Parser: Intro @cdecl attribute and gate it behind feature flag CDecl

Author: xymus

include/swift/AST/Attr.h

@@ -694,22 +694,29 @@ 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/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

@@ -2036,6 +2036,9 @@ ERROR(cdecl_empty_name,none,
       "@_cdecl symbol name cannot be empty", ())
 ERROR(cdecl_throws,none,
       "raising errors from @_cdecl functions is not supported", ())
+ERROR(cdecl_feature_required,none,
+      "@cdecl requires '-enable-experimental-feature CDecl'",
+      ())
 
 // @_used and @_section
 ERROR(section_linkage_markers_disabled,none,

include/swift/Basic/Features.def

@@ -519,6 +519,9 @@ 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

@@ -1809,7 +1809,9 @@ 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/FeatureSet.cpp

@@ -392,6 +392,11 @@ 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/TypeCheckAttr.cpp

@@ -2367,6 +2367,11 @@ void AttributeChecker::visitCDeclAttr(CDeclAttr *attr) {
       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/Serialization/Deserialization.cpp

@@ -5993,9 +5993,10 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
 
       case decls_block::CDecl_DECL_ATTR: {
         bool isImplicit;
+        bool isUnderscored = false;
         serialization::decls_block::CDeclDeclAttrLayout::readRecord(
-            scratch, isImplicit);
-        Attr = new (ctx) CDeclAttr(blobData, isImplicit);
+            scratch, isImplicit); // TODO isUnderscored
+        Attr = new (ctx) CDeclAttr(blobData, isImplicit, isUnderscored);
         break;
       }