getDeclKind();
 
-  std::reverse(Contexts.begin(), Contexts.end());
+  if (Kind == Decl::TranslationUnit)
+    collectAllContextsImpl(static_cast<TranslationUnitDecl *>(this), Contexts);
+  else if (Kind == Decl::Namespace)
+    collectAllContextsImpl(static_cast<NamespaceDecl *>(this), Contexts);
+  else
+    Contexts.push_back(this);
 }
 
 std::pair<Decl *, Decl *>
Index: clang/lib/AST/Decl.cpp
===================================================================
--- clang/lib/AST/Decl.cpp
+++ clang/lib/AST/Decl.cpp
@@ -102,7 +102,7 @@
 
 TranslationUnitDecl::TranslationUnitDecl(ASTContext &ctx)
     : Decl(TranslationUnit, nullptr, SourceLocation()),
-      DeclContext(TranslationUnit), Ctx(ctx) {}
+      DeclContext(TranslationUnit), redeclarable_base(ctx), Ctx(ctx) {}
 
 //===----------------------------------------------------------------------===//
 // NamedDecl Implementation
Index: clang/lib/AST/ASTContext.cpp
===================================================================
--- clang/lib/AST/ASTContext.cpp
+++ clang/lib/AST/ASTContext.cpp
@@ -966,7 +966,7 @@
 
 ASTContext::ASTContext(LangOptions &LOpts, SourceManager &SM,
                        IdentifierTable &idents, SelectorTable &sels,
-                       Builtin::Context &builtins)
+                       Builtin::Context &builtins, TranslationUnitKind TUKind)
     : ConstantArrayTypes(this_()), FunctionProtoTypes(this_()),
       TemplateSpecializationTypes(this_()),
       DependentTemplateSpecializationTypes(this_()), AutoTypes(this_()),
@@ -978,11 +978,10 @@
                                         LangOpts.XRayAttrListFiles, SM)),
       ProfList(new ProfileList(LangOpts.ProfileListFiles, SM)),
       PrintingPolicy(LOpts), Idents(idents), Selectors(sels),
-      BuiltinInfo(builtins), DeclarationNames(*this), Comments(SM),
-      CommentCommandTraits(BumpAlloc, LOpts.CommentOpts),
+      BuiltinInfo(builtins), TUKind(TUKind), DeclarationNames(*this),
+      Comments(SM), CommentCommandTraits(BumpAlloc, LOpts.CommentOpts),
       CompCategories(this_()), LastSDM(nullptr, 0) {
-  TUDecl = TranslationUnitDecl::Create(*this);
-  TraversalScope = {TUDecl};
+  addTranslationUnitDecl();
 }
 
 ASTContext::~ASTContext() {
@@ -1196,9 +1195,10 @@
 BuiltinTemplateDecl *
 ASTContext::buildBuiltinTemplateDecl(BuiltinTemplateKind BTK,
                                      const IdentifierInfo *II) const {
-  auto *BuiltinTemplate = BuiltinTemplateDecl::Create(*this, TUDecl, II, BTK);
+  auto *BuiltinTemplate =
+      BuiltinTemplateDecl::Create(*this, getTranslationUnitDecl(), II, BTK);
   BuiltinTemplate->setImplicit();
-  TUDecl->addDecl(BuiltinTemplate);
+  getTranslationUnitDecl()->addDecl(BuiltinTemplate);
 
   return BuiltinTemplate;
 }
@@ -1485,7 +1485,7 @@
   // MSVC predeclares struct _GUID, and we need it to create MSGuidDecls.
   if (LangOpts.MicrosoftExt || LangOpts.Borland) {
     MSGuidTagDecl = buildImplicitRecord("_GUID");
-    TUDecl->addDecl(MSGuidTagDecl);
+    getTranslationUnitDecl()->addDecl(MSGuidTagDecl);
   }
 }
 
@@ -6607,7 +6607,7 @@
 QualType ASTContext::getObjCSuperType() const {
   if (ObjCSuperType.isNull()) {
     RecordDecl *ObjCSuperTypeDecl = buildImplicitRecord("objc_super");
-    TUDecl->addDecl(ObjCSuperTypeDecl);
+    getTranslationUnitDecl()->addDecl(ObjCSuperTypeDecl);
     ObjCSuperType = getTagDeclType(ObjCSuperTypeDecl);
   }
   return ObjCSuperType;
Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -1375,7 +1375,7 @@
   /// initializers for tentative definitions in C) once parsing has
   /// completed. Modules and precompiled headers perform different kinds of
   /// checks.
-  TranslationUnitKind TUKind;
+  const TranslationUnitKind TUKind;
 
   llvm::BumpPtrAllocator BumpAlloc;
 
