[Package CMO] Diagnose missing library-evolution flag.

This PR ensures library-evolution is enabled for Package CMO; without it,
it previously fell back to regular CMO, which caused mismatching serialization
attributes if importing another module that had Package CMO enbaled, causing
an assert fail for loadable types.

Resolves rdar://135308288

Author: elsh

include/swift/AST/DiagnosticsFrontend.def

@@ -576,6 +576,9 @@ ERROR(no_allocations_without_embedded,none,
 ERROR(no_swift_sources_with_embedded,none,
       "embedded swift cannot be enabled in a compiler built without Swift sources", ())
 
+WARNING(package_cmo_requires_library_evolution, none,
+        "Library evolution must be enabled for Package CMO", ())
+
 ERROR(experimental_not_supported_in_production,none,
       "experimental feature '%0' cannot be enabled in production compiler",
       (StringRef))

lib/Frontend/CompilerInvocation.cpp

@@ -2688,6 +2688,8 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
       Diags.diagnose(SourceLoc(), diag::ignoring_option_requires_option,
                      "-package-cmo",
                      "-allow-non-resilient-access");
+    } else if (!FEOpts.EnableLibraryEvolution) {
+      Diags.diagnose(SourceLoc(), diag::package_cmo_requires_library_evolution);
     } else {
       Opts.EnableSerializePackage = true;
       Opts.CMOMode = CrossModuleOptimizationMode::Default;

lib/SILOptimizer/IPO/CrossModuleOptimization.cpp

@@ -947,9 +947,15 @@ void CrossModuleOptimization::makeSubstUsableFromInline(
 class CrossModuleOptimizationPass: public SILModuleTransform {
   void run() override {
     auto &M = *getModule();
-    if (M.getSwiftModule()->isResilient() &&
-        !M.getSwiftModule()->serializePackageEnabled())
+    if (M.getSwiftModule()->serializePackageEnabled()) {
+      assert(M.getSwiftModule()->isResilient() &&
+             "Package CMO requires library-evolution");
+    } else if (M.getSwiftModule()->isResilient()) {
+      // If no Package CMO flags are passed and library
+      // evolution is enabled, just return.
       return;
+    }
+
     if (!M.isWholeModule())
       return;
 
@@ -974,6 +980,9 @@ class CrossModuleOptimizationPass: public SILModuleTransform {
       return;
     }
 
+    if (isPackageCMOEnabled(M.getSwiftModule()))
+      assert(conservative && "Package CMO requires conservative CMO mode");
+
     CrossModuleOptimization CMO(M, conservative, everything);
     CMO.serializeFunctionsInModule(PM);
 

test/SILOptimizer/package-cmo-require-library-evolution.swift

@@ -0,0 +1,38 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-build-swift %s \
+// RUN: -module-name=Lib -package-name Pkg \
+// RUN: -emit-module -o %t/Lib.swiftmodule -I%t \
+// RUN: -Xfrontend -package-cmo \
+// RUN: -Xfrontend -allow-non-resilient-access \
+// RUN: -O -wmo 2>&1 | %FileCheck %s --check-prefix CHECK-WARNING
+// CHECK-WARNING: warning: Library evolution must be enabled for Package CMO
+
+// RUN: llvm-bcanalyzer %t/Lib.swiftmodule | %FileCheck %s -check-prefix=CHECK-BC
+// CHECK-BC-NOT: SERIALIZE_PACKAGE_ENABLED
+
+// RUN: rm -rf %t/Lib.swiftmodule
+
+// RUN: %target-build-swift %s \
+// RUN: -module-name=Lib -package-name Pkg \
+// RUN: -emit-module -o %t/Lib.swiftmodule -I%t \
+// RUN: -Xfrontend -package-cmo \
+// RUN: -Xfrontend -allow-non-resilient-access \
+// RUN: -enable-library-evolution \
+// RUN: -O -wmo
+
+// RUN: llvm-bcanalyzer %t/Lib.swiftmodule | %FileCheck %s -check-prefix=CHECK-OK
+// CHECK-OK: SERIALIZE_PACKAGE_ENABLED
+
+// REQUIRES: swift_in_compiler
+
+public struct S {
+  public init() {}
+  package var x: Int {
+    get { return 0 }
+    set {}
+  }
+  package func f() -> Int {
+    return 1
+  }
+}