[LifetimeDependenceMutableAccessors] add a long feature flag

Author: glessard

include/swift/Basic/Features.def

@@ -420,6 +420,7 @@ EXPERIMENTAL_FEATURE(LifetimeDependence, true)
 
 /// Enable inout lifetime dependence - @lifetime(&arg)
 EXPERIMENTAL_FEATURE(InoutLifetimeDependence, true)
+EXPERIMENTAL_FEATURE(LifetimeDependenceMutableAccessors, true)
 
 /// Enable the `@_staticExclusiveOnly` attribute.
 EXPERIMENTAL_FEATURE(StaticExclusiveOnly, true)

lib/AST/FeatureSet.cpp

@@ -294,6 +294,23 @@ static bool usesFeatureInoutLifetimeDependence(Decl *decl) {
   }
 }
 
+static bool usesFeatureLifetimeDependenceMutableAccessors(Decl *decl) {
+  if (!isa<VarDecl>(decl)) {
+    return false;
+  }
+  auto var = cast<VarDecl>(decl);
+  if (!var->isGetterMutating()) {
+    return false;
+  }
+  if (auto dc = var->getDeclContext()) {
+    if (auto nominal = dc->getSelfNominalTypeDecl()) {
+      auto sig = nominal->getGenericSignature();
+      return !var->getInterfaceType()->isEscapable(sig);
+    }
+  }
+  return false;
+}
+
 UNINTERESTING_FEATURE(DynamicActorIsolation)
 UNINTERESTING_FEATURE(NonfrozenEnumExhaustivity)
 UNINTERESTING_FEATURE(ClosureIsolation)