[Concurrency] NonisolatedNonsendingByDefault: Migration applies only to the current module declarations

Prevent migration from handling declarations that come from a
module different from the current one, this is primarily a
problem for swiftinterfaces that can get rebuilt when the module
is imported by a model that has migration mode enabled.

Resolves: rdar://152687353

Author: xedin

lib/Sema/NonisolatedNonsendingByDefaultMigration.cpp

@@ -153,6 +153,10 @@ void NonisolatedNonsendingByDefaultMigrationTarget::diagnose() const {
 
   const auto featureName = feature.getName();
   if (decl) {
+    // Only diagnose declarations from the current module.
+    if (decl->getModuleContext() != ctx.MainModule)
+      return;
+
     // Diagnose the function, but slap the attribute on the storage declaration
     // instead if the function is an accessor.
     auto *functionDecl = dyn_cast<AbstractFunctionDecl>(decl);

test/Concurrency/attr_execution/migration_mode_with_interface.swift

@@ -0,0 +1,44 @@
+// RUN: %empty-directory(%t/src)
+// RUN: split-file %s %t/src
+
+/// Build the library
+// RUN: %target-swift-frontend -emit-module %t/src/Lib.swift \
+// RUN:   -target %target-swift-5.1-abi-triple \
+// RUN:   -module-name Lib -swift-version 5 -enable-library-evolution \
+// RUN:   -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -emit-module-interface-path %t/Lib.swiftinterface
+
+// RUN: rm %t/Lib.swiftmodule
+
+// Build the client using interface
+// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 5 -I %t %t/src/Test.swift -enable-upcoming-feature NonisolatedNonsendingByDefault:migrate
+
+// REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
+
+//--- Lib.swift
+public struct Counter: AsyncSequence {
+  public typealias Element = Int
+
+  public init(howHigh: Int) {
+  }
+
+  public struct AsyncIterator: AsyncIteratorProtocol {
+    public mutating func next() async -> Int? {
+      nil
+    }
+  }
+  
+  public func makeAsyncIterator() -> AsyncIterator {
+    AsyncIterator()
+  }
+}
+
+//--- Test.swift
+import Lib
+
+func count(n: Int) async {
+  // expected-warning@-1 {{feature 'NonisolatedNonsendingByDefault' will cause nonisolated async global function 'count' to run on the caller's actor; use '@concurrent' to preserve behavior}}
+  for await _ in Counter(howHigh: n) {
+  }
+}