[6.0][IRGen] Fetch the correct IGM for synthesized files

lib/IRGen/IRGen.cpp

@@ -1431,7 +1431,7 @@ static void performParallelIRGeneration(IRGenDescriptor desc) {
       }
 
       if (auto *synthSFU = File->getSynthesizedFile()) {
-        CurrentIGMPtr IGM = irgen.getGenModule(synthSFU);
+        CurrentIGMPtr IGM = irgen.getGenModule(&synthSFU->getFileUnit());
         IGM->emitSynthesizedFileUnit(*synthSFU);
       }
     } else {

test/Macros/extension_emission.swift

@@ -0,0 +1,61 @@
+// REQUIRES: swift_swift_parser
+
+// This test ensures that code generated by extension macros gets emitted into the correct LLVM module
+// rdar://128870792
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %host-build-swift -swift-version 5 -emit-library -o %t/%target-library-name(MacroDefinition) -parse-as-library -module-name=MacroDefinition %t/macro.swift -g -no-toolchain-stdlib-rpath
+
+// RUN: %target-swift-frontend -num-threads 1 -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) -enable-library-evolution %t/b.swift %t/a.swift -emit-ir -o %t/b.ll -o %t/a.ll
+
+// RUN: %FileCheck -check-prefix=CHECK-A %s < %t/a.ll
+
+// RUN: %FileCheck -check-prefix=CHECK-B %s < %t/b.ll
+
+//--- macro.swift
+
+import SwiftSyntax
+import SwiftSyntaxBuilder
+@_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacros
+
+public struct SomeExtensionMacro: ExtensionMacro {
+  public static func expansion(of node: AttributeSyntax, attachedTo declaration: some DeclGroupSyntax, providingExtensionsOf type: some TypeSyntaxProtocol, conformingTo protocols: [TypeSyntax], in context: some MacroExpansionContext) throws -> [ExtensionDeclSyntax] {
+    let decl: DeclSyntax =
+    """
+    extension \(type.trimmed) {
+      struct Storage {
+        let x: Int
+      }
+
+      func alsoUseStorage(_ x: Storage?) {
+        print(type(of: x))
+      }
+    }
+    """
+    guard let extensionDecl = decl.as(ExtensionDeclSyntax.self) else {
+      return []
+    }
+
+    return [extensionDecl]
+  }
+}
+
+//--- a.swift
+
+@attached(
+  extension,
+  names: named(Storage), named(alsoUseStorage)
+)
+macro someExtension() = #externalMacro(module: "MacroDefinition", type: "SomeExtensionMacro")
+
+// CHECK-A: @"$s1b1SV7StorageVMn" = {{.*}}constant
+@someExtension
+struct S {}
+
+//--- b.swift
+
+// This file needs no content, it just needs to exist
+
+// CHECK-B-NOT: @"$s1b1SV7StorageVMn"