AST: A conformance is weak-linked if its protocol, conforming type or extension is @_weakLinked

Fixes <rdar://problem/46316197>.

Author: slavapestov

include/swift/AST/ProtocolConformance.h

@@ -356,6 +356,9 @@ class RootProtocolConformance : public ProtocolConformance {
 
   bool isInvalid() const;
 
+  /// Whether this conformance is weak-imported.
+  bool isWeakImported(ModuleDecl *fromModule) const;
+
   bool hasWitness(ValueDecl *requirement) const;
   Witness getWitness(ValueDecl *requirement, LazyResolver *resolver) const;
 

lib/AST/ProtocolConformance.cpp

@@ -392,6 +392,29 @@ SourceLoc RootProtocolConformance::getLoc() const {
   ROOT_CONFORMANCE_SUBCLASS_DISPATCH(getLoc, ())
 }
 
+bool RootProtocolConformance::isWeakImported(ModuleDecl *fromModule) const {
+  auto *dc = getDeclContext();
+  if (dc->getParentModule() == fromModule)
+    return false;
+
+  // If the protocol is weak imported, so are any conformances to it.
+  if (getProtocol()->isWeakImported(fromModule))
+    return true;
+
+  // If the conforming type is weak imported, so are any of its conformances.
+  if (auto *nominal = getType()->getAnyNominal())
+    if (nominal->isWeakImported(fromModule))
+      return true;
+
+  // If the conformance is declared in an extension with the @_weakLinked
+  // attribute, it is weak imported.
+  if (auto *ext = dyn_cast<ExtensionDecl>(dc))
+    if (ext->isWeakImported(fromModule))
+      return true;
+
+  return false;
+}
+
 bool RootProtocolConformance::hasWitness(ValueDecl *requirement) const {
   ROOT_CONFORMANCE_SUBCLASS_DISPATCH(hasWitness, (requirement))
 }

lib/IRGen/Linking.cpp

@@ -917,6 +917,11 @@ bool LinkEntity::isWeakImported(ModuleDecl *module) const {
   case Kind::DynamicallyReplaceableFunctionImpl:
     return getDecl()->isWeakImported(module);
 
+  case Kind::ProtocolWitnessTable:
+  case Kind::ProtocolConformanceDescriptor:
+    return getProtocolConformance()->getRootConformance()
+                                   ->isWeakImported(module);
+
   // TODO: Revisit some of the below, for weak conformances.
   case Kind::TypeMetadataPattern:
   case Kind::TypeMetadataInstantiationCache:
@@ -925,12 +930,10 @@ bool LinkEntity::isWeakImported(ModuleDecl *module) const {
   case Kind::TypeMetadataCompletionFunction:
   case Kind::ExtensionDescriptor:
   case Kind::AnonymousDescriptor:
-  case Kind::ProtocolWitnessTable:
   case Kind::ProtocolWitnessTablePattern:
   case Kind::GenericProtocolWitnessTableInstantiationFunction:
   case Kind::AssociatedTypeWitnessTableAccessFunction:
   case Kind::ReflectionAssociatedTypeDescriptor:
-  case Kind::ProtocolConformanceDescriptor:
   case Kind::ProtocolWitnessTableLazyAccessFunction:
   case Kind::ProtocolWitnessTableLazyCacheVariable:
   case Kind::ValueWitness:

test/IRGen/Inputs/weak_import_native_helper.swift

@@ -118,3 +118,6 @@ public protocol ProtocolWithWeakMembers {
 extension ProtocolWithWeakMembers {
   @_weakLinked public func f() {}
 }
+
+public protocol BaseP {}
+@_weakLinked extension S : BaseP {}

test/IRGen/weak_import_native.swift

@@ -170,3 +170,8 @@ class WeakGenericSub: GenericC<Int> {
     // CHECK-DAG: declare extern_weak swiftcc {{.+}} @"$s25weak_import_native_helper8GenericCCfd"
   }
 }
+
+protocol RefinesP : BaseP {}
+
+// CHECK-DAG: @"$s25weak_import_native_helper1SVAA5BasePAAWP" = extern_weak global i8*
+extension S : RefinesP {}