[6.2] Accept @cdecl global functions and enums, behind experimental feature flags

include/swift/AST/ASTBridging.h

@@ -949,11 +949,12 @@ BridgedBackDeployedAttr BridgedBackDeployedAttr_createParsed(
     BridgedSourceRange cRange, BridgedPlatformKind cPlatform,
     BridgedVersionTuple cVersion);
 
-SWIFT_NAME("BridgedCDeclAttr.createParsed(_:atLoc:range:name:)")
+SWIFT_NAME("BridgedCDeclAttr.createParsed(_:atLoc:range:name:underscored:)")
 BridgedCDeclAttr BridgedCDeclAttr_createParsed(BridgedASTContext cContext,
                                                BridgedSourceLoc cAtLoc,
                                                BridgedSourceRange cRange,
-                                               BridgedStringRef cName);
+                                               BridgedStringRef cName,
+                                               bool underscored);
 
 SWIFT_NAME(
     "BridgedCustomAttr.createParsed(_:atLoc:type:initContext:argumentList:)")

include/swift/AST/Attr.h

@@ -691,22 +691,29 @@ 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

@@ -8172,6 +8172,11 @@ 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

@@ -365,9 +365,10 @@ SIMPLE_DECL_ATTR(_show_in_interface, ShowInInterface,
   62)
 
 DECL_ATTR(_cdecl, CDecl,
-  OnFunc | OnAccessor,
+  OnFunc | OnAccessor | OnEnum,
   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/DiagnosticsCommon.h

@@ -48,6 +48,7 @@ namespace swift {
   } // end namespace detail
 
   enum class StaticSpellingKind : uint8_t;
+  enum class ForeignLanguage : uint8_t;
 
   namespace diag {
 

include/swift/AST/DiagnosticsParse.def

@@ -1529,6 +1529,9 @@ ERROR(attr_expected_comma,none,
 ERROR(attr_expected_string_literal,none,
 "expected string literal in '%0' attribute", (StringRef))
 
+ERROR(attr_expected_cname,none,
+      "expected C identifier in '%0' attribute", (StringRef))
+
 ERROR(attr_expected_option_such_as,none,
       "expected '%0' option such as '%1'", (StringRef, StringRef))
 

include/swift/AST/DiagnosticsSema.def

@@ -2051,11 +2051,17 @@ 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_incompatible_with_objc,none,
+      "cannot apply both '@cdecl' and '@objc' to %kindonly0",
+      (const Decl *))
+ERROR(cdecl_feature_required,none,
+      "@cdecl requires '-enable-experimental-feature CDecl'",
+      ())
 
 // @_used and @_section
 ERROR(section_linkage_markers_disabled,none,
@@ -3736,9 +3742,9 @@ ERROR(enum_with_raw_type_case_with_argument,none,
 NOTE(enum_raw_type_here,none,
      "declared raw type %0 here", (Type))
 ERROR(objc_enum_no_raw_type,none,
-      "'@objc' enum must declare an integer raw type", ())
+      "'%0' enum must declare an integer raw type", (DeclAttribute))
 ERROR(objc_enum_raw_type_not_integer,none,
-      "'@objc' enum raw type %0 is not an integer type", (Type))
+      "'%0' enum raw type %1 is not an integer type", (DeclAttribute, Type))
 ERROR(enum_non_integer_raw_value_auto_increment,none,
       "enum case must declare a raw value when the preceding raw value is not an integer", ())
 ERROR(enum_non_integer_convertible_raw_type_no_value,none,
@@ -5513,10 +5519,14 @@ FIXIT(insert_globalactor_attr, "@%0 ", (Type))
 ERROR(main_function_must_be_mainActor,none,
       "main() must be '@MainActor'", ())
 
+// Keep aligned with enum ForeignLanguage
+#define FOREIGN_LANG_SELECT "select{C|Objective-C}"
+
 ERROR(not_objc_function_async,none,
       "'async' %0 cannot be represented in Objective-C", (DescriptiveDeclKind))
 NOTE(not_objc_function_type_async,none,
-      "'async' function types cannot be represented in Objective-C", ())
+     "'async' function types cannot be represented "
+     "in %" FOREIGN_LANG_SELECT "0", (ForeignLanguage))
 ERROR(actor_isolated_objc,none,
       "actor-isolated %kind0 cannot be '@objc'",
        (const ValueDecl *))
@@ -6523,7 +6533,7 @@ ERROR(objc_cannot_infer_name_raw_identifier,none,
       (DescriptiveDeclKind))
 
 // 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}"
 
