start working on lifetime capture

clang/include/clang/Basic/Attr.td

@@ -1869,6 +1869,46 @@ def LifetimeBound : DeclOrTypeAttr {
   let SimpleHandler = 1;
 }
 
+def LifetimeCaptureBy : DeclOrTypeAttr {
+  let Spellings = [Clang<"lifetime_capture_by", 0>];
+  let Subjects = SubjectList<[ParmVar, ImplicitObjectParameter], ErrorDiag>;
+  let Args = [VariadicParamOrParamIdxArgument<"Params">];
+  let Documentation = [LifetimeBoundDocs];
+  let LangOpts = [CPlusPlus];
+
+  // let SimpleHandler = 1;
+  // let LateParsed = LateAttrParseStandard;
+  // let HasCustomParsing = 1;
+  // let ParseArgumentsAsUnevaluated = 1;
+
+  let AdditionalMembers = [{
+private:
+  SmallVector<IdentifierInfo*, 1> ArgIdents;
+  SmallVector<SourceLocation, 1> ArgLocs;
+
+public:
+  static const int INVALID = -2;
+  static const int UNKNOWN = -1;
+  static const int GLOBAL = -1;
+  static const int THIS = 0;
+
+  void setArgs(SmallVector<IdentifierInfo*, 1> Idents,
+               SmallVector<SourceLocation, 1> Locs) { 
+    assert(Idents.size() == Locs.size());
+    assert(Idents.size() == params_Size);
+    ArgIdents = std::move(Idents);
+    ArgLocs = std::move(Locs);
+  }
+
+  const SmallVector<IdentifierInfo*, 1>& getArgIdents() const { return ArgIdents; }
+  const SmallVector<SourceLocation, 1>& getArgLocs() const { return ArgLocs; }
+  void setParamIdx(size_t Idx, int Val) { 
+    assert(Idx < params_Size);
+    params_[Idx] = Val;
+  }
+}];
+}
+
 def TrivialABI : InheritableAttr {
   // This attribute does not have a C [[]] spelling because it requires the
   // CPlusPlus language option.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3382,6 +3382,18 @@ def err_callback_callee_is_variadic : Error<
   "'callback' attribute callee may not be variadic">;
 def err_callback_implicit_this_not_available : Error<
   "'callback' argument at position %0 references unavailable implicit 'this'">;
+
+def err_capture_by_attribute_multiple : Error<
+  "multiple 'lifetime_capture' attributes specified">;
+def err_capture_by_attribute_no_entity : Error<
+  "'lifetime_capture_by' attribute specifies no capturing entity">;
+def err_capture_by_implicit_this_not_available : Error<
+  "'lifetime_capture_by' argument references unavailable implicit 'this'">;
+def err_capture_by_attribute_argument_unknown : Error<
+  "'lifetime_capture_by' attribute argument %0 is not a known function parameter"
+  ". Must be a function parameter of one of 'this', 'global' or 'unknown'">;
+def err_capture_by_references_itself : Error<"'lifetime_capture_by' argument references itself">;
+
 def err_init_method_bad_return_type : Error<
   "init methods must return an object pointer type, not %0">;
 def err_attribute_invalid_size : Error<
@@ -10185,6 +10197,10 @@ def warn_dangling_pointer_assignment : Warning<
    "object backing the pointer %0 "
    "will be destroyed at the end of the full-expression">,
    InGroup<DanglingAssignment>;
+def warn_dangling_reference_captured : Warning<
+   "object captured by the '%0' "
+   "will be destroyed at the end of the full-expression">,
+   InGroup<DanglingAssignment>;
 
 // For non-floating point, expressions of the form x == x or x != x
 // should result in a warning, since these always evaluate to a constant.

clang/include/clang/Sema/Sema.h

@@ -1830,6 +1830,10 @@ class Sema final : public SemaBase {
   /// Add [[gsl::Pointer]] attributes for std:: types.
   void inferGslPointerAttribute(TypedefNameDecl *TD);
 
+  LifetimeCaptureByAttr *ParseLifetimeCaptureByAttr(const ParsedAttr &AL,
+                                                    StringRef ParamName);
+  void LazyProcessLifetimeCaptureByParams(FunctionDecl *FD);
+
   /// Add _Nullable attributes for std:: types.
   void inferNullableClassAttribute(CXXRecordDecl *CRD);
 
@@ -2384,6 +2388,9 @@ class Sema final : public SemaBase {
   bool BuiltinVectorMath(CallExpr *TheCall, QualType &Res);
   bool BuiltinVectorToScalarMath(CallExpr *TheCall);
 
+  void checkLifetimeCaptureBy(FunctionDecl *FDecl, bool IsMemberFunction,
+                              const Expr *ThisArg, ArrayRef<const Expr *> Args);
+
   /// Handles the checks for format strings, non-POD arguments to vararg
   /// functions, NULL arguments passed to non-NULL parameters, diagnose_if
   /// attributes and AArch64 SME attributes.

