[gardening]: refactor checkGlobalIsolation function & add comments

Author: jamieQ

lib/Sema/TypeCheckConcurrency.cpp

@@ -6198,56 +6198,66 @@ void swift::checkOverrideActorIsolation(ValueDecl *value) {
 
 void swift::checkGlobalIsolation(VarDecl *var) {
   const auto isolation = getActorIsolation(var);
-  if (var->getLoc() &&
-      var->getASTContext().LangOpts.hasFeature(Feature::GlobalConcurrency) &&
-      !isolation.isGlobalActor() &&
-      (isolation != ActorIsolation::NonisolatedUnsafe)) {
-    if (var->isGlobalStorage()) {
-      auto *diagVar = var;
-      if (auto *originalVar = var->getOriginalWrappedProperty()) {
-        diagVar = originalVar;
-      }
 
-      bool diagnosed = false;
-      if (var->isLet()) {
-        auto type = var->getInterfaceType();
-        diagnosed = diagnoseIfAnyNonSendableTypes(
-            type, SendableCheckContext(var->getDeclContext()),
-            /*inDerivedConformance=*/Type(), /*typeLoc=*/SourceLoc(),
-            /*diagnoseLoc=*/var->getLoc(), diag::shared_immutable_state_decl,
-            diagVar);
-      } else {
-        diagVar->diagnose(diag::shared_mutable_state_decl, diagVar)
-            .warnUntilSwiftVersion(6);
-        diagnosed = true;
-      }
-
-      // If we diagnosed this global, tack on notes to suggest potential courses
-      // of action.
-      if (diagnosed) {
-        if (!var->isLet()) {
-          auto diag =
-              diagVar->diagnose(diag::shared_state_make_immutable, diagVar);
-          SourceLoc fixItLoc = getFixItLocForVarToLet(diagVar);
-          if (fixItLoc.isValid()) {
-            diag.fixItReplace(fixItLoc, "let");
-          }
-        }
+  // Skip this if the relevant features aren't supported.
+  if (!var->getLoc() ||
+      !var->getASTContext().LangOpts.hasFeature(Feature::GlobalConcurrency))
+    return;
 
-        auto mainActor = var->getASTContext().getMainActorType();
-        if (mainActor) {
-          diagVar
-              ->diagnose(diag::add_globalactor_to_decl, mainActor->getString(),
-                         diagVar, mainActor)
-              .fixItInsert(diagVar->getAttributeInsertionLoc(false),
-                           diag::insert_globalactor_attr, mainActor);
-        }
-        diagVar->diagnose(diag::shared_state_nonisolated_unsafe, diagVar)
-            .fixItInsert(diagVar->getAttributeInsertionLoc(true),
-                         "nonisolated(unsafe) ");
-      }
-    }
+  // Skip if the decl is global actor-isolated or is unsafely opted-out.
+  if (isolation.isGlobalActor() || isolation.isNonisolatedUnsafe())
+    return;
+
+  // We're only concerned with global storage.
+  if (!var->isGlobalStorage())
+    return;
+
+  // At this point, we've found global state that may need to be diagnosed.
+  auto *diagVar = var;
+
+  // Look through property wrappers.
+  if (auto *originalVar = var->getOriginalWrappedProperty())
+    diagVar = originalVar;
+
+  bool diagnosed = false;
+  if (var->isLet()) {
+    // `let` variables are okay if they are of Sendable type.
+    auto type = var->getInterfaceType();
+    diagnosed = diagnoseIfAnyNonSendableTypes(
+        type, SendableCheckContext(var->getDeclContext()),
+        /*inDerivedConformance=*/Type(), /*typeLoc=*/SourceLoc(),
+        /*diagnoseLoc=*/var->getLoc(), diag::shared_immutable_state_decl,
+        diagVar);
+  } else {
+    diagVar->diagnose(diag::shared_mutable_state_decl, diagVar)
+        .warnUntilSwiftVersion(6);
+    diagnosed = true;
   }
+
+  // If we didn't find anything to report, we're done.
+  if (!diagnosed)
+    return;
+
+  // If we diagnosed this global, tack on notes to suggest potential courses
+  // of action.
+  if (!var->isLet()) {
+    auto diag = diagVar->diagnose(diag::shared_state_make_immutable, diagVar);
+    SourceLoc fixItLoc = getFixItLocForVarToLet(diagVar);
+    if (fixItLoc.isValid())
+      diag.fixItReplace(fixItLoc, "let");
+  }
+
+  auto mainActor = var->getASTContext().getMainActorType();
+  if (mainActor) {
+    diagVar
+        ->diagnose(diag::add_globalactor_to_decl, mainActor->getString(),
+                   diagVar, mainActor)
+        .fixItInsert(diagVar->getAttributeInsertionLoc(false),
+                     diag::insert_globalactor_attr, mainActor);
+  }
+  diagVar->diagnose(diag::shared_state_nonisolated_unsafe, diagVar)
+      .fixItInsert(diagVar->getAttributeInsertionLoc(true),
+                   "nonisolated(unsafe) ");
 }
 
 bool swift::contextRequiresStrictConcurrencyChecking(