[clang] Introduce [[clang::lifetime_capture_by(X)]]

clang/docs/ReleaseNotes.rst

@@ -449,6 +449,9 @@ Attribute Changes in Clang
 - Fix a bug where clang doesn't automatically apply the ``[[gsl::Owner]]`` or
   ``[[gsl::Pointer]]`` to STL explicit template specialization decls. (#GH109442)
 
+- Clang now supports ``[[clang::lifetime_capture_by(X)]]``. Similar to lifetimebound, this can be
+  used to specify when a reference to a function parameter is captured by another capturing entity ``X``.
+
 Improvements to Clang's diagnostics
 -----------------------------------
 

clang/include/clang/Basic/Attr.td

@@ -1889,6 +1889,39 @@ 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 = [LifetimeCaptureByDocs];
+  let AdditionalMembers = [{
+private:
+  SmallVector<IdentifierInfo*, 1> ArgIdents;
+  SmallVector<SourceLocation, 1> ArgLocs;
+
+public:
+  static constexpr int THIS = 0;
+  static constexpr int INVALID = -1;
+  static constexpr int UNKNOWN = -2;
+  static constexpr int GLOBAL = -3;
+
+  void setArgs(SmallVector<IdentifierInfo*>&& Idents,
+               SmallVector<SourceLocation>&& Locs) { 
+    assert(Idents.size() == Locs.size());
+    assert(Idents.size() == params_Size);
+    ArgIdents = std::move(Idents);
+    ArgLocs = std::move(Locs);
+  }
+
+  ArrayRef<IdentifierInfo*> getArgIdents() const { return ArgIdents; }
+  ArrayRef<SourceLocation> 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/AttrDocs.td

@@ -3918,6 +3918,75 @@ have their lifetimes extended.
   }];
 }
 
+def LifetimeCaptureByDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+    Similar to `lifetimebound`_, the ``lifetime_capture_by(X)`` attribute on a function
+parameter or implicit object parameter indicates that that objects that are referred to
+by that parameter may also be referred to by the capturing entity ``X``.
+
+By default, a reference is considered to refer to its referenced object, a
+pointer is considered to refer to its pointee, a ``std::initializer_list<T>``
+is considered to refer to its underlying array, and aggregates (arrays and
+simple ``struct``\s) are considered to refer to all objects that their
+transitive subobjects refer to.
+
+The capturing entity ``X`` can be one of the following:
+- Another (named) function parameter. 
+
+  .. code-block:: c++
+
+    void addToSet(std::string_view a [[clang::lifetime_capture_by(s)]], std::set<std::string_view>& s) {
+      s.insert(a);
+    }
+
+- ``this`` (in case of member functions).
+
+  .. code-block:: c++
+
+    class S {
+      void addToSet(std::string_view a [[clang::lifetime_capture_by(this)]]) {
+        s.insert(a);
+      }
+      std::set<std::string_view> s;
+    };
+
+- 'global', 'unknown' (without quotes).
+
+  .. code-block:: c++
+
+    std::set<std::string_view> s;
+    void addToSet(std::string_view a [[clang::lifetime_capture_by(global)]]) {
+      s.insert(a);
+    }
+    void addSomewhere(std::string_view a [[clang::lifetime_capture_by(unknown)]]);
+
+The attribute can be applied to the implicit ``this`` parameter of a member
+function by writing the attribute after the function type:
+
+.. code-block:: c++
+
+  struct S {
+    const char *data(std::set<S*>& s) [[clang::lifetime_capture_by(s)]] {
+      s.insert(this);
+    }
+  };
+
+The attribute supports specifying more than one capturing entities:
+
+.. code-block:: c++
+
+  void addToSets(std::string_view a [[clang::lifetime_capture_by(s1, s2)]],
+                 std::set<std::string_view>& s1,
+                 std::set<std::string_view>& s2) {
+    s1.insert(a);
+    s2.insert(a);
+  }
+
+.. _`lifetimebound`: https://clang.llvm.org/docs/AttributeReference.html#lifetimebound
+  }];
+}
+
 def TrivialABIDocs : Documentation {
   let Category = DocCatDecl;
   let Content = [{

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3383,6 +3383,20 @@ 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, 'this', 'global' or 'unknown'">;
+def err_capture_by_references_itself : Error<"'lifetime_capture_by' argument references itself">;
+def err_capture_by_param_uses_reserved_name : Error<
+  "parameter cannot be named '%select{global|unknown}0' while using 'lifetime_capture_by(%select{global|unknown}0)'">;
+
 def err_init_method_bad_return_type : Error<
   "init methods must return an object pointer type, not %0">;
 def err_attribute_invalid_size : Error<

clang/include/clang/Sema/Sema.h

@@ -1760,6 +1760,14 @@ class Sema final : public SemaBase {
   /// Add [[gsl::Pointer]] attributes for std:: types.
   void inferGslPointerAttribute(TypedefNameDecl *TD);
 
+  LifetimeCaptureByAttr *ParseLifetimeCaptureByAttr(const ParsedAttr &AL,
+                                                    StringRef ParamName);
+  // Processes the argument 'X' in [[clang::lifetime_capture_by(X)]]. Since 'X'
+  // can be the name of a function parameter, we need to parse the function
+  // declaration and rest of the parameters before processesing 'X'. Therefore
+  // do this lazily instead of processing while parsing the annotation itself.
+  void LazyProcessLifetimeCaptureByParams(FunctionDecl *FD);
+
   /// Add _Nullable attributes for std:: types.
   void inferNullableClassAttribute(CXXRecordDecl *CRD);
 

clang/lib/AST/TypePrinter.cpp

@@ -25,6 +25,7 @@
 #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"
@@ -1909,6 +1910,19 @@ void TypePrinter::printAttributedAfter(const AttributedType *T,
     OS << " [[clang::lifetimebound]]";
     return;
   }
+  if (T->getAttrKind() == attr::LifetimeCaptureBy) {
+    OS << " [[clang::lifetime_capture_by(";
+    if (auto *attr = dyn_cast_or_null<LifetimeCaptureByAttr>(T->getAttr())) {
+      auto Idents = attr->getArgIdents();
+      for (unsigned I = 0; I < Idents.size(); ++I) {
+        OS << Idents[I]->getName();
+        if (I != Idents.size() - 1)
+          OS << ", ";
+      }
+    }
+    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
@@ -1976,6 +1990,7 @@ 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/SemaDecl.cpp

@@ -16687,6 +16687,7 @@ void Sema::AddKnownFunctionAttributes(FunctionDecl *FD) {
     }
   }
 
+  LazyProcessLifetimeCaptureByParams(FD);
   inferLifetimeBoundAttribute(FD);
   AddKnownFunctionAttributesForReplaceableGlobalAllocationFunction(FD);
 

clang/lib/Sema/SemaDeclAttr.cpp

@@ -14,6 +14,7 @@
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTMutationListener.h"
 #include "clang/AST/CXXInheritance.h"
+#include "clang/AST/Decl.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
@@ -3867,6 +3868,113 @@ static void handleCallbackAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
       S.Context, AL, EncodingIndices.data(), EncodingIndices.size()));
 }
 
+LifetimeCaptureByAttr *Sema::ParseLifetimeCaptureByAttr(const ParsedAttr &AL,
+                                                        StringRef ParamName) {
+  // Atleast one capture by is required.
+  if (AL.getNumArgs() == 0) {
+    Diag(AL.getLoc(), diag::err_capture_by_attribute_no_entity)
+        << AL.getRange();
+    return nullptr;
+  }
+  SmallVector<IdentifierInfo *, 1> ParamIdents;
+  SmallVector<SourceLocation, 1> ParamLocs;
+  for (unsigned I = 0; I < AL.getNumArgs(); ++I) {
+    if (AL.isArgExpr(I)) {
+      Expr *E = AL.getArgAsExpr(I);
+      Diag(E->getExprLoc(), diag::err_capture_by_attribute_argument_unknown)
+          << E << E->getExprLoc();
+      continue;
+    }
+    assert(AL.isArgIdent(I));
+    IdentifierLoc *IdLoc = AL.getArgAsIdent(I);
+    if (IdLoc->Ident->getName() == ParamName) {
+      Diag(IdLoc->Loc, diag::err_capture_by_references_itself) << IdLoc->Loc;
+      continue;
+    }
+    ParamIdents.push_back(IdLoc->Ident);
+    ParamLocs.push_back(IdLoc->Loc);
+  }
+  SmallVector<int, 1> FakeParamIndices(ParamIdents.size(),
+                                       LifetimeCaptureByAttr::INVALID);
+  LifetimeCaptureByAttr *CapturedBy = ::new (Context) LifetimeCaptureByAttr(
+      Context, AL, FakeParamIndices.data(), FakeParamIndices.size());
+  CapturedBy->setArgs(std::move(ParamIdents), std::move(ParamLocs));
+  return CapturedBy;
+}
+
+static void HandleLifetimeCaptureByAttr(Sema &S, Decl *D,
+                                        const ParsedAttr &AL) {
+  // Do not allow multiple attributes.
+  if (D->hasAttr<LifetimeCaptureByAttr>()) {
+    S.Diag(AL.getLoc(), diag::err_capture_by_attribute_multiple)
+        << AL.getRange();
+    return;
+  }
+  auto *PVD = dyn_cast<ParmVarDecl>(D);
+  assert(PVD);
+  auto *CaptureByAttr = S.ParseLifetimeCaptureByAttr(AL, PVD->getName());
+  if (CaptureByAttr)
+    D->addAttr(CaptureByAttr);
+}
+
+void Sema::LazyProcessLifetimeCaptureByParams(FunctionDecl *FD) {
+  bool HasImplicitThisParam = isInstanceMethod(FD);
+
+  llvm::StringMap<int> NameIdxMapping;
+  NameIdxMapping["global"] = LifetimeCaptureByAttr::GLOBAL;
+  NameIdxMapping["unknown"] = LifetimeCaptureByAttr::UNKNOWN;
+  int Idx = 0;
+  if (HasImplicitThisParam) {
+    NameIdxMapping["this"] = 0;
+    Idx++;
+  }
+  for (const ParmVarDecl *PVD : FD->parameters())
+    NameIdxMapping[PVD->getName()] = Idx++;
+  auto DisallowReservedParams = [&](StringRef Reserved) {
+    for (const ParmVarDecl *PVD : FD->parameters())
+      if (PVD->getName() == Reserved)
+        Diag(PVD->getLocation(), diag::err_capture_by_param_uses_reserved_name)
+            << (PVD->getName() == "unknown");
+  };
+  auto HandleCaptureBy = [&](LifetimeCaptureByAttr *CapturedBy) {
+    if (!CapturedBy)
+      return;
+    const auto &Entities = CapturedBy->getArgIdents();
+    for (size_t I = 0; I < Entities.size(); ++I) {
+      StringRef Name = Entities[I]->getName();
+      auto It = NameIdxMapping.find(Name);
+      if (It == NameIdxMapping.end()) {
+        auto Loc = CapturedBy->getArgLocs()[I];
+        if (!HasImplicitThisParam && Name == "this")
+          Diag(Loc, diag::err_capture_by_implicit_this_not_available) << Loc;
+        else
+          Diag(Loc, diag::err_capture_by_attribute_argument_unknown)
+              << Entities[I] << Loc;
+        continue;
+      }
+      if (Name == "unknown" || Name == "global")
+        DisallowReservedParams(Name);
+      CapturedBy->setParamIdx(I, It->second);
+    }
+  };
+  for (ParmVarDecl *PVD : FD->parameters())
+    HandleCaptureBy(PVD->getAttr<LifetimeCaptureByAttr>());
+  if (!HasImplicitThisParam)
+    return;
+  TypeSourceInfo *TSI = FD->getTypeSourceInfo();
+  if (!TSI)
+    return;
+  AttributedTypeLoc ATL;
+  for (TypeLoc TL = TSI->getTypeLoc();
+       (ATL = TL.getAsAdjusted<AttributedTypeLoc>());
+       TL = ATL.getModifiedLoc()) {
+    auto *A = ATL.getAttrAs<LifetimeCaptureByAttr>();
+    if (!A)
+      continue;
+    HandleCaptureBy(const_cast<LifetimeCaptureByAttr *>(A));
+  }
+}
+
 static bool isFunctionLike(const Type &T) {
   // Check for explicit function types.
   // 'called_once' is only supported in Objective-C and it has
@@ -6644,6 +6752,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_Callback:
     handleCallbackAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_LifetimeCaptureBy:
+    HandleLifetimeCaptureByAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_CalledOnce:
     handleCalledOnceAttr(S, D, AL);
     break;

clang/lib/Sema/SemaType.cpp

@@ -8609,6 +8609,15 @@ static void HandleLifetimeBoundAttr(TypeProcessingState &State,
   }
 }
 
+static void HandleLifetimeCaptureByAttr(TypeProcessingState &State,
+                                        QualType &CurType, ParsedAttr &PA) {
+  if (State.getDeclarator().isDeclarationOfFunction()) {
+    auto *Attr = State.getSema().ParseLifetimeCaptureByAttr(PA, "this");
+    if (Attr)
+      CurType = State.getAttributedType(Attr, CurType, CurType);
+  }
+}
+
 static void HandleHLSLParamModifierAttr(TypeProcessingState &State,
                                         QualType &CurType,
                                         const ParsedAttr &Attr, Sema &S) {
@@ -8770,6 +8779,10 @@ static void processTypeAttrs(TypeProcessingState &state, QualType &type,
       if (TAL == TAL_DeclChunk)
         HandleLifetimeBoundAttr(state, type, attr);
       break;
+    case ParsedAttr::AT_LifetimeCaptureBy:
+      if (TAL == TAL_DeclChunk)
+        HandleLifetimeCaptureByAttr(state, type, attr);
+      break;
 
     case ParsedAttr::AT_NoDeref: {
       // FIXME: `noderef` currently doesn't work correctly in [[]] syntax.

clang/test/AST/attr-lifetime-capture-by.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 %s -ast-dump | FileCheck %s
+
+// Verify that we print the [[clang::lifetime_capture_by(X)]] attribute.
+
+struct S {
+    void foo(int &a, int &b) [[clang::lifetime_capture_by(a, b, global)]];
+};
+
+// CHECK: CXXMethodDecl {{.*}}clang::lifetime_capture_by(a, b, global)