 ERROR(objc_invalid_on_var,none,
       "property cannot be %" OBJC_ATTR_SELECT "0 "
@@ -6556,25 +6566,36 @@ NOTE(not_objc_error_protocol_composition,none,
      "protocol-constrained type containing 'Error' cannot be represented "
      "in Objective-C", ())
 NOTE(not_objc_empty_tuple,none,
-      "empty tuple type cannot be represented in Objective-C", ())
+     "empty tuple type cannot be represented in %" FOREIGN_LANG_SELECT "0",
+     (ForeignLanguage))
 NOTE(not_objc_non_trivial_cxx_class,none,
-     "non-trivial C++ classes cannot be represented in Objective-C", ())
+     "non-trivial C++ classes cannot be represented in "
+     "%" FOREIGN_LANG_SELECT "0",
+     (ForeignLanguage))
 NOTE(not_objc_tuple,none,
-      "tuples cannot be represented in Objective-C", ())
+     "tuples cannot be represented in %" FOREIGN_LANG_SELECT "0",
+     (ForeignLanguage))
 NOTE(not_objc_swift_class,none,
      "classes not annotated with '@objc' cannot be represented "
      "in Objective-C", ())
 NOTE(not_objc_swift_struct,none,
-      "Swift structs cannot be represented in Objective-C", ())
+      "Swift structs cannot be represented in %" FOREIGN_LANG_SELECT "0",
+     (ForeignLanguage))
 NOTE(not_objc_swift_enum,none,
       "non-'@objc' enums cannot be represented in Objective-C", ())
+NOTE(not_cdecl_or_objc_swift_enum,none,
+     "Swift enums not marked '@cdecl'%select{| or '@objc'}0 cannot be "
+     "represented in %" FOREIGN_LANG_SELECT "0",
+     (ForeignLanguage))
 NOTE(not_objc_generic_type_param,none,
-     "generic type parameters cannot be represented in Objective-C", ())
+     "generic type parameters cannot be represented in "
+     "%" FOREIGN_LANG_SELECT "0", (ForeignLanguage))
 NOTE(not_objc_function_type_param,none,
-     "function types cannot be represented in Objective-C unless their "
-     "parameters and returns can be", ())
+     "function types cannot be represented in %" FOREIGN_LANG_SELECT "0 "
+     "unless their parameters and returns can be", (ForeignLanguage))
 NOTE(not_objc_function_type_throwing,none,
-      "throwing function types cannot be represented in Objective-C", ())
+     "throwing function types cannot be represented in "
+     "%" FOREIGN_LANG_SELECT "0", (ForeignLanguage))
 NOTE(objc_inferring_on_objc_protocol_member,none,
      "inferring '@objc' because the declaration is a member of "
      "an '@objc' protocol", ())
@@ -6584,6 +6605,11 @@ NOTE(objc_witness_objc_requirement,none,
      "satisfying requirement for %kind0 in protocol %1",
      (const ValueDecl *, const ProtocolDecl *))
 
+NOTE(cdecl_incompatible_with_protocols,none,
+     "protocols cannot be represented in C", ())
+NOTE(cdecl_incompatible_with_classes,none,
+     "classes cannot be represented in C", ())
+
 ERROR(no_opaque_return_type_of,none,
       "unable to resolve type for _opaqueReturnTypeOf attribute", ())
 
@@ -6596,20 +6622,24 @@ ERROR(objc_addressor, none,
 ERROR(objc_coroutine_accessor, none,
       "'read' and 'modify' accessors are not allowed to be marked '@objc'", ())
 ERROR(objc_invalid_on_func_variadic,none,
-      "method cannot be %" OBJC_ATTR_SELECT "0 because it has a variadic "
-      "parameter", (unsigned))
+      "%kindonly0 cannot be %" OBJC_ATTR_SELECT "1 because it has a variadic "
+      "parameter", (const AbstractFunctionDecl*, 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, ForeignLanguage))
 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, ForeignLanguage))
 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, ForeignLanguage))
 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, ForeignLanguage))
 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))
@@ -6735,6 +6765,7 @@ 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/Type.h

@@ -161,7 +161,7 @@ inline SubstOptions operator|(SubstFlags lhs, SubstFlags rhs) {
 
 /// Enumeration describing foreign languages to which Swift may be
 /// bridged.
-enum class ForeignLanguage {
+enum class ForeignLanguage : uint8_t {
   C,
   ObjectiveC,
 };

include/swift/AST/TypeCheckRequests.h

@@ -4875,6 +4875,44 @@ class TypeCheckObjCImplementationRequest
   bool isCached() const { return true; }
 };
 
+/// Check @cdecl functions for compatibility with the foreign language.
+class TypeCheckCDeclFunctionRequest
+    : public SimpleRequest<TypeCheckCDeclFunctionRequest,
+                           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; }
+};
+
+/// Check @cdecl enums for compatibility with C.
+class TypeCheckCDeclEnumRequest
+    : public SimpleRequest<TypeCheckCDeclEnumRequest,
+                           evaluator::SideEffect(EnumDecl *ED,
+                                                 CDeclAttr *attr),
+                           RequestFlags::Cached> {
+public:
+  using SimpleRequest::SimpleRequest;
+
+private:
+  friend SimpleRequest;
+
+  evaluator::SideEffect
+  evaluate(Evaluator &evaluator, EnumDecl *ED, 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

@@ -554,6 +554,12 @@ SWIFT_REQUEST(TypeChecker, IsNonUserModuleRequest,
 SWIFT_REQUEST(TypeChecker, TypeCheckObjCImplementationRequest,
               unsigned(ExtensionDecl *),
               Cached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, TypeCheckCDeclFunctionRequest,
+              evaluator::SideEffect(FunctionDecl *, CDeclAttr *),
+              Cached, NoLocationInfo)
+SWIFT_REQUEST(TypeChecker, TypeCheckCDeclEnumRequest,
+              evaluator::SideEffect(EnumDecl *, CDeclAttr *),
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, HasInitAccessorRequest,
               bool(AbstractStorageDecl *), Cached,
               NoLocationInfo)

include/swift/Basic/Features.def

@@ -520,6 +520,9 @@ EXPERIMENTAL_FEATURE(ClosureBodyMacro, true)
 /// Allow declarations of Swift runtime symbols using @_silgen_name.
 EXPERIMENTAL_FEATURE(AllowRuntimeSymbolDeclarations, true)
 
+/// Allow use of `@cdecl`
+EXPERIMENTAL_FEATURE(CDecl, true)
+
 /// Optimize copies of ObjectiveC blocks.
 EXPERIMENTAL_FEATURE(CopyBlockOptimization, true)
 

include/swift/PrintAsClang/ClangMacros.def

@@ -176,20 +176,46 @@ CLANG_MACRO_CONDITIONAL("SWIFT_ENUM_ATTR", "(_extensibility)", \
                         "__attribute__((enum_extensibility(_extensibility)))")
 
 CLANG_MACRO_BODY("SWIFT_ENUM", \
-    "# define SWIFT_ENUM(_type, _name, _extensibility) " \
+    "# if (defined(__cplusplus) && __cplusplus >= 201103L) || " \
+    "     (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) || " \
+    "     __has_feature(objc_fixed_enum)\n" \
+    "#  define SWIFT_ENUM(_type, _name, _extensibility) " \
         "enum _name : _type _name; " \
         "enum SWIFT_ENUM_ATTR(_extensibility) SWIFT_ENUM_EXTRA _name : _type\n" \
-    "# if __has_feature(generalized_swift_name)\n" \
-    "#  define SWIFT_ENUM_NAMED(_type, _name, SWIFT_NAME, _extensibility) " \
-        "enum _name : _type _name SWIFT_COMPILE_NAME(SWIFT_NAME); " \
-        "enum SWIFT_COMPILE_NAME(SWIFT_NAME) SWIFT_ENUM_ATTR(_extensibility) " \
-            "SWIFT_ENUM_EXTRA _name : _type\n" \
+    "#  if __has_feature(generalized_swift_name)\n" \
+    "#   define SWIFT_ENUM_NAMED(_type, _name, SWIFT_NAME, _extensibility) " \
+         "enum _name : _type _name SWIFT_COMPILE_NAME(SWIFT_NAME); " \
+         "enum SWIFT_COMPILE_NAME(SWIFT_NAME) SWIFT_ENUM_ATTR(_extensibility) " \
+             "SWIFT_ENUM_EXTRA _name : _type\n" \
+    "#  else\n" \
+    "#   define SWIFT_ENUM_NAMED(_type, _name, SWIFT_NAME, _extensibility) " \
+         "SWIFT_ENUM(_type, _name, _extensibility)\n" \
+    "#  endif\n" \
     "# else\n" \
+    "#  define SWIFT_ENUM(_type, _name, _extensibility) _type _name; enum \n" \
     "#  define SWIFT_ENUM_NAMED(_type, _name, SWIFT_NAME, _extensibility) " \
         "SWIFT_ENUM(_type, _name, _extensibility)\n" \
     "# endif")
 CLANG_MACRO_DEFINED("SWIFT_ENUM_NAMED")
 
+CLANG_MACRO_BODY("SWIFT_ENUM_TAG", \
+    "# if (defined(__cplusplus) && __cplusplus >= 201103L) || " \
+    "     (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) || " \
+    "     __has_feature(objc_fixed_enum)\n" \
+    "#  define SWIFT_ENUM_TAG enum\n" \
+    "# else\n" \
+    "#  define SWIFT_ENUM_TAG\n" \
+    "# endif")
+
+CLANG_MACRO_BODY("SWIFT_ENUM_FWD_DECL", \
+    "# if (defined(__cplusplus) && __cplusplus >= 201103L) || " \
+    "     (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202311L) || " \
+    "     __has_feature(objc_fixed_enum)\n" \
+    "#  define SWIFT_ENUM_FWD_DECL(_type, _name) enum _name : _type _name;\n" \
+    "# else\n" \
+    "#  define SWIFT_ENUM_FWD_DECL(_type, _name) _type _name;\n" \
+    "# endif")
+
 CLANG_MACRO("SWIFT_UNAVAILABLE", , "__attribute__((unavailable))")
 CLANG_MACRO("SWIFT_UNAVAILABLE_MSG", "(msg)", "__attribute__((unavailable(msg)))")
 

lib/AST/Attr.cpp

@@ -1799,7 +1799,9 @@ 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: