[Concurrency] Make OptionalIsolatedParameters a conditionally suppressible

language feature, and suppress it for `Clock.measure`.

This allows the _Concurrency swiftinterface file to continue building with
compilers that do not support `OptionalIsolatedParameters`. The feature
suppression drops the `isolated` keyword and replaces `#isolation` with
`nil`.

Author: hborla

include/swift/AST/PrintOptions.h

@@ -379,6 +379,9 @@ struct PrintOptions {
   /// Suppress the @isolated(any) attribute.
   bool SuppressIsolatedAny = false;
 
+  /// Suppress 'isolated' and '#isolation' on isolated parameters with optional type.
+  bool SuppressOptionalIsolatedParams = false;
+
   /// List of attribute kinds that should not be printed.
   std::vector<AnyAttrKind> ExcludeAttrList = {
       DeclAttrKind::Transparent, DeclAttrKind::Effects,

include/swift/Basic/Features.def

@@ -164,7 +164,7 @@ LANGUAGE_FEATURE(FreestandingMacros, 397, "freestanding declaration macros")
 SUPPRESSIBLE_LANGUAGE_FEATURE(RetroactiveAttribute, 364, "@retroactive")
 SUPPRESSIBLE_LANGUAGE_FEATURE(ExtensionMacroAttr, 0, "@attached(extension)")
 LANGUAGE_FEATURE(TypedThrows, 413, "Typed throws")
-LANGUAGE_FEATURE(OptionalIsolatedParameters, 420, "Optional isolated parameters")
+CONDITIONALLY_SUPPRESSIBLE_LANGUAGE_FEATURE(OptionalIsolatedParameters, 420, "Optional isolated parameters")
 SUPPRESSIBLE_LANGUAGE_FEATURE(Extern, 0, "@_extern")
 LANGUAGE_FEATURE(ExpressionMacroDefaultArguments, 422, "Expression macro as caller-side default argument")
 LANGUAGE_FEATURE(BuiltinStoreRaw, 0, "Builtin.storeRaw")

lib/AST/ASTPrinter.cpp

@@ -3083,6 +3083,14 @@ static void suppressingFeatureIsolatedAny(PrintOptions &options,
   action();
 }
 
+static void suppressingFeatureOptionalIsolatedParameters(
+    PrintOptions &options,
+    llvm::function_ref<void()> action) {
+  llvm::SaveAndRestore<bool> scope(
+      options.SuppressOptionalIsolatedParams, true);
+  action();
+}
+
 static void suppressingFeatureAssociatedTypeImplements(PrintOptions &options,
                                      llvm::function_ref<void()> action) {
   unsigned originalExcludeAttrCount = options.ExcludeAttrList.size();
@@ -3660,8 +3668,11 @@ static void printParameterFlags(ASTPrinter &printer,
     break;
   }
 
-  if (flags.isIsolated())
-    printer.printKeyword("isolated", options, " ");
+  if (flags.isIsolated()) {
+    if (!(param && param->getInterfaceType()->isOptional() &&
+          options.SuppressOptionalIsolatedParams))
+      printer.printKeyword("isolated", options, " ");
+  }
 
   if (flags.hasResultDependsOn())
     printer.printKeyword("_resultDependsOn", options, " ");
@@ -3809,6 +3820,16 @@ void PrintAST::printOneParameter(const ParamDecl *param,
   }
 
   if (param->isDefaultArgument() && Options.PrintDefaultArgumentValue) {
+    auto defaultArgKind = param->getDefaultArgumentKind();
+    if (param->isIsolated() &&
+        defaultArgKind == DefaultArgumentKind::ExpressionMacro &&
+        Options.SuppressOptionalIsolatedParams) {
+      // If we're suppressing optional isolated parameters, print
+      // 'nil' instead of '#isolation'
+      Printer << " = nil";
+      return;
+    }
+
     Printer.callPrintStructurePre(PrintStructureKind::DefaultArgumentClause);
     SWIFT_DEFER {
       Printer.printStructurePost(PrintStructureKind::DefaultArgumentClause);

stdlib/public/Concurrency/Clock.swift

@@ -68,6 +68,7 @@ extension Clock {
   ///       }
   @available(SwiftStdlib 5.7, *)
   @_alwaysEmitIntoClient
+  @_allowFeatureSuppression(OptionalIsolatedParameters)
   public func measure(
     isolation: isolated (any Actor)? = #isolation,
     _ work: () async throws -> Void