ModuleInterface: Guard declarations that use the $CustomAvailability feature.

include/swift/Basic/Features.def

@@ -490,7 +490,7 @@ SUPPRESSIBLE_EXPERIMENTAL_FEATURE(ExecutionAttribute, false)
 ADOPTABLE_EXPERIMENTAL_FEATURE(AsyncCallerExecution, false)
 
 /// Allow custom availability domains to be defined and referenced.
-SUPPRESSIBLE_EXPERIMENTAL_FEATURE(CustomAvailability, true)
+EXPERIMENTAL_FEATURE(CustomAvailability, true)
 
 /// Allow public enumerations to be extensible by default
 /// regardless of whether the module they are declared in

lib/AST/ASTPrinter.cpp

@@ -3301,14 +3301,6 @@ suppressingFeatureAddressableTypes(PrintOptions &options,
   action();
 }
 
-static void
-suppressingFeatureCustomAvailability(PrintOptions &options,
-                                     llvm::function_ref<void()> action) {
-  // FIXME: [availability] Save and restore a bit controlling whether
-  // @available attributes for custom domains are printed.
-  action();
-}
-
 static void
 suppressingFeatureExecutionAttribute(PrintOptions &options,
                                     llvm::function_ref<void()> action) {

lib/AST/FeatureSet.cpp

@@ -468,8 +468,10 @@ static bool usesFeatureCoroutineAccessors(Decl *decl) {
 }
 
 static bool usesFeatureCustomAvailability(Decl *decl) {
-  // FIXME: [availability] Check whether @available attributes for custom
+  for (auto attr : decl->getSemanticAvailableAttrs()) {
-  // domains are attached to the decl.
+    if (attr.getDomain().isCustom())
+      return true;
+  }
   return false;
 }
 

test/ModuleInterface/availability_custom_domains.swift

@@ -0,0 +1,32 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-emit-module-interface(%t/Test.swiftinterface) %s \
+// RUN:   -I %S/../Inputs/custom-modules/availability-domains \
+// RUN:   -enable-experimental-feature CustomAvailability \
+// RUN:   -module-name Test
+
+// RUN: %target-swift-typecheck-module-from-interface(%t/Test.swiftinterface) \
+// RUN:   -I %S/../Inputs/custom-modules/availability-domains \
+// RUN:   -module-name Test
+
+// RUN: %FileCheck %s < %t/Test.swiftinterface
+
+// REQUIRES: swift_feature_CustomAvailability
+
+import Oceans // re-exports Rivers
+
+// CHECK:      #if compiler(>=5.3) && $CustomAvailability
+// CHECK-NEXT: @available(Colorado)
+// CHECK-NEXT: public func availableInColorado()
+// CHECK-NEXT: #endif
+@available(Colorado)
+public func availableInColorado() { }
+
+// CHECK:      #if compiler(>=5.3) && $CustomAvailability
+// CHECK-NEXT: @available(Arctic, unavailable)
+// CHECK-NEXT: @available(Pacific)
+// CHECK-NEXT: public func unavailableInArcticButAvailableInPacific()
+// CHECK-NEXT: #endif
+@available(Arctic, unavailable)
+@available(Pacific)
+public func unavailableInArcticButAvailableInPacific() { }