[Basic] Add an experimental feature flag to guard use of ~Sendable

Author: xedin

include/swift/AST/PrintOptions.h

@@ -406,6 +406,9 @@ struct PrintOptions {
   /// Suppress @inline(always) attribute and emit @inline(__always) instead.
   bool SuppressInlineAlways = false;
 
+  /// Suppress printing of ~Sendable in inheritance and requirement lists.
+  bool SuppressTildeSendable = false;
+
   /// Whether to print the \c{/*not inherited*/} comment on factory initializers.
   bool PrintFactoryInitializerComment = true;
 

include/swift/Basic/Features.def

@@ -559,6 +559,9 @@ SUPPRESSIBLE_EXPERIMENTAL_FEATURE(InlineAlways, false)
 /// Allow use of 'Swift' (Swift runtime version) in @available attributes
 EXPERIMENTAL_FEATURE(SwiftRuntimeAvailability, true)
 
+/// Allow use of `~Sendable`.
+SUPPRESSIBLE_EXPERIMENTAL_FEATURE(TildeSendable, false)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/ASTPrinter.cpp

@@ -3285,6 +3285,12 @@ suppressingFeatureLifetimes(PrintOptions &options,
   action();
 }
 
+static void suppressingFeatureTildeSendable(PrintOptions &options,
+                                            llvm::function_ref<void()> action) {
+  llvm::SaveAndRestore<bool> scope(options.SuppressTildeSendable, true);
+  action();
+}
+
 namespace {
 struct ExcludeAttrRAII {
   std::vector<AnyAttrKind> &ExcludeAttrList;
@@ -3302,7 +3308,7 @@ struct ExcludeAttrRAII {
     ExcludeAttrList.resize(OriginalExcludeAttrCount);
   }
 };
-}
+} // namespace
 
 static void
 suppressingFeatureCoroutineAccessors(PrintOptions &options,

lib/AST/FeatureSet.cpp

@@ -450,6 +450,26 @@ static bool usesFeatureInlineAlways(Decl *decl) {
 
 UNINTERESTING_FEATURE(SwiftRuntimeAvailability)
 
+static bool usesFeatureTildeSendable(Decl *decl) {
+  auto *TD = dyn_cast<TypeDecl>(decl);
+  if (!TD)
+    return false;
+
+  return llvm::any_of(
+      TD->getInherited().getEntries(), [&decl](const auto &entry) {
+        if (!entry.isSuppressed())
+          return false;
+
+        auto T = entry.getType();
+        if (!T)
+          return false;
+
+        auto &C = decl->getASTContext();
+        return C.getProtocol(KnownProtocolKind::Sendable) == T->getAnyNominal();
+      });
+}
+
+
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet
 // ----------------------------------------------------------------------------