Index: clang/include/clang/Lex/Preprocessor.h
===================================================================
--- clang/include/clang/Lex/Preprocessor.h
+++ clang/include/clang/Lex/Preprocessor.h
@@ -264,9 +264,11 @@
   /// avoid tearing the Lexer and etc. down).
   bool IncrementalProcessing = false;
 
+public:
   /// The kind of translation unit we are processing.
-  TranslationUnitKind TUKind;
+  const TranslationUnitKind TUKind;
 
+private:
   /// The code-completion handler.
   CodeCompletionHandler *CodeComplete = nullptr;
 
Index: clang/include/clang/Interpreter/PartialTranslationUnit.h
===================================================================
--- clang/include/clang/Interpreter/PartialTranslationUnit.h
+++ clang/include/clang/Interpreter/PartialTranslationUnit.h
@@ -11,11 +11,10 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef LLVM_CLANG_INTERPRETER_TRANSACTION_H
-#define LLVM_CLANG_INTERPRETER_TRANSACTION_H
+#ifndef LLVM_CLANG_INTERPRETER_PARTIALTRANSLATIONUNIT_H
+#define LLVM_CLANG_INTERPRETER_PARTIALTRANSLATIONUNIT_H
 
 #include <memory>
-#include <vector>
 
 namespace llvm {
 class Module;
@@ -23,17 +22,16 @@
 
 namespace clang {
 
-class DeclGroupRef;
+class TranslationUnitDecl;
 
 /// The class keeps track of various objects created as part of processing
 /// incremental inputs.
-struct Transaction {
-  /// The decls created for the input.
-  std::vector<clang::DeclGroupRef> Decls;
+struct PartialTranslationUnit {
+  TranslationUnitDecl *TUPart = nullptr;
 
   /// The llvm IR produced for the input.
   std::unique_ptr<llvm::Module> TheModule;
 };
 } // namespace clang
 
-#endif // LLVM_CLANG_INTERPRETER_TRANSACTION_H
+#endif // LLVM_CLANG_INTERPRETER_PARTIALTRANSLATIONUNIT_H
Index: clang/include/clang/Interpreter/Interpreter.h
===================================================================
--- clang/include/clang/Interpreter/Interpreter.h
+++ clang/include/clang/Interpreter/Interpreter.h
@@ -14,7 +14,7 @@
 #ifndef LLVM_CLANG_INTERPRETER_INTERPRETER_H
 #define LLVM_CLANG_INTERPRETER_INTERPRETER_H
 
-#include "clang/Interpreter/Transaction.h"
+#include "clang/Interpreter/PartialTranslationUnit.h"
 
 #include "llvm/Support/Error.h"
 
@@ -55,14 +55,14 @@
   static llvm::Expected<std::unique_ptr<Interpreter>>
   create(std::unique_ptr<CompilerInstance> CI);
   const CompilerInstance *getCompilerInstance() const;
-  llvm::Expected<Transaction &> Parse(llvm::StringRef Code);
-  llvm::Error Execute(Transaction &T);
+  llvm::Expected<PartialTranslationUnit &> Parse(llvm::StringRef Code);
+  llvm::Error Execute(PartialTranslationUnit &T);
   llvm::Error ParseAndExecute(llvm::StringRef Code) {
-    auto ErrOrTransaction = Parse(Code);
-    if (auto Err = ErrOrTransaction.takeError())
+    auto ErrOrPTU = Parse(Code);
+    if (auto Err = ErrOrPTU.takeError())
       return Err;
-    if (ErrOrTransaction->TheModule)
-      return Execute(*ErrOrTransaction);
+    if (ErrOrPTU->TheModule)
+      return Execute(*ErrOrPTU);
     return llvm::Error::success();
   }
 };
Index: clang/include/clang/Basic/LangOptions.h
===================================================================
--- clang/include/clang/Basic/LangOptions.h
+++ clang/include/clang/Basic/LangOptions.h
@@ -685,7 +685,10 @@
   TU_Prefix,
 
   /// The translation unit is a module.
-  TU_Module
+  TU_Module,
+
+  /// The translation unit is a partial translation unit, growing incrementally.
+  TU_Partial
 };
 
 } // namespace clang
Index: clang/include/clang/AST/Redeclarable.h
===================================================================
--- clang/include/clang/AST/Redeclarable.h
+++ clang/include/clang/AST/Redeclarable.h
@@ -193,6 +193,7 @@
 public:
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
+  friend class IncrementalParser;
 
