Basic/AST: Define the CustomAvailability experimental feature.

Author: tshortli

include/swift/Basic/Features.def

@@ -447,6 +447,9 @@ SUPPRESSIBLE_EXPERIMENTAL_FEATURE(ABIAttribute, true)
 /// calling context.
 EXPERIMENTAL_FEATURE(NonIsolatedAsyncInheritsIsolationFromContext, false)
 
+/// Allow custom availability domains to be defined and referenced.
+SUPPRESSIBLE_EXPERIMENTAL_FEATURE(CustomAvailability, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTPrinter.cpp

@@ -3239,6 +3239,14 @@ 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();
+}
+
 /// Suppress the printing of a particular feature.
 static void suppressingFeature(PrintOptions &options, Feature feature,
                                llvm::function_ref<void()> action) {

lib/AST/FeatureSet.cpp

@@ -407,6 +407,12 @@ static bool usesFeatureCoroutineAccessors(Decl *decl) {
   }
 }
 
+static bool usesFeatureCustomAvailability(Decl *decl) {
+  // FIXME: [availability] Check whether @available attributes for custom
+  // domains are attached to the decl.
+  return false;
+}
+
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet
 // ----------------------------------------------------------------------------