[ScanDependencies] Do not count optional dependencies when not needed

If a testable module is loaded from a non-testable import, ignore its
optional dependencies because the consumer should not use them. This
matches the behavior of the implicit build or the behavior how
forwarding module is created.

Author: cachemeifyoucan

include/swift/Serialization/SerializedModuleLoader.h

@@ -176,7 +176,7 @@ class SerializedModuleLoaderBase : public ModuleLoader {
   static BinaryModuleImports
   getImportsOfModule(const ModuleFileSharedCore &loadedModule,
                      ModuleLoadingBehavior transitiveBehavior,
-                     StringRef packageName);
+                     StringRef packageName, bool isTestableImport);
 
   /// Load the module file into a buffer and also collect its module name.
   static std::unique_ptr<llvm::MemoryBuffer>

lib/Serialization/SerializedModuleLoader.cpp

@@ -395,7 +395,8 @@ std::error_code SerializedModuleLoaderBase::openModuleFile(
 SerializedModuleLoaderBase::BinaryModuleImports
 SerializedModuleLoaderBase::getImportsOfModule(
     const ModuleFileSharedCore &loadedModuleFile,
-    ModuleLoadingBehavior transitiveBehavior, StringRef packageName) {
+    ModuleLoadingBehavior transitiveBehavior, StringRef packageName,
+    bool isTestableImport) {
   llvm::StringSet<> importedModuleNames;
   std::string importedHeader = "";
   for (const auto &dependency : loadedModuleFile.getDependencies()) {
@@ -410,8 +411,7 @@ SerializedModuleLoaderBase::getImportsOfModule(
         loadedModuleFile.getTransitiveLoadingBehavior(
             dependency,
             /*debuggerMode*/ false,
-            /*isPartialModule*/ false, packageName,
-            loadedModuleFile.isTestable());
+            /*isPartialModule*/ false, packageName, isTestableImport);
     if (dependencyTransitiveBehavior > transitiveBehavior)
       continue;
 
@@ -475,12 +475,12 @@ SerializedModuleLoaderBase::scanModuleFile(Twine modulePath, bool isFramework,
   //       optional.
   auto binaryModuleImports =
       getImportsOfModule(*loadedModuleFile, ModuleLoadingBehavior::Required,
-                         Ctx.LangOpts.PackageName);
+                         Ctx.LangOpts.PackageName, isTestableImport);
 
   // Lookup optional imports of this module also
   auto binaryModuleOptionalImports =
       getImportsOfModule(*loadedModuleFile, ModuleLoadingBehavior::Optional,
-                         Ctx.LangOpts.PackageName);
+                         Ctx.LangOpts.PackageName, isTestableImport);
 
   auto importedModuleSet = binaryModuleImports.moduleImports;
   std::vector<std::string> importedModuleNames;

test/ScanDependencies/testable-dependencies.swift

@@ -56,12 +56,16 @@
 // RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib -enable-testing \
 // RUN:   -o %t/deps5.json -I %t/regular -swift-version 5 -Rmodule-loading
 
-/// Regular import a testable module with no interface, will try to import binary module but fail to look up the dependency.
+/// Regular import a testable module with no interface, don't load optional dependencies.
 // RUN: rm %t/testable/A.swiftinterface
 // RUN: %target-swift-frontend -scan-dependencies -module-load-mode prefer-interface -module-name Test %t/main.swift \
 // RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib -enable-testing \
-// RUN:   -o %t/deps6.json -I %t/testable -swift-version 5 -Rmodule-loading 2>&1 | %FileCheck %s --check-prefix ERROR
-// ERROR: error: Unable to find module dependency: 'B'
+// RUN:   -o %t/deps6.json -I %t/testable -swift-version 5 -Rmodule-loading
+// RUN: %{python} %S/../CAS/Inputs/SwiftDepsExtractor.py %t/deps6.json Test directDependencies | %FileCheck %s --check-prefix TEST6
+// TEST6:  "swiftPrebuiltExternal": "A"
+// RUN: %{python} %S/../CAS/Inputs/SwiftDepsExtractor.py %t/deps6.json swiftPrebuiltExternal:A directDependencies | %FileCheck %s --check-prefix TEST6-A
+// TEST6-A: []
+
 
 //--- main.swift
 import A