   Redeclarable(const ASTContext &Ctx)
       : RedeclLink(LatestDeclLink(Ctx)),
Index: clang/include/clang/AST/Decl.h
===================================================================
--- clang/include/clang/AST/Decl.h
+++ clang/include/clang/AST/Decl.h
@@ -79,7 +79,22 @@
 class VarTemplateDecl;
 
 /// The top declaration context.
-class TranslationUnitDecl : public Decl, public DeclContext {
+class TranslationUnitDecl : public Decl, public DeclContext,
+                            public Redeclarable<TranslationUnitDecl> {
+  using redeclarable_base = Redeclarable<TranslationUnitDecl>;
+
+  TranslationUnitDecl *getNextRedeclarationImpl() override {
+    return getNextRedeclaration();
+  }
+
+  TranslationUnitDecl *getPreviousDeclImpl() override {
+    return getPreviousDecl();
+  }
+
+  TranslationUnitDecl *getMostRecentDeclImpl() override {
+    return getMostRecentDecl();
+  }
+
   ASTContext &Ctx;
 
   /// The (most recently entered) anonymous namespace for this
@@ -91,6 +106,16 @@
   virtual void anchor();
 
 public:
+  using redecl_range = redeclarable_base::redecl_range;
+  using redecl_iterator = redeclarable_base::redecl_iterator;
+
+  using redeclarable_base::redecls_begin;
+  using redeclarable_base::redecls_end;
+  using redeclarable_base::redecls;
+  using redeclarable_base::getPreviousDecl;
+  using redeclarable_base::getMostRecentDecl;
+  using redeclarable_base::isFirstDecl;
+
   ASTContext &getASTContext() const { return Ctx; }
 
   NamespaceDecl *getAnonymousNamespace() const { return AnonymousNamespace; }
Index: clang/include/clang/AST/ASTContext.h
===================================================================
--- clang/include/clang/AST/ASTContext.h
+++ clang/include/clang/AST/ASTContext.h
@@ -459,6 +459,7 @@
   friend class ASTWriter;
   template <class> friend class serialization::AbstractTypeReader;
   friend class CXXRecordDecl;
+  friend class IncrementalParser;
 
   /// A mapping to contain the template or declaration that
   /// a variable declaration describes or was instantiated from,
@@ -556,7 +557,7 @@
   ImportDecl *FirstLocalImport = nullptr;
   ImportDecl *LastLocalImport = nullptr;
 
-  TranslationUnitDecl *TUDecl;
+  TranslationUnitDecl *TUDecl = nullptr;
   mutable ExternCContextDecl *ExternCContext = nullptr;
   mutable BuiltinTemplateDecl *MakeIntegerSeqDecl = nullptr;
   mutable BuiltinTemplateDecl *TypePackElementDecl = nullptr;
@@ -613,6 +614,7 @@
   IdentifierTable &Idents;
   SelectorTable &Selectors;
   Builtin::Context &BuiltinInfo;
+  const TranslationUnitKind TUKind;
   mutable DeclarationNameTable DeclarationNames;
   IntrusiveRefCntPtr<ExternalASTSource> ExternalSource;
   ASTMutationListener *Listener = nullptr;
@@ -992,7 +994,18 @@
   /// Get the initializations to perform when importing a module, if any.
   ArrayRef<Decl*> getModuleInitializers(Module *M);
 
-  TranslationUnitDecl *getTranslationUnitDecl() const { return TUDecl; }
+  TranslationUnitDecl *getTranslationUnitDecl() const {
+    return TUDecl->getMostRecentDecl();
+  }
+  void addTranslationUnitDecl() {
+    assert(!TUDecl || TUKind == TU_Partial);
+    TranslationUnitDecl *NewTUDecl = TranslationUnitDecl::Create(*this);
+    if (TraversalScope.empty() || TraversalScope.back() == TUDecl)
+      TraversalScope = {NewTUDecl};
+    if (TUDecl)
+      NewTUDecl->setPreviousDecl(TUDecl);
+    TUDecl = NewTUDecl;
+  }
 
   ExternCContextDecl *getExternCContextDecl() const;
   BuiltinTemplateDecl *getMakeIntegerSeqDecl() const;
@@ -1069,7 +1082,8 @@
   llvm::DenseSet<const VarDecl *> CUDADeviceVarODRUsedByHost;
 
   ASTContext(LangOptions &LOpts, SourceManager &SM, IdentifierTable &idents,
-             SelectorTable &sels, Builtin::Context &builtins);
+             SelectorTable &sels, Builtin::Context &builtins,
+             TranslationUnitKind TUKind);
   ASTContext(const ASTContext &) = delete;
   ASTContext &operator=(const ASTContext &) = delete;
   ~ASTContext();
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