clang/lib/AST/TypePrinter.cpp

@@ -12,6 +12,7 @@
 
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Attr.h"
+#include "clang/AST/Attrs.inc"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/DeclCXX.h"
@@ -25,6 +26,7 @@
 #include "clang/AST/TextNodeDumper.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/AddressSpaces.h"
+#include "clang/Basic/AttrKinds.h"
 #include "clang/Basic/ExceptionSpecificationType.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Basic/LLVM.h"
@@ -1907,6 +1909,24 @@ void TypePrinter::printAttributedAfter(const AttributedType *T,
     OS << " [[clang::lifetimebound]]";
     return;
   }
+  if (T->getAttrKind() == attr::LifetimeCaptureBy) {
+    OS << " [[clang::lifetime_capture_by(...)";
+    // const LifetimeCaptureByAttr* A= T->getAs<LifetimeCaptureByAttr>();
+    // bool valid = true;
+    // for (int I : A->params())
+    //   valid &= I != -2;
+    // if (valid) {
+    //   OS << "invalid)";
+    //   return;
+    // }
+    // for (size_t I = 0; I < A->params_size(); ++I) {
+    //   OS << A->getArgIdents()[I]->getName()
+    //      << "(idx: " << *(A->params_begin() + I) << ")";
+    //   if (I != A->params_size() - 1)
+    //     OS << ", ";
+    // }
+    return;
+  }
 
   // The printing of the address_space attribute is handled by the qualifier
   // since it is still stored in the qualifier. Return early to prevent printing
@@ -1966,6 +1986,7 @@ void TypePrinter::printAttributedAfter(const AttributedType *T,
   case attr::SizedBy:
   case attr::SizedByOrNull:
   case attr::LifetimeBound:
+  case attr::LifetimeCaptureBy:
   case attr::TypeNonNull:
   case attr::TypeNullable:
   case attr::TypeNullableResult:

clang/lib/Sema/CheckExprLifetime.cpp

@@ -41,10 +41,14 @@ enum LifetimeKind {
   /// a default member initializer), the program is ill-formed.
   LK_MemInitializer,
 
-  /// The lifetime of a temporary bound to this entity probably ends too soon,
+  /// The lifetime of a temporary bound to this entity may end too soon,
   /// because the entity is a pointer and we assign the address of a temporary
   /// object to it.
   LK_Assignment,
+
+  /// The lifetime of a temporary bound to this entity probably ends too soon,
+  /// because the entity may capture the reference to a temporary object.
+  LK_LifetimeCapture,
 };
 using LifetimeResult =
     llvm::PointerIntPair<const InitializedEntity *, 3, LifetimeKind>;
@@ -189,6 +193,7 @@ struct IndirectLocalPathEntry {
     VarInit,
     LValToRVal,
     LifetimeBoundCall,
+    LifetimeCapture,
     TemporaryCopy,
     LambdaCaptureInit,
     GslReferenceInit,
@@ -898,6 +903,7 @@ static SourceRange nextPathEntryRange(const IndirectLocalPath &Path, unsigned I,
     case IndirectLocalPathEntry::AddressOf:
     case IndirectLocalPathEntry::LValToRVal:
     case IndirectLocalPathEntry::LifetimeBoundCall:
+    case IndirectLocalPathEntry::LifetimeCapture:
     case IndirectLocalPathEntry::TemporaryCopy:
     case IndirectLocalPathEntry::GslReferenceInit:
     case IndirectLocalPathEntry::GslPointerInit:
@@ -928,6 +934,7 @@ static bool pathOnlyHandlesGslPointer(IndirectLocalPath &Path) {
     case IndirectLocalPathEntry::VarInit:
     case IndirectLocalPathEntry::AddressOf:
     case IndirectLocalPathEntry::LifetimeBoundCall:
+    case IndirectLocalPathEntry::LifetimeCapture:
       continue;
     case IndirectLocalPathEntry::GslPointerInit:
     case IndirectLocalPathEntry::GslReferenceInit:
@@ -948,21 +955,22 @@ static bool isAssignmentOperatorLifetimeBound(CXXMethodDecl *CMD) {
 }
 
 static bool shouldRunGSLAssignmentAnalysis(const Sema &SemaRef,
-                                           const AssignedEntity &Entity) {
+                                           const CapturingEntity &Entity) {
   bool EnableGSLAssignmentWarnings = !SemaRef.getDiagnostics().isIgnored(
       diag::warn_dangling_lifetime_pointer_assignment, SourceLocation());
   return (EnableGSLAssignmentWarnings &&
-          (isRecordWithAttr<PointerAttr>(Entity.LHS->getType()) ||
+          (isRecordWithAttr<PointerAttr>(Entity.Expression->getType()) ||
            isAssignmentOperatorLifetimeBound(Entity.AssignmentOperator)));
 }
 
 static void checkExprLifetimeImpl(Sema &SemaRef,
                                   const InitializedEntity *InitEntity,
                                   const InitializedEntity *ExtendingEntity,
                                   LifetimeKind LK,
-                                  const AssignedEntity *AEntity, Expr *Init) {
-  assert((AEntity && LK == LK_Assignment) ||
-         (InitEntity && LK != LK_Assignment));
+                                  const CapturingEntity *CEntity, Expr *Init) {
+  assert(InitEntity || CEntity);
+  assert(!CEntity || LK == LK_Assignment || LK == LK_LifetimeCapture);
+  assert(!InitEntity || LK != LK_Assignment);
   // If this entity doesn't have an interesting lifetime, don't bother looking
   // for temporaries within its initializer.
   if (LK == LK_FullExpression)
@@ -1046,6 +1054,17 @@ static void checkExprLifetimeImpl(Sema &SemaRef,
       break;
     }
 
+    case LK_LifetimeCapture: {
+      if (!MTE)
+        return false;
+      assert(shouldLifetimeExtendThroughPath(Path) ==
+                 PathLifetimeKind::NoExtend &&
+             "No lifetime extension for in function calls");
+      SemaRef.Diag(DiagLoc, diag::warn_dangling_reference_captured)
+          << CEntity->Expression << DiagRange;
+      return false;
+    }
+
     case LK_Assignment: {
       if (!MTE || pathContainsInit(Path))
         return false;
@@ -1056,7 +1075,7 @@ static void checkExprLifetimeImpl(Sema &SemaRef,
                    IsGslPtrValueFromGslTempOwner
                        ? diag::warn_dangling_lifetime_pointer_assignment
                        : diag::warn_dangling_pointer_assignment)
-          << AEntity->LHS << DiagRange;
+          << CEntity->Expression << DiagRange;
       return false;
     }
     case LK_MemInitializer: {
@@ -1199,6 +1218,7 @@ static void checkExprLifetimeImpl(Sema &SemaRef,
         break;
 
       case IndirectLocalPathEntry::LifetimeBoundCall:
+      case IndirectLocalPathEntry::LifetimeCapture:
       case IndirectLocalPathEntry::TemporaryCopy:
       case IndirectLocalPathEntry::GslPointerInit:
       case IndirectLocalPathEntry::GslReferenceInit:
@@ -1243,8 +1263,10 @@ static void checkExprLifetimeImpl(Sema &SemaRef,
   };
 
   llvm::SmallVector<IndirectLocalPathEntry, 8> Path;
-  if (LK == LK_Assignment && shouldRunGSLAssignmentAnalysis(SemaRef, *AEntity))
+  if (LK == LK_Assignment && shouldRunGSLAssignmentAnalysis(SemaRef, *CEntity))
     Path.push_back({IndirectLocalPathEntry::GslPointerAssignment, Init});
+  else if (LK == LK_LifetimeCapture)
+    Path.push_back({IndirectLocalPathEntry::LifetimeCapture, Init});
 
   if (Init->isGLValue())
     visitLocalsRetainedByReferenceBinding(Path, Init, RK_ReferenceBinding,
@@ -1256,7 +1278,7 @@ static void checkExprLifetimeImpl(Sema &SemaRef,
         /*RevisitSubinits=*/!InitEntity);
 }
 
-void checkExprLifetime(Sema &SemaRef, const InitializedEntity &Entity,
+void checkInitLifetime(Sema &SemaRef, const InitializedEntity &Entity,
                        Expr *Init) {
   auto LTResult = getEntityLifetime(&Entity);
   LifetimeKind LK = LTResult.getInt();
@@ -1265,20 +1287,26 @@ void checkExprLifetime(Sema &SemaRef, const InitializedEntity &Entity,
                         /*AEntity*/ nullptr, Init);
 }
 
-void checkExprLifetime(Sema &SemaRef, const AssignedEntity &Entity,
-                       Expr *Init) {
+void checkAssignmentLifetime(Sema &SemaRef, const CapturingEntity &Entity,
+                             Expr *RHS) {
   bool EnableDanglingPointerAssignment = !SemaRef.getDiagnostics().isIgnored(
       diag::warn_dangling_pointer_assignment, SourceLocation());
   bool RunAnalysis = (EnableDanglingPointerAssignment &&
-                      Entity.LHS->getType()->isPointerType()) ||
+                      Entity.Expression->getType()->isPointerType()) ||
                      shouldRunGSLAssignmentAnalysis(SemaRef, Entity);
 
   if (!RunAnalysis)
     return;
 
   checkExprLifetimeImpl(SemaRef, /*InitEntity=*/nullptr,
                         /*ExtendingEntity=*/nullptr, LK_Assignment, &Entity,
-                        Init);
+                        RHS);
 }
 
+void checkCaptureLifetime(Sema &SemaRef, const CapturingEntity &Entity,
+                          Expr *Captured) {
+  checkExprLifetimeImpl(SemaRef, /*InitEntity=*/nullptr,
+                        /*ExtendingEntity=*/nullptr, LK_LifetimeCapture,
+                        &Entity, Captured);
+}
 } // namespace clang::sema

clang/lib/Sema/CheckExprLifetime.h

@@ -18,22 +18,36 @@
 
 namespace clang::sema {
 
-/// Describes an entity that is being assigned.
-struct AssignedEntity {
-  // The left-hand side expression of the assignment.
-  Expr *LHS = nullptr;
+struct CapturingEntity {
+  // The expression of the entity which captures another entity.
+  // For example:
+  //  1. In an assignment, this would be the left-hand side expression.
+  //    std::string_view sv;
+  //    sv = std::string(); // Here 'sv' is the 'Entity'.
+  //
+  //  2. In an function call involving a lifetime capture, this would be the
+  //  argument capturing the lifetime of another argument.
+  //    void addToSet(std::string_view s [[clang::lifetime_capture_by(sv)]],
+  //                  set<std::string_view>& setsv);
+  //    set<std::string_view> ssv;
+  //    addToSet(std::string(), ssv); // Here 'ssv' is the 'Entity'.
+  Expr *Expression = nullptr;
   CXXMethodDecl *AssignmentOperator = nullptr;
 };
 
 /// Check that the lifetime of the given expr (and its subobjects) is
 /// sufficient for initializing the entity, and perform lifetime extension
 /// (when permitted) if not.
-void checkExprLifetime(Sema &SemaRef, const InitializedEntity &Entity,
+void checkInitLifetime(Sema &SemaRef, const InitializedEntity &Entity,
                        Expr *Init);
 
 /// Check that the lifetime of the given expr (and its subobjects) is
 /// sufficient for assigning to the entity.
-void checkExprLifetime(Sema &SemaRef, const AssignedEntity &Entity, Expr *Init);
+void checkAssignmentLifetime(Sema &SemaRef, const CapturingEntity &Entity,
+                             Expr *RHS);
+
+void checkCaptureLifetime(Sema &SemaRef, const CapturingEntity &Entity,
+                          Expr *Captured);
 
 } // namespace clang::sema
 

clang/lib/Sema/SemaChecking.cpp

@@ -11,10 +11,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "CheckExprLifetime.h"
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/AttrIterator.h"
+#include "clang/AST/Attrs.inc"
 #include "clang/AST/CharUnits.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
@@ -3203,6 +3205,29 @@ void Sema::CheckArgAlignment(SourceLocation Loc, NamedDecl *FDecl,
         << ParamName << (FDecl != nullptr) << FDecl;
 }
 
+void Sema::checkLifetimeCaptureBy(FunctionDecl *FD, bool IsMemberFunction,
+                                  const Expr *ThisArg,
+                                  ArrayRef<const Expr *> Args) {
+  auto GetArgAt = [&](int Idx) {
+    if (IsMemberFunction && Idx == 0)
+      return const_cast<Expr *>(ThisArg);
+    return const_cast<Expr *>(Args[Idx - int(IsMemberFunction)]);
+  };
+  for (unsigned I = 0; I < FD->getNumParams(); ++I) {
+    auto *CapturedByAttr =
+        FD->getParamDecl(I)->getAttr<LifetimeCaptureByAttr>();
+    if (!CapturedByAttr)
+      continue;
+    for (int CapturingParamIdx : CapturedByAttr->params()) {
+      Expr *Capturing = GetArgAt(CapturingParamIdx);
+      Expr *Captured = GetArgAt(I + IsMemberFunction);
+      CapturingEntity CE{Capturing};
+      // Ensure that 'Captured' lives atleast as long as the 'Capturing' entity.
+      checkCaptureLifetime(*this, CE, Captured);
+    }
+  }
+}
+
 void Sema::checkCall(NamedDecl *FDecl, const FunctionProtoType *Proto,
                      const Expr *ThisArg, ArrayRef<const Expr *> Args,
                      bool IsMemberFunction, SourceLocation Loc,
@@ -3244,6 +3269,8 @@ void Sema::checkCall(NamedDecl *FDecl, const FunctionProtoType *Proto,
     }
   }
 
+  if (FD)
+    checkLifetimeCaptureBy(FD, IsMemberFunction, ThisArg, Args);
   if (FDecl || Proto) {
     CheckNonNullArguments(*this, FDecl, Proto, Args, Loc);