[Frontend] Don't set "current" compiler version for modules built from swiftinterface

If swiftinterface doesn't have `-swift-compiler-version` flag
it means that it was generated with an older version of the
Swift compiler. In such cases it would be incorrect to use
"current" compiler version because the field is intended to
indicate the compiler the swiftinterface was built with.

Author: xedin

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -304,7 +304,12 @@ bool ArgsToFrontendOptionsConverter::convert(
       Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
                      A->getAsString(Args), A->getValue());
     }
-  } else {
+  // If swiftinterface doesn't have a flag, let's not set the version
+  // to the current compiler version here. This helps us to identify
+  // swiftmodules built from swiftinterface generated before introduction
+  // of `-swift-compiler-version` flag.
+  } else if (Opts.InputMode !=
+             FrontendOptions::ParseInputMode::SwiftModuleInterface) {
     Opts.SwiftCompilerVersion.tryParse(version::getCompilerVersion());
   }
 

lib/Frontend/Frontend.cpp

@@ -1489,8 +1489,10 @@ ModuleDecl *CompilerInstance::getMainModule() const {
     if (Invocation.getSILOptions().EnableSerializePackage)
       MainModule->setSerializePackageEnabled();
 
-    MainModule->setSwiftCompilerVersion(
-        Invocation.getFrontendOptions().SwiftCompilerVersion);
+    if (auto compilerVersion =
+            Invocation.getFrontendOptions().SwiftCompilerVersion) {
+      MainModule->setSwiftCompilerVersion(compilerVersion);
+    }
 
     // Register the main module with the AST context.
     Context->addLoadedModule(MainModule);

lib/Serialization/Serialization.cpp

@@ -1141,8 +1141,10 @@ void Serializer::writeHeader() {
       }
 
       llvm::VersionTuple compilerVersion = M->getSwiftCompilerVersion();
-      options_block::SwiftCompilerVersionLayout SwiftCompilerVersion(Out);
-      SwiftCompilerVersion.emit(ScratchRecord, compilerVersion.getAsString());
+      if (compilerVersion) {
+        options_block::SwiftCompilerVersionLayout SwiftCompilerVersion(Out);
+        SwiftCompilerVersion.emit(ScratchRecord, compilerVersion.getAsString());
+      }
 
       if (M->isConcurrencyChecked()) {
         options_block::IsConcurrencyCheckedLayout IsConcurrencyChecked(Out);

test/ModuleInterface/swift-compiler-version-option.swift

@@ -2,6 +2,7 @@
 
 /// Build the libraries.
 // RUN: %target-swift-frontend %s \
+// RUN:   -module-name Lib \
 // RUN:   -emit-module-path %t/Lib.swiftmodule \
 // RUN:   -emit-module-interface-path %t/Lib.swiftinterface \
 // RUN:   -enable-library-evolution \
@@ -20,6 +21,16 @@
 // CHECK-MODULE-OPTION: <SWIFT_COMPILER_VERSION abbrevid={{.*}}/> blob data = '{{.*}}'
 // CHECK-MODULE-OPTION: </OPTIONS_BLOCK>
 
+// Drop and rebuilt swiftmodule to make sure that the version is inferred from the interface file.
+// RUN: rm %t/Lib.swiftmodule
+// RUN: %target-swift-frontend -compile-module-from-interface %t/Lib.swiftinterface -o %t/Lib.swiftmodule -module-name Lib
+
+/// Check option in swiftmodule
+// RUN: llvm-bcanalyzer --dump %t/Lib.swiftmodule | %FileCheck --check-prefix=CHECK-REBUILT-MODULE-OPTION %s
+// CHECK-REBUILT-MODULE-OPTION: <OPTIONS_BLOCK
+// CHECK-REBUILT-MODULE-OPTION: <SWIFT_COMPILER_VERSION abbrevid={{.*}}/> blob data = '{{.*}}'
+// CHECK-REBUILT-MODULE-OPTION: </OPTIONS_BLOCK>
+
 public struct S {
     public var test: Int = 42
 }