Sema: Intro experimental @cdecl and basic C compatibility check

include/swift/AST/Attr.h

@@ -690,22 +690,29 @@ class SectionAttr : public DeclAttribute {
 /// Defines the @_cdecl attribute.
 class CDeclAttr : public DeclAttribute {
 public:
-  CDeclAttr(StringRef Name, SourceLoc AtLoc, SourceRange Range, bool Implicit)
-      : DeclAttribute(DeclAttrKind::CDecl, AtLoc, Range, Implicit), Name(Name) {
+  CDeclAttr(StringRef Name, SourceLoc AtLoc, SourceRange Range, bool Implicit,
+            bool Underscored)
+      : DeclAttribute(DeclAttrKind::CDecl, AtLoc, Range, Implicit),
+        Name(Name), Underscored(Underscored) {
   }
 
-  CDeclAttr(StringRef Name, bool Implicit)
-    : CDeclAttr(Name, SourceLoc(), SourceRange(), Implicit) {}
+  CDeclAttr(StringRef Name, bool Implicit, bool Underscored)
+    : CDeclAttr(Name, SourceLoc(), SourceRange(), Implicit, Underscored) {}
 
   /// The symbol name.
   const StringRef Name;
 
+  /// Is this the version of the attribute that's underscored?
+  /// Used to preserve retro compatibility with early adopters.
+  const bool Underscored;
+
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DeclAttrKind::CDecl;
   }
 
   CDeclAttr *clone(ASTContext &ctx) const {
-    return new (ctx) CDeclAttr(Name, AtLoc, Range, isImplicit());
+    return new (ctx) CDeclAttr(Name, AtLoc, Range, isImplicit(),
+                               Underscored);
   }
 
   bool isEquivalent(const CDeclAttr *other, Decl *attachedTo) const {

include/swift/AST/Decl.h

@@ -8128,6 +8128,11 @@ class AbstractFunctionDecl : public GenericContext, public ValueDecl {
   /// instance method.
   bool isObjCInstanceMethod() const;
 
+  /// Get the foreign language targeted by a @cdecl-style attribute, if any.
+  /// Used to abstract away the change in meaning of @cdecl vs @_cdecl while
+  /// formalizing the attribute.
+  std::optional<ForeignLanguage> getCDeclKind() const;
+
   /// Determine whether the name of an argument is an API name by default
   /// depending on the function context.
   bool argumentNameIsAPIByDefault() const;

include/swift/AST/DeclAttr.def

@@ -368,6 +368,7 @@ DECL_ATTR(_cdecl, CDecl,
   OnFunc | OnAccessor,
   LongAttribute | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
   63)
+DECL_ATTR_ALIAS(cdecl, CDecl)
 
 SIMPLE_DECL_ATTR(usableFromInline, UsableFromInline,
   OnAbstractFunction | OnVar | OnSubscript | OnNominalType | OnTypeAlias,

include/swift/AST/DiagnosticsSema.def

@@ -2044,11 +2044,14 @@ WARNING(wrap_objc_implementation_will_become_error,none,
         (DiagnosticInfo *))
 
 ERROR(cdecl_not_at_top_level,none,
-      "@_cdecl can only be applied to global functions", ())
+      "%0 can only be applied to global functions", (DeclAttribute))
 ERROR(cdecl_empty_name,none,
-      "@_cdecl symbol name cannot be empty", ())
+      "%0 symbol name cannot be empty", (DeclAttribute))
 ERROR(cdecl_throws,none,
-      "raising errors from @_cdecl functions is not supported", ())
+      "raising errors from %0 functions is not supported", (DeclAttribute))
+ERROR(cdecl_feature_required,none,
+      "@cdecl requires '-enable-experimental-feature CDecl'",
+      ())
 
 // @_used and @_section
 ERROR(section_linkage_markers_disabled,none,
@@ -6499,7 +6502,10 @@ ERROR(objc_cannot_infer_name_raw_identifier,none,
       (const ValueDecl *))
 
 // If you change this, also change enum ObjCReason
-#define OBJC_ATTR_SELECT "select{marked @_cdecl|marked dynamic|marked @objc|marked @objcMembers|marked @IBOutlet|marked @IBAction|marked @IBSegueAction|marked @NSManaged|a member of an @objc protocol|implicitly @objc|an @objc override|an implementation of an @objc requirement|marked @IBInspectable|marked @GKInspectable|in an @objc extension of a class (without @nonobjc)|in an @objc @implementation extension of a class (without final or @nonobjc)|marked @objc by an access note}"
+#define OBJC_ATTR_SELECT "select{marked @cdecl|marked @_cdecl|marked dynamic|marked @objc|marked @objcMembers|marked @IBOutlet|marked @IBAction|marked @IBSegueAction|marked @NSManaged|a member of an @objc protocol|implicitly @objc|an @objc override|an implementation of an @objc requirement|marked @IBInspectable|marked @GKInspectable|in an @objc extension of a class (without @nonobjc)|in an @objc @implementation extension of a class (without final or @nonobjc)|marked @objc by an access note}"
+
+// Keep aligned with enum ForeignLanguage
+#define FOREIGN_LANG_SELECT "select{C|Objective-C}"
 
 ERROR(objc_invalid_on_var,none,
       "property cannot be %" OBJC_ATTR_SELECT "0 "
@@ -6579,17 +6585,21 @@ ERROR(objc_invalid_on_func_variadic,none,
       "method cannot be %" OBJC_ATTR_SELECT "0 because it has a variadic "
       "parameter", (unsigned))
 ERROR(objc_invalid_on_func_inout,none,
-      "method cannot be %" OBJC_ATTR_SELECT "0 because inout "
-      "parameters cannot be represented in Objective-C", (unsigned))
+      "%kindonly0 cannot be %" OBJC_ATTR_SELECT "1 because inout "
+      "parameters cannot be represented in %" FOREIGN_LANG_SELECT "2",
+      (const AbstractFunctionDecl*, unsigned, unsigned))
 ERROR(objc_invalid_on_func_param_type,none,
-      "method cannot be %" OBJC_ATTR_SELECT "1 because the type of the "
-      "parameter %0 cannot be represented in Objective-C", (unsigned, unsigned))
+      "%kindonly0 cannot be %" OBJC_ATTR_SELECT "2 because the type of the "
+      "parameter %1 cannot be represented in %" FOREIGN_LANG_SELECT "3",
+      (const AbstractFunctionDecl*, unsigned, unsigned, unsigned))
 ERROR(objc_invalid_on_func_single_param_type,none,
-      "method cannot be %" OBJC_ATTR_SELECT "0 because the type of the "
-      "parameter cannot be represented in Objective-C", (unsigned))
+      "%kindonly0 cannot be %" OBJC_ATTR_SELECT "1 because the type of the "
+      "parameter cannot be represented in %" FOREIGN_LANG_SELECT "2",
+      (const AbstractFunctionDecl*, unsigned, unsigned))
 ERROR(objc_invalid_on_func_result_type,none,
-      "method cannot be %" OBJC_ATTR_SELECT "0 because its result type "
-      "cannot be represented in Objective-C", (unsigned))
+      "%kindonly0 cannot be %" OBJC_ATTR_SELECT "1 because its result type "
+      "cannot be represented in %" FOREIGN_LANG_SELECT "2",
+      (const AbstractFunctionDecl*, unsigned, unsigned))
 ERROR(objc_invalid_on_foreign_class,none,
       "method cannot be %" OBJC_ATTR_SELECT "0 because Core Foundation "
       "types are not classes in Objective-C", (unsigned))
@@ -6715,6 +6725,7 @@ ERROR(nonobjc_not_allowed,none,
 #undef OBJC_DIAG_SELECT_2
 #undef OBJC_DIAG_SELECT
 #undef OBJC_ATTR_SELECT
+#undef FOREIGN_LANG_SELECT
 
 //------------------------------------------------------------------------------
 // MARK: @exclusivity

include/swift/AST/TypeCheckRequests.h

@@ -4853,6 +4853,25 @@ class TypeCheckObjCImplementationRequest
   bool isCached() const { return true; }
 };
 
+/// Check @cdecl-style attributes for compatibility with the foreign language.
+class TypeCheckCDeclAttributeRequest
+    : public SimpleRequest<TypeCheckCDeclAttributeRequest,
+                           evaluator::SideEffect(FuncDecl *FD,
+                                                 CDeclAttr *attr),
+                           RequestFlags::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  evaluator::SideEffect
+  evaluate(Evaluator &evaluator, FuncDecl *FD, CDeclAttr *attr) const;
+
+public:
+  bool isCached() const { return true; }
+};
+
 void simple_display(llvm::raw_ostream &out, ASTNode node);
 void simple_display(llvm::raw_ostream &out, Type value);
 void simple_display(llvm::raw_ostream &out, const TypeRepr *TyR);

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -550,6 +550,9 @@ SWIFT_REQUEST(TypeChecker, IsNonUserModuleRequest,
 SWIFT_REQUEST(TypeChecker, TypeCheckObjCImplementationRequest,
               unsigned(ExtensionDecl *),
               Cached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, TypeCheckCDeclAttributeRequest,
+              evaluator::SideEffect(FuncDecl *, CDeclAttr *),
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, HasInitAccessorRequest,
               bool(AbstractStorageDecl *), Cached,
               NoLocationInfo)

include/swift/Basic/Features.def

@@ -513,6 +513,9 @@ EXPERIMENTAL_FEATURE(ClosureBodyMacro, true)
 /// Allow declarations of Swift runtime symbols using @_silgen_name.
 EXPERIMENTAL_FEATURE(AllowRuntimeSymbolDeclarations, true)
 
+/// Allow use of `@cdecl`
+EXPERIMENTAL_FEATURE(CDecl, false)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/Attr.cpp

@@ -1783,7 +1783,9 @@ StringRef DeclAttribute::getAttrName() const {
   case DeclAttrKind::Alignment:
     return "_alignment";
   case DeclAttrKind::CDecl:
-    return "_cdecl";
+    if (cast<CDeclAttr>(this)->Underscored)
+      return "_cdecl";
+    return "cdecl";
   case DeclAttrKind::SwiftNativeObjCRuntimeBase:
     return "_swift_native_objc_runtime_base";
   case DeclAttrKind::Semantics:

lib/AST/Bridging/DeclAttributeBridging.cpp

@@ -250,7 +250,7 @@ BridgedCDeclAttr BridgedCDeclAttr_createParsed(BridgedASTContext cContext,
                                                BridgedStringRef cName) {
   return new (cContext.unbridged())
       CDeclAttr(cName.unbridged(), cAtLoc.unbridged(), cRange.unbridged(),
-                /*Implicit=*/false);
+                /*Implicit=*/false, /*Underscored*/true);
 }
 
 BridgedCustomAttr BridgedCustomAttr_createParsed(

lib/AST/Decl.cpp

@@ -10449,6 +10449,15 @@ bool AbstractFunctionDecl::isObjCInstanceMethod() const {
   return isInstanceMember() || isa<ConstructorDecl>(this);
 }
 
+std::optional<ForeignLanguage> AbstractFunctionDecl::getCDeclKind() const {
+  auto attr = getAttrs().getAttribute<CDeclAttr>();
+  if (!attr)
+    return std::nullopt;
+
+  return attr->Underscored ? ForeignLanguage::ObjectiveC
+                           : ForeignLanguage::C;
+}
+
 bool AbstractFunctionDecl::needsNewVTableEntry() const {
   auto &ctx = getASTContext();
   return evaluateOrDefault(

lib/AST/FeatureSet.cpp

@@ -393,6 +393,11 @@ static bool usesFeatureClosureBodyMacro(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureCDecl(Decl *decl) {
+  auto attr = decl->getAttrs().getAttribute<CDeclAttr>();
+  return attr && !attr->Underscored;
+}
+
 static bool usesFeatureMemorySafetyAttributes(Decl *decl) {
   if (decl->getAttrs().hasAttribute<SafeAttr>() ||
       decl->getAttrs().hasAttribute<UnsafeAttr>())

