[Serialization] Allow loading modules built on some SDK variants

When restricting loading swiftmodules to the SDK used to build them,
an exception should be made for modules built against an SDK that is a subset
of the SDK used when loading the module. In such a case, the swiftmodule
file is more reliable.

rdar://92827584

Author: xymus

lib/Serialization/ModuleFile.cpp

@@ -156,11 +156,18 @@ Status ModuleFile::associateWithFileContext(FileUnit *file, SourceLoc diagLoc,
       return error(status);
   }
 
+  // The loaded module was built with a compatible SDK if either:
+  // * it was the same SDK
+  // * or one who's name is a prefix of the clients' SDK name. This expects
+  // that a module built with macOS11 can be used with the macOS11.secret SDK.
+  // This is generally the case as SDKs with suffixes are a superset of the
+  // short SDK name equivalent. While this is accepted, this is still not a
+  // recommended configuration and may lead to unreadable swiftmodules.
   StringRef moduleSDK = Core->SDKName;
   StringRef clientSDK = ctx.LangOpts.SDKName;
   if (ctx.SearchPathOpts.EnableSameSDKCheck &&
       !moduleSDK.empty() && !clientSDK.empty() &&
-      moduleSDK != clientSDK) {
+      !clientSDK.startswith(moduleSDK)) {
     status = Status::SDKMismatch;
     return error(status);
   }

test/Serialization/restrict-swiftmodule-to-sdk.swift

@@ -9,8 +9,16 @@
 // RUN: %target-swift-frontend -typecheck %t/Client.swift -swift-version 5 -target-sdk-name A -I %t/build -parse-stdlib -module-cache-path %t/cache
 
 /// Build Client against SDK B, this should fail at loading Lib against a different SDK than A.
-// RUN: not %target-swift-frontend -typecheck %t/Client.swift -swift-version 5 -target-sdk-name B -I %t/build -parse-stdlib -module-cache-path %t/cache 2>&1 | %FileCheck %s
-// CHECK: cannot load module 'Lib' built with SDK 'A' when using SDK 'B': {{.*}}Lib.swiftmodule
+// RUN: not %target-swift-frontend -typecheck %t/Client.swift -swift-version 5 -target-sdk-name B -I %t/build -parse-stdlib -module-cache-path %t/cache 2>&1 | %FileCheck %s -check-prefix=CHECK-AvsB
+// CHECK-AvsB: cannot load module 'Lib' built with SDK 'A' when using SDK 'B': {{.*}}Lib.swiftmodule
+
+/// Build Client against SDK A.Secret, this should accept the SDK as being a super set of A.
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -swift-version 5 -target-sdk-name A.Secret -I %t/build -parse-stdlib -module-cache-path %t/cache
+
+/// Build Lib against SDK C.Secret and Client against SDK C, this should be rejected.
+// RUN: %target-swift-frontend -emit-module %t/Lib.swift -swift-version 5 -target-sdk-name C.Secret -o %t/build -parse-stdlib -module-cache-path %t/cache
+// RUN: not %target-swift-frontend -typecheck %t/Client.swift -swift-version 5 -target-sdk-name C -I %t/build -parse-stdlib -module-cache-path %t/cache 2>&1 | %FileCheck %s -check-prefix=CHECK-C
+// CHECK-C: cannot load module 'Lib' built with SDK 'C.Secret' when using SDK 'C': {{.*}}Lib.swiftmodule
 
 // BEGIN Lib.swift
 public func foo() {}