Introduce ModuleDescriptor#isFriend(ModuleDescriptor)

Use it to allow friend modules to see internal members

core/descriptors/src/org/jetbrains/jet/lang/descriptors/ModuleDescriptor.kt

@@ -36,6 +36,8 @@ public trait ModuleDescriptor : DeclarationDescriptor {
 
     public val platformToKotlinClassMap: PlatformToKotlinClassMap
 
+    public fun isFriend(other: ModuleDescriptor): Boolean
+
     override fun substitute(substitutor: TypeSubstitutor): ModuleDescriptor {
         return this
     }

core/descriptors/src/org/jetbrains/jet/lang/descriptors/Visibilities.java

@@ -76,7 +76,7 @@ public class Visibilities {
         @Override
         protected boolean isVisible(@NotNull DeclarationDescriptorWithVisibility what, @NotNull DeclarationDescriptor from) {
             DeclarationDescriptor fromOrModule = from instanceof PackageViewDescriptor ? ((PackageViewDescriptor) from).getModule() : from;
-            return DescriptorUtils.areInSameModule(what, fromOrModule);
+            return isInFriendModule(what, fromOrModule);
         }
     };
 
@@ -119,6 +119,10 @@ public class Visibilities {
         return findInvisibleMember(what, from) == null;
     }
 
+    private static boolean isInFriendModule(@NotNull DeclarationDescriptor what, @NotNull DeclarationDescriptor from) {
+        return DescriptorUtils.getContainingModule(what).isFriend(DescriptorUtils.getContainingModule(from));
+    }
+
     @Nullable
     public static DeclarationDescriptorWithVisibility findInvisibleMember(
             @NotNull DeclarationDescriptorWithVisibility what,

core/descriptors/src/org/jetbrains/jet/lang/descriptors/impl/ModuleDescriptorImpl.kt

@@ -24,6 +24,7 @@ import org.jetbrains.jet.lang.resolve.ImportPath
 import org.jetbrains.jet.lang.descriptors.annotations.Annotations
 import org.jetbrains.jet.lang.descriptors.ModuleDescriptor
 import kotlin.properties.Delegates
+import java.util.LinkedHashSet
 
 public class ModuleDescriptorImpl(
         moduleName: Name,
@@ -84,4 +85,14 @@ public class ModuleDescriptorImpl(
     }
 
     override fun getPackageFragmentProvider() = packageFragmentProviderForWholeModuleWithDependencies
+
+    private val friendModules = LinkedHashSet<ModuleDescriptor>()
+
+    override fun isFriend(other: ModuleDescriptor) = other == this || other in friendModules
+
+    public fun addFriend(friend: ModuleDescriptorImpl): Unit {
+        assert(friend != this, "Attempt to make module $id a friend to itself")
+        assert(!isSealed, "Attempt to add friend module ${friend.id} to sealed module $id")
+        friendModules.add(friend)
+    }
 }