cross-module-optimization: Don't serialize functions which reference implementationOnly-imported functions

The check for implementationOnly imports was already done for types, but it was missing for functions.
Fixes a crash when implementationOnly-importing a C module.

https://bugs.swift.org/browse/SR-15048
rdar://81701218

Author: eeckstein

include/swift/AST/Module.h

@@ -645,9 +645,10 @@ class ModuleDecl : public DeclContext, public TypeDecl {
   /// This assumes that \p module was imported.
   bool isImportedImplementationOnly(const ModuleDecl *module) const;
 
-  /// Returns true if a function, which is using \p nominal, can be serialized
-  /// by cross-module-optimization.
-  bool canBeUsedForCrossModuleOptimization(NominalTypeDecl *nominal) const;
+  /// Returns true if decl context or its content can be serialized by
+  /// cross-module-optimization.
+  /// The \p ctxt can e.g. be a NominalType or the context of a function.
+  bool canBeUsedForCrossModuleOptimization(DeclContext *ctxt) const;
 
   /// Finds all top-level decls of this module.
   ///

lib/AST/Module.cpp

@@ -2179,14 +2179,15 @@ bool ModuleDecl::isImportedImplementationOnly(const ModuleDecl *module) const {
 }
 
 bool ModuleDecl::
-canBeUsedForCrossModuleOptimization(NominalTypeDecl *nominal) const {
-  ModuleDecl *moduleOfNominal = nominal->getParentModule();
+canBeUsedForCrossModuleOptimization(DeclContext *ctxt) const {
+  ModuleDecl *moduleOfCtxt = ctxt->getParentModule();
 
-  // If the nominal is defined in the same module, it's fine.
-  if (moduleOfNominal == this)
+  // If the context defined in the same module - or is the same module, it's
+  // fine.
+  if (moduleOfCtxt == this)
     return true;
 
-  // See if nominal is imported in a "regular" way, i.e. not with
+  // See if context is imported in a "regular" way, i.e. not with
   // @_implementationOnly or @_spi.
   ModuleDecl::ImportFilter filter = {
     ModuleDecl::ImportFilterKind::Exported,
@@ -2196,7 +2197,7 @@ canBeUsedForCrossModuleOptimization(NominalTypeDecl *nominal) const {
 
   auto &imports = getASTContext().getImportCache();
   for (auto &desc : results) {
-    if (imports.isImportedBy(moduleOfNominal, desc.importedModule))
+    if (imports.isImportedBy(moduleOfCtxt, desc.importedModule))
       return true;
   }
   return false;

lib/SILOptimizer/IPO/CrossModuleSerializationSetup.cpp

@@ -386,6 +386,11 @@ bool CrossModuleSerializationSetup::canUseFromInline(SILFunction *func,
   if (!func)
     return false;
 
+  if (DeclContext *funcCtxt = func->getDeclContext()) {
+    if (!M.getSwiftModule()->canBeUsedForCrossModuleOptimization(funcCtxt))
+      return false;
+  }
+
   switch (func->getLinkage()) {
   case SILLinkage::PublicNonABI:
     return func->isSerialized() != IsNotSerialized;

test/SILOptimