diff --git a/include/swift/AST/DiagnosticsFrontend.def b/include/swift/AST/DiagnosticsFrontend.def
index 7fa4d6fefd823..dc5d82d37657a 100644
--- a/include/swift/AST/DiagnosticsFrontend.def
+++ b/include/swift/AST/DiagnosticsFrontend.def
@@ -440,6 +440,7 @@ NOTE(sdk_version_pbm_version,none,
 NOTE(compiled_module_ignored_reason,none,
      "compiled module '%0' was ignored because %select{%error"
      "|it belongs to a framework in the SDK"
+     "|it's a library module in the SDK"
      "|loading from module interfaces is preferred"
      "|it's a compiler host module"
      "|the module name is blocklisted,"
diff --git a/lib/Frontend/ModuleInterfaceLoader.cpp b/lib/Frontend/ModuleInterfaceLoader.cpp
index b5854d422990b..afca2671a31af 100644
--- a/lib/Frontend/ModuleInterfaceLoader.cpp
+++ b/lib/Frontend/ModuleInterfaceLoader.cpp
@@ -237,6 +237,7 @@ struct ModuleRebuildInfo {
   enum class ReasonIgnored {
     NotIgnored,
     PublicFramework,
+    PublicLibrary,
     InterfacePreferred,
     CompilerHostModule,
     Blocklisted,
@@ -762,6 +763,28 @@ class ModuleInterfaceLoaderImpl {
     return pathStartsWith(frameworksPath, path);
   }
 
+  bool isInSystemSubFrameworks(StringRef path) {
+    StringRef sdkPath = ctx.SearchPathOpts.getSDKPath();
+    if (sdkPath.empty()) return false;
+
+    SmallString<128> frameworksPath;
+    llvm::sys::path::append(frameworksPath,
+                            sdkPath, "System", "Library", "SubFrameworks");
+
+    return pathStartsWith(frameworksPath, path);
+  }
+
+  bool isInSystemLibraries(StringRef path) {
+    StringRef sdkPath = ctx.SearchPathOpts.getSDKPath();
+    if (sdkPath.empty()) return false;
+
+    SmallString<128> frameworksPath;
+    llvm::sys::path::append(frameworksPath,
+                            sdkPath, "usr", "lib", "swift");
+
+    return pathStartsWith(frameworksPath, path);
+  }
+
   std::pair<std::string, std::string> getCompiledModuleCandidates() {
     using ReasonIgnored = ModuleRebuildInfo::ReasonIgnored;
     using ReasonModuleInterfaceIgnored =
@@ -813,10 +836,15 @@ class ModuleInterfaceLoaderImpl {
 
       // Don't use the adjacent swiftmodule for frameworks from the public
       // Frameworks folder of the SDK.
-      if (isInSystemFrameworks(modulePath, /*publicFramework*/true)) {
+      if (isInSystemFrameworks(modulePath, /*publicFramework*/true) ||
+          isInSystemSubFrameworks(modulePath)) {
         shouldLoadAdjacentModule = false;
         rebuildInfo.addIgnoredModule(modulePath,
                                      ReasonIgnored::PublicFramework);
+      } else if (isInSystemLibraries(modulePath) && moduleName != STDLIB_NAME) {
+        shouldLoadAdjacentModule = false;
+        rebuildInfo.addIgnoredModule(modulePath,
+                                     ReasonIgnored::PublicLibrary);
       } else if (isInResourceHostDir(modulePath)) {
         shouldLoadAdjacentModule = false;
         rebuildInfo.addIgnoredModule(modulePath,
diff --git a/test/ModuleInterface/ignore-adjacent-swiftmodules.swift b/test/ModuleInterface/ignore-adjacent-swiftmodules.swift
index 77d9ffda8b712..106c17c3891e0 100644
--- a/test/ModuleInterface/ignore-adjacent-swiftmodules.swift
+++ b/test/ModuleInterface/ignore-adjacent-swiftmodules.swift
@@ -1,32 +1,82 @@
 // RUN: %empty-directory(%t)
-// RUN: %empty-directory(%t/cache)
+// RUN: %empty-directory(%t/cache0)
+// RUN: %empty-directory(%t/cache1)
+// RUN: cp -r %S/../Sema/Inputs/public-private-sdk %t/sdk
 // REQUIRES: VENDOR=apple
 
