[Serialization] Mark SPI only dependencies as required in swiftmodules

rdar://108326858

Author: xymus

lib/Serialization/Serialization.cpp

@@ -1214,8 +1214,9 @@ void Serializer::writeInputBlock(const SerializationOptions &options) {
   // distinguish them.
   ImportSet publicImportSet =
       getImportsAsSet(M, ModuleDecl::ImportFilterKind::Exported);
-  ImportSet privateImportSet =
-      getImportsAsSet(M, ModuleDecl::ImportFilterKind::Default);
+  ImportSet defaultImportSet =
+      getImportsAsSet(M, {ModuleDecl::ImportFilterKind::Default,
+                          ModuleDecl::ImportFilterKind::SPIOnly});
   ImportSet packageOnlyImportSet =
       getImportsAsSet(M, ModuleDecl::ImportFilterKind::PackageOnly);
   ImportSet internalOrBelowImportSet =
@@ -1265,7 +1266,7 @@ void Serializer::writeInputBlock(const SerializationOptions &options) {
     // form here.
     if (publicImportSet.count(import))
       stableImportControl = ImportControl::Exported;
-    else if (privateImportSet.count(import))
+    else if (defaultImportSet.count(import))
       stableImportControl = ImportControl::Normal;
     else if (packageOnlyImportSet.count(import))
       stableImportControl = ImportControl::PackageOnly;

test/Serialization/spi-only-dependency.swift

@@ -0,0 +1,40 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+//--- SPIDependency.swift
+
+// RUN: %target-swift-frontend -emit-module %t/SPIDependency.swift -o %t \
+// RUN:   -enable-library-evolution -swift-version 5
+
+//--- Lib.swift
+
+// RUN: %target-swift-frontend -emit-module %t/Lib.swift -o %t -I %t \
+// RUN:   -enable-library-evolution -swift-version 5 \
+// RUN:   -emit-module-path %t/Lib.swiftmodule \
+// RUN:   -emit-module-interface-path %t/Lib.swiftinterface \
+// RUN:   -emit-private-module-interface-path %t/Lib.private.swiftinterface \
+// RUN:   -experimental-spi-only-imports
+
+@_spiOnly import SPIDependency
+
+//--- Client.swift
+
+/// SPIDependency is visible when using the swiftmodule or private interface.
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
+// RUN:   -experimental-spi-only-imports \
+// RUN:   -Rmodule-loading 2>&1 | %FileCheck -check-prefix=VISIBLE-DEP %s
+// VISIBLE-DEP: loaded module 'SPIDependency'
+
+// RUN: rm %t/Lib.swiftmodule
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
+// RUN:   -experimental-spi-only-imports \
+// RUN:   -Rmodule-loading 2>&1 | %FileCheck -check-prefix=VISIBLE-DEP %s
+
+/// SPIDependency is not visible when using the public swiftinterface.
+// RUN: rm %t/Lib.private.swiftinterface
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t \
+// RUN:   -experimental-spi-only-imports \
+// RUN:   -Rmodule-loading 2>&1 | %FileCheck -check-prefix=VISIBLE-DEP-NOT %s
+// VISIBLE-DEP-NOT-NOT: loaded module 'SPIDependency'
+
+import Lib