[Caching] Give swiftmodule loaded from CAS a path identifier

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -2603,7 +2603,7 @@ bool ExplicitCASModuleLoader::findModule(
   // that are not located on disk.
   auto moduleBuf = loadCachedCompileResultFromCacheKey(
       Impl.CAS, Impl.Cache, Ctx.Diags, moduleCASID,
-      file_types::ID::TY_SwiftModuleFile);
+      file_types::ID::TY_SwiftModuleFile, moduleInfo.modulePath);
   if (!moduleBuf) {
     // We cannot read the module content, diagnose.
     Ctx.Diags.diagnose(SourceLoc(), diag::error_opening_explicit_module_file,

lib/FrontendTool/FrontendTool.cpp

@@ -410,6 +410,13 @@ static bool buildModuleFromInterface(CompilerInstance &Instance) {
   bool IgnoreAdjacentModules = Instance.hasASTContext() &&
                                Instance.getASTContext().IgnoreAdjacentModules;
 
+  // When caching is enabled, the explicit module build dependencies are
+  // discovered by dependency scanner and the swiftmodule is already rebuilt
+  // ignoring candidate module. There is no need to serialized dependencies for
+  // validation purpose.
+  bool ShouldSerializeDeps =
+      !Instance.getInvocation().getCASOptions().EnableCaching;
+
   // If an explicit interface build was requested, bypass the creation of a new
   // sub-instance from the interface which will build it in a separate thread,
   // and isntead directly use the current \c Instance for compilation.
@@ -422,8 +429,7 @@ static bool buildModuleFromInterface(CompilerInstance &Instance) {
     return ModuleInterfaceLoader::buildExplicitSwiftModuleFromSwiftInterface(
         Instance, Invocation.getClangModuleCachePath(),
         FEOpts.BackupModuleInterfaceDir, PrebuiltCachePath, ABIPath, InputPath,
-        Invocation.getOutputFilename(),
-        /* shouldSerializeDeps */ true,
+        Invocation.getOutputFilename(), ShouldSerializeDeps,
         Invocation.getSearchPathOptions().CandidateCompiledModules);
 
   return ModuleInterfaceLoader::buildSwiftModuleFromSwiftInterface(

test/CAS/module_trace.swift

@@ -0,0 +1,58 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -emit-module -module-name B -o %t/B.swiftmodule -swift-version 5 \
+// RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
+// RUN:   -emit-module-interface-path %t/ignore/B.swiftinterface -enable-library-evolution \
+// RUN:   %t/B.swift
+
+// RUN: %target-swift-frontend -emit-module -module-name A -o %t/ignore/A.swiftmodule -swift-version 5 \
+// RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
+// RUN:   -emit-module-interface-path %t/A.swiftinterface -enable-library-evolution -I %t \
+// RUN:   %t/A.swift
+
+// RUN: %target-swift-frontend -scan-dependencies -module-name Test -module-cache-path %t/clang-module-cache %t/main.swift \
+// RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
+// RUN:   -o %t/deps.json -I %t -cache-compile-job -cas-path %t/cas -swift-version 5
+
+// RUN: %{python} %S/Inputs/BuildCommandExtractor.py %t/deps.json A > %t/A.cmd
+// RUN: %swift_frontend_plain @%t/A.cmd
+
+// RUN: %FileCheck %s --input-file=%t/A.cmd
+
+// CHECK-NOT: -candidate-module-file
+
+// RUN: %{python} %S/Inputs/GenerateExplicitModuleMap.py %t/deps.json > %t/map.json
+// RUN: llvm-cas --cas %t/cas --make-blob --data %t/map.json > %t/map.casid
+
+// RUN: %{python} %S/Inputs/BuildCommandExtractor.py %t/deps.json Test > %t/MyApp.cmd
+
+// RUN: %target-swift-frontend \
+// RUN:   -c -cache-compile-job -cas-path %t/cas \
+// RUN:   -disable-implicit-swift-modules -o %t/test.o\
+// RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
+// RUN:   -module-name Test -explicit-swift-module-map-file @%t/map.casid \
+// RUN:   -emit-reference-dependencies-path %t/test.swiftdeps -emit-dependencies \
+// RUN:   -primary-file %t/main.swift @%t/MyApp.cmd -emit-loaded-module-trace -emit-loaded-module-trace-path %t/test.trace.json 2>&1 \
+// RUN:     | %FileCheck %s --check-prefix=WARNING --allow-empty
+
+// WARNING-NOT: WARNING:
+
+// RUN: %FileCheck %s --check-prefix=TRACE --input-file=%t/test.trace.json
+// TRACE-DAG: A.swiftinterface
+
+// RUN: %{python} %S/../Inputs/process_fine_grained_swiftdeps_with_fingerprints.py %swift-dependency-tool %t/test.swiftdeps > %t/test-processed.swiftdeps
+// RUN: %FileCheck %s --check-prefix=SWIFTDEPS --input-file=%t/test-processed.swiftdeps
+// SWIFTDEPS: A.swiftinterface
+// SWIFTDEPS: B.swiftmdoule
+
+
+//--- main.swift
+import A
+
+//--- A.swift
+import B
+func test() {}
+
+//--- B.swift
+func b() {}