ModuleInterface: teach module build action to use fall-back interface if building canonical interface failed

include/swift/AST/ModuleLoader.h

@@ -157,8 +157,6 @@ class ModuleInterfaceChecker {
                                ArrayRef<std::string> candidates,
                                StringRef outPath) = 0;
   virtual ~ModuleInterfaceChecker() = default;
-  virtual std::string getBackupPublicModuleInterfacePath(StringRef moduleName,
-                                                         StringRef interfacePath) = 0;
 };
 
 /// Abstract interface to run an action in a sub ASTContext.

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -367,8 +367,6 @@ class ModuleInterfaceCheckerImpl: public ModuleInterfaceChecker {
                                ArrayRef<std::string> candidates,
                                StringRef outPath) override;
   bool isCached(StringRef DepPath);
-  std::string getBackupPublicModuleInterfacePath(StringRef moduleName,
-                                                 StringRef interfacePath) override;
 };
 
 /// A ModuleLoader that runs a subordinate \c CompilerInvocation and

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -425,9 +425,17 @@ class ModuleInterfaceLoaderImpl {
   }
 
   std::string getBackupPublicModuleInterfacePath() {
+    return getBackupPublicModuleInterfacePath(ctx.SourceMgr, backupInterfaceDir,
+                                              moduleName, interfacePath);
+  }
+
+  static std::string getBackupPublicModuleInterfacePath(SourceManager &SM,
+                                                        StringRef backupInterfaceDir,
+                                                        StringRef moduleName,
+                                                        StringRef interfacePath) {
     if (backupInterfaceDir.empty())
       return std::string();
-    auto &fs = *ctx.SourceMgr.getFileSystem();
+    auto &fs = *SM.getFileSystem();
     auto fileName = llvm::sys::path::filename(interfacePath);
     {
       llvm::SmallString<256> path(backupInterfaceDir);
@@ -1145,21 +1153,6 @@ ModuleInterfaceCheckerImpl::getCompiledModuleCandidatesForInterface(
   return results;
 }
 
-std::string
-ModuleInterfaceCheckerImpl::getBackupPublicModuleInterfacePath(StringRef moduleName,
-                                                               StringRef interfacePath) {
-  // Derive .swiftmodule path from the .swiftinterface path.
-  auto newExt = file_types::getExtension(file_types::TY_SwiftModuleFile);
-  llvm::SmallString<32> modulePath = interfacePath;
-  llvm::sys::path::replace_extension(modulePath, newExt);
-  ModuleInterfaceLoaderImpl Impl(Ctx, modulePath, interfacePath, moduleName,
-                                 CacheDir, PrebuiltCacheDir, BackupInterfaceDir,
-                                 SourceLoc(), Opts,
-                                 RequiresOSSAModules, nullptr,
-                                 ModuleLoadingMode::PreferSerialized);
-  return Impl.getBackupPublicModuleInterfacePath();
-}
-
 bool ModuleInterfaceCheckerImpl::tryEmitForwardingModule(
     StringRef moduleName, StringRef interfacePath,
     ArrayRef<std::string> candidates, StringRef outputPath) {
@@ -1212,9 +1205,29 @@ bool ModuleInterfaceLoader::buildSwiftModuleFromSwiftInterface(
                                  LoaderOpts.disableInterfaceLock);
   // FIXME: We really only want to serialize 'important' dependencies here, if
   //        we want to ship the built swiftmodules to another machine.
-  return builder.buildSwiftModule(OutPath, /*shouldSerializeDeps*/true,
-                                  /*ModuleBuffer*/nullptr, nullptr,
-                                  SearchPathOpts.CandidateCompiledModules);
+  auto failed = builder.buildSwiftModule(OutPath, /*shouldSerializeDeps*/true,
+                                         /*ModuleBuffer*/nullptr, nullptr,
+                                         SearchPathOpts.CandidateCompiledModules);
+  if (!failed)
+    return false;
+  auto backInPath =
+    ModuleInterfaceLoaderImpl::getBackupPublicModuleInterfacePath(SourceMgr,
+      BackupInterfaceDir, ModuleName, InPath);
+  if (backInPath.empty())
+    return true;
+  assert(failed);
+  assert(!backInPath.empty());
+  ModuleInterfaceBuilder backupBuilder(SourceMgr, &Diags, astDelegate, backInPath,
+                                       ModuleName, CacheDir, PrebuiltCacheDir,
+                                       BackupInterfaceDir,
+                                       LoaderOpts.disableInterfaceLock);
+  // Ensure we can rebuild module after user changed the original interface file.
+  backupBuilder.addExtraDependency(InPath);
+  // FIXME: We really only want to serialize 'important' dependencies here, if
+  //        we want to ship the built swiftmodules to another machine.
+  return backupBuilder.buildSwiftModule(OutPath, /*shouldSerializeDeps*/true,
+                                        /*ModuleBuffer*/nullptr, nullptr,
+                                        SearchPathOpts.CandidateCompiledModules);
 }
 
 void ModuleInterfaceLoader::collectVisibleTopLevelModuleNames(

test/ModuleInterface/build-alternative-interface-explicit.swift

@@ -0,0 +1,14 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/sources)
+// RUN: %empty-directory(%t/inputs)
+// RUN: %empty-directory(%t/alternative-inputs)
+// RUN: %empty-directory(%t/module-cache)
+// RUN: %empty-directory(%t/outputs)
+
+// RUN: echo "public func foo() {}" > %t/sources/Foo.swift
+// RUN: %target-swift-frontend-typecheck -emit-module-interface-path %t/inputs/Foo.swiftinterface %t/sources/Foo.swift -module-name Foo -disable-implicit-concurrency-module-import -enable-library-evolution -module-cache-path %t/module-cache -I %t/inputs -swift-version 5
+// RUN: cp %t/inputs/Foo.swiftinterface %t/alternative-inputs/Foo.swiftinterface
+// RUN: echo "mess_mess_mess" >> %t/inputs/Foo.swiftinterface
+// RUN: not %target-swift-frontend -compile-module-from-interface -module-name Foo %t/inputs/Foo.swiftinterface -o %t/outputs/Foo.swiftmodule
+
+// RUN: %target-swift-frontend -compile-module-from-interface -module-name Foo %t/inputs/Foo.swiftinterface -o %t/outputs/Foo.swiftmodule -backup-module-interface-path %t/alternative-inputs