 /// Prepare the SDK.
-// RUN: cp -r %S/../Sema/Inputs/public-private-sdk %t/sdk
+//// stdlib
+// RUN: %target-swift-frontend -emit-module -module-name Swift -enable-library-evolution -swift-version 5 \
+// RUN:   -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -Osize \
+// RUN:   %t/sdk/usr/lib/swift/Swift.swiftmodule/source.swift \
+// RUN:   -o %t/sdk/usr/lib/swift/Swift.swiftmodule/%target-swiftmodule-name \
+// RUN:   -emit-module-interface-path %t/sdk/usr/lib/swift/Swift.swiftmodule/%target-swiftinterface-name \
+// RUN:   -parse-stdlib
+// RUN: %target-swift-typecheck-module-from-interface(%t/sdk/usr/lib/swift/Swift.swiftmodule/%target-swiftinterface-name) -module-name Swift -parse-stdlib
+
+//// Public framework
 // RUN: %target-swift-frontend -emit-module -module-name PublicSwift -enable-library-evolution -swift-version 5 \
+// RUN:   -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -Osize -sdk %t/sdk \
 // RUN:   %t/sdk/System/Library/Frameworks/PublicSwift.framework/Modules/PublicSwift.swiftmodule/source.swift \
 // RUN:   -o %t/sdk/System/Library/Frameworks/PublicSwift.framework/Modules/PublicSwift.swiftmodule/%target-swiftmodule-name \
 // RUN:   -emit-module-interface-path %t/sdk/System/Library/Frameworks/PublicSwift.framework/Modules/PublicSwift.swiftmodule/%target-swiftinterface-name
 // RUN: %target-swift-typecheck-module-from-interface(%t/sdk/System/Library/Frameworks/PublicSwift.framework/Modules/PublicSwift.swiftmodule/%target-swiftinterface-name) -module-name PublicSwift
+
+//// Private framework
 // RUN: %target-swift-frontend -emit-module -module-name PrivateSwift -enable-library-evolution -swift-version 5 \
+// RUN:   -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -Osize -sdk %t/sdk \
 // RUN:   %t/sdk/System/Library/PrivateFrameworks/PrivateSwift.framework/Modules/PrivateSwift.swiftmodule/source.swift \
 // RUN:   -o %t/sdk/System/Library/PrivateFrameworks/PrivateSwift.framework/Modules/PrivateSwift.swiftmodule/%target-swiftmodule-name \
 // RUN:   -emit-module-interface-path %t/sdk/System/Library/PrivateFrameworks/PrivateSwift.framework/Modules/PrivateSwift.swiftmodule/%target-swiftinterface-name
 // RUN: %target-swift-typecheck-module-from-interface(%t/sdk/System/Library/PrivateFrameworks/PrivateSwift.framework/Modules/PrivateSwift.swiftmodule/%target-swiftinterface-name) -module-name PrivateSwift
 
+//// Public library
+// RUN: %target-swift-frontend -emit-module -module-name PublicSwiftLibrary -enable-library-evolution -swift-version 5 \
+// RUN:   -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -Osize -sdk %t/sdk \
+// RUN:   %t/sdk/usr/lib/swift/PublicSwiftLibrary.swiftmodule/source.swift \
+// RUN:   -o %t/sdk/usr/lib/swift/PublicSwiftLibrary.swiftmodule/%target-swiftmodule-name \
+// RUN:   -emit-module-interface-path %t/sdk/usr/lib/swift/PublicSwiftLibrary.swiftmodule/%target-swiftinterface-name
+// RUN: %target-swift-typecheck-module-from-interface(%t/sdk/usr/lib/swift/PublicSwiftLibrary.swiftmodule/%target-swiftinterface-name) -module-name PublicSwiftLibrary
+
+//// Public subframework
+// RUN: %target-swift-frontend -emit-module -module-name SubSwift -enable-library-evolution -swift-version 5 \
+// RUN:   -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import -Osize -sdk %t/sdk \
+// RUN:   %t/sdk/System/Library/SubFrameworks/SubSwift.framework/Modules/SubSwift.swiftmodule/source.swift \
+// RUN:   -o %t/sdk/System/Library/SubFrameworks/SubSwift.framework/Modules/SubSwift.swiftmodule/%target-swiftmodule-name \
+// RUN:   -emit-module-interface-path %t/sdk/System/Library/SubFrameworks/SubSwift.framework/Modules/SubSwift.swiftmodule/%target-swiftinterface-name
+// RUN: %target-swift-typecheck-module-from-interface(%t/sdk/System/Library/SubFrameworks/SubSwift.framework/Modules/SubSwift.swiftmodule/%target-swiftinterface-name) -module-name SubSwift
+
 /// Break the swiftmodules.
 // RUN: echo "This is a malformed swiftmodule" > %t/sdk/System/Library/Frameworks/PublicSwift.framework/Modules/PublicSwift.swiftmodule/%target-swiftmodule-name
 // RUN: echo "This is a malformed swiftmodule" > %t/sdk/System/Library/PrivateFrameworks/PrivateSwift.framework/Modules/PrivateSwift.swiftmodule/%target-swiftmodule-name
+// RUN: echo "This is a malformed swiftmodule" > %t/sdk/System/Library/SubFrameworks/SubSwift.framework/Modules/SubSwift.swiftmodule/%target-swiftmodule-name
+// RUN: echo "This is a malformed swiftmodule" > %t/sdk/usr/lib/swift/PublicSwiftLibrary.swiftmodule/%target-swiftmodule-name
 
