Adding a test.

Author: qiongsiwu

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -628,6 +628,17 @@ struct SwiftInterfaceInfo {
   std::optional<version::Version> CompilerToolsVersion;
 };
 
+/// A small helper class that expands the module name to a full output path
+/// with context hash.
+/// The cannonical way to use this class is the following:
+/// 1. Create an instance through the constructor.
+/// 2. Optional: prune the extra args if necessary.
+/// 3. Obtain the expanded name that contains a full path, and a hash.
+/// expandedName is computed only once, and never updated.
+/// Currently, there is no use case where we first obtain expandedName,
+/// then prune the extra args, and finally obtain expandedName again.
+/// Pruning always happens before the first time we call getExpandedName().
+/// Therefore it is correct to calculate expandedName only once.
 class InterfaceModuleNameExpander {
 public:
   using ArgListTy = std::vector<std::string>;

lib/DependencyScan/ScanDependencies.cpp

@@ -445,7 +445,7 @@ class ExplicitModuleDependencyResolver {
 
         newArgs.push_back(*it);
       }
-      args = newArgs;
+      args = std::move(newArgs);
       return;
     };
 

test/ScanDependencies/eliminate_unused_vfs.swift

@@ -5,9 +5,16 @@
 // RUN: split-file %s %t
 
 // RUN: sed -e "s|OUT_DIR|%t/redirects|g"  -e "s|IN_DIR|%S/Inputs/CHeaders|g" %t/overlay_template.yaml > %t/overlay.yaml
+// RUN: sed -e "s|OUT_DIR|%t/redirects|g"  -e "s|IN_DIR|%S/Inputs/CHeaders1|g" %t/overlay_template.yaml > %t/overlay1.yaml
 
 // RUN: %target-swift-frontend -scan-dependencies -module-load-mode prefer-interface -module-cache-path %t/module-cache %t/test.swift -o %t/deps.json -I %S/Inputs/CHeaders -I %S/Inputs/Swift -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -file-compilation-dir %t -Xcc -ivfsoverlay -Xcc %t/overlay.yaml
-// RUN: %validate-json %t/deps.json | %FileCheck %s
+// RUN: %validate-json %t/deps.json > %t/validated_deps.json
+// RUN: cat %t/validated_deps.json | %FileCheck %s
+
+// RUN: %target-swift-frontend -scan-dependencies -module-load-mode prefer-interface -module-cache-path %t/module-cache %t/test.swift -o %t/deps1.json -I %S/Inputs/CHeaders -I %S/Inputs/Swift -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -file-compilation-dir %t -Xcc -ivfsoverlay -Xcc %t/overlay1.yaml
+// RUN: %validate-json %t/deps1.json > %t/validated_deps1.json
+// RUN: cat %t/validated_deps.json %t/validated_deps1.json \
+// RUN:   | %FileCheck %s --check-prefix=MOD-HASH
 
 //--- overlay_template.yaml
 {
@@ -61,3 +68,27 @@ import F
 // CHECK-LABEL: "modulePath": "{{.*}}{{/|\\}}F-{{.*}}.pcm",
 // CHECK-NOT: "-ivfsoverlay",
 // CHECK-NOT: "{{.*}}{{/|\\}}preserve_used_vfs.swift.tmp{{/|\\}}overlay.yaml",
+
+/// Check that the dependency swift module hashes are identical when the vfs overlays are ignored.
+// MOD-HASH: "mainModuleName": "deps",
+// MOD-HASH: "linkLibraries": [],
+// MOD-HASH-NEXT: "details": {
+// MOD-HASH-NEXT:   "swift": {
+// MOD-HASH-NEXT:   "moduleInterfacePath": "{{.*}}{{/|\\}}F.swiftinterface",
+// MOD-HASH:   "commandLine": [
+// MOD-HASH:     "-o",
+// MOD-HASH-NEXT:     "{{.*}}{{/|\\}}F-[[FHASH:.*]].swiftmodule",
+// MOD-HASH:     "-swift-module-file=Swift={{.*}}{{/|\\}}Swift-[[SHASH:.*]].swiftmodule",
+// MOD-HASH:     "-swift-module-file=SwiftOnoneSupport={{.*}}{{/|\\}}SwiftOnoneSupport-[[SOSHASH:.*]].swiftmodule",
+// MOD-HASH:     ],
+// MOD-HASH: "mainModuleName": "deps1",
+// MOD-HASH: "linkLibraries": [],
+// MOD-HASH: "details": {
+// MOD-HASH-NEXT:   "swift": {
+// MOD-HASH-NEXT:   "moduleInterfacePath": "{{.*}}{{/|\\}}F.swiftinterface",
+// MOD-HASH:   "commandLine": [
+// MOD-HASH:     "-o",
+// MOD-HASH-NEXT:     "{{.*}}{{/|\\}}F-[[FHASH]].swiftmodule",
+// MOD-HASH:     "-swift-module-file=Swift={{.*}}{{/|\\}}Swift-[[SHASH]].swiftmodule",
+// MOD-HASH:     "-swift-module-file=SwiftOnoneSupport={{.*}}{{/|\\}}SwiftOnoneSupport-[[SOSHASH]].swiftmodule",
+// MOD-HASH:     ],