lib/Parse/ParseDecl.cpp

@@ -3074,10 +3074,12 @@ ParserStatus Parser::parseNewDeclAttribute(DeclAttributes &Attributes,
       if (DK == DeclAttrKind::SILGenName)
         Attributes.add(new (Context) SILGenNameAttr(AsmName.value(), Raw, AtLoc,
                                                 AttrRange, /*Implicit=*/false));
-      else if (DK == DeclAttrKind::CDecl)
+      else if (DK == DeclAttrKind::CDecl) {
+        bool underscored = AttrName.starts_with("_");
         Attributes.add(new (Context) CDeclAttr(AsmName.value(), AtLoc,
-                                               AttrRange, /*Implicit=*/false));
-      else if (DK == DeclAttrKind::Expose) {
+                                               AttrRange, /*Implicit=*/false,
+                                               /*isUnderscored*/underscored));
+      } else if (DK == DeclAttrKind::Expose) {
         for (auto *EA : Attributes.getAttributes<ExposeAttr>()) {
           // A single declaration cannot have two @_exported attributes with
           // the same exposure kind.

lib/Sema/CodeSynthesis.cpp

@@ -637,8 +637,9 @@ configureInheritedDesignatedInitAttributes(ClassDecl *classDecl,
   std::optional<ForeignAsyncConvention> asyncConvention;
   std::optional<ForeignErrorConvention> errorConvention;
   if (superclassCtor->isObjC() &&
-      !isRepresentableInObjC(ctor, ObjCReason::MemberOfObjCSubclass,
-                             asyncConvention, errorConvention))
+      !isRepresentableInLanguage(ctor, ObjCReason::MemberOfObjCSubclass,
+                                 asyncConvention, errorConvention,
+                                 ForeignLanguage::ObjectiveC))
     ctor->getAttrs().add(new (ctx) NonObjCAttr(/*isImplicit=*/true));
 }
 

lib/Sema/TypeCheckAttr.cpp

@@ -2368,17 +2368,24 @@ static bool canDeclareSymbolName(StringRef symbol, ModuleDecl *fromModule) {
 void AttributeChecker::visitCDeclAttr(CDeclAttr *attr) {
   // Only top-level func decls are currently supported.
   if (D->getDeclContext()->isTypeContext())
-    diagnose(attr->getLocation(), diag::cdecl_not_at_top_level);
+    diagnose(attr->getLocation(), diag::cdecl_not_at_top_level,
+             attr);
 
   // The name must not be empty.
   if (attr->Name.empty())
-    diagnose(attr->getLocation(), diag::cdecl_empty_name);
+    diagnose(attr->getLocation(), diag::cdecl_empty_name,
+             attr);
 
   // The standard library can use @_cdecl to implement runtime functions.
   if (!canDeclareSymbolName(attr->Name, D->getModuleContext())) {
       diagnose(attr->getLocation(), diag::reserved_runtime_symbol_name,
                attr->Name);
   }
+
+  if (!attr->Underscored &&
+      !Ctx.LangOpts.hasFeature(Feature::CDecl)) {
+    Ctx.Diags.diagnose(attr->getLocation(), diag::cdecl_feature_required);
+  }
 }
 
 void AttributeChecker::visitExposeAttr(ExposeAttr *attr) {

lib/Sema/TypeCheckCaptures.cpp

@@ -789,8 +789,10 @@ CaptureInfo CaptureInfoRequest::evaluate(Evaluator &evaluator,
         std::optional<ForeignAsyncConvention> asyncConvention;
         std::optional<ForeignErrorConvention> errorConvention;
         if (!AFD->isObjC() &&
-            isRepresentableInObjC(AFD, ObjCReason::MemberOfObjCMembersClass,
-                                  asyncConvention, errorConvention)) {
+            isRepresentableInLanguage(AFD,
+                                      ObjCReason::MemberOfObjCMembersClass,
+                                      asyncConvention, errorConvention,
+                                      ForeignLanguage::ObjectiveC)) {
           AFD->diagnose(
                    diag::objc_generic_extension_using_type_parameter_try_objc)
             .fixItInsert(AFD->getAttributeInsertionLoc(false), "@objc ");