-/// There should be no attempt at loading the malformed PublicSwift swiftmodule.
-/// This means no notes about:
+/// Check the loading behavior from attempts at loading the malformed swiftmodules,
+/// printing the notes:
 /// * compiled module is out of date
 /// * unable to load compiled module '*': malformed
+
+/// Check diagnostics in the local file:
 // RUN: %target-swift-frontend -typecheck %s -sdk %t/sdk \
-// RUN:   -module-name Main -module-cache-path %t/cache \
-// RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ \
-// RUN:   -verify -Rmodule-interface-rebuild
+// RUN:   -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import \
+// RUN:   -module-name Main -module-cache-path %t/cache0 \
+// RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ -resource-dir "" \
+// RUN:   -verify -verify-ignore-unknown -Rmodule-interface-rebuild -diagnostic-style=llvm
+
+/// Check diagnostic for implicit imports:
+// RUN: echo "This is a malformed swiftmodule" > %t/sdk/usr/lib/swift/Swift.swiftmodule/%target-swiftmodule-name
+// RUN: %target-swift-frontend -typecheck %s -sdk %t/sdk \
+// RUN:   -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import \
+// RUN:   -module-name Main -module-cache-path %t/cache1 \
+// RUN:   -F %t/sdk/System/Library/PrivateFrameworks/ -resource-dir "" \
+// RUN:   -Rmodule-interface-rebuild -Rmodule-loading -diagnostic-style=llvm 2> %t/out
+// RUN: %FileCheck --input-file %t/out %s
+
+import Swift
+// CHECK: rebuilding module 'Swift' from interface
+// CHECK-NEXT: compiled module is out of date
+// CHECK-NEXT: : malformed
 
 import PublicSwift // expected-remark {{rebuilding module 'PublicSwift' from interface}}
 // expected-note @-1 {{was ignored because it belongs to a framework in the SDK}}
@@ -36,3 +86,9 @@ import PrivateSwift
 // expected-remark @-1 {{rebuilding module 'PrivateSwift' from interface}}
 // expected-note @-2 {{compiled module is out of date}}
 // expected-note @-3 {{: malformed}}
+
+import PublicSwiftLibrary // expected-remark {{rebuilding module 'PublicSwiftLibrary' from interface}}
+// expected-note @-1 {{was ignored because it's a library module in the SDK}}
+
+import SubSwift // expected-remark {{rebuilding module 'SubSwift' from interface}}
+// expected-note @-1 {{was ignored because it belongs to a framework in the SDK}}
diff --git a/test/Sema/Inputs/public-private-sdk/System/Library/SubFrameworks/SubSwift.framework/Modules/SubSwift.swiftmodule/source.swift b/test/Sema/Inputs/public-private-sdk/System/Library/SubFrameworks/SubSwift.framework/Modules/SubSwift.swiftmodule/source.swift
new file mode 100644
index 0000000000000..6d71fe5d9cef6
--- /dev/null
+++ b/test/Sema/Inputs/public-private-sdk/System/Library/SubFrameworks/SubSwift.framework/Modules/SubSwift.swiftmodule/source.swift
@@ -0,0 +1 @@
+public func foo() {}
diff --git a/test/Sema/Inputs/public-private-sdk/usr/lib/swift/PublicSwiftLibrary.swiftmodule/source.swift b/test/Sema/Inputs/public-private-sdk/usr/lib/swift/PublicSwiftLibrary.swiftmodule/source.swift
new file mode 100644
index 0000000000000..6d71fe5d9cef6
--- /dev/null
+++ b/test/Sema/Inputs/public-private-sdk/usr/lib/swift/PublicSwiftLibrary.swiftmodule/source.swift
@@ -0,0 +1 @@
+public func foo() {}
diff --git a/test/Sema/Inputs/public-private-sdk/usr/lib/swift/Swift.swiftmodule/source.swift b/test/Sema/Inputs/public-private-sdk/usr/lib/swift/Swift.swiftmodule/source.swift
new file mode 100644
index 0000000000000..6d71fe5d9cef6
--- /dev/null
+++ b/test/Sema/Inputs/public-private-sdk/usr/lib/swift/Swift.swiftmodule/source.swift
@@ -0,0 +1 @@
+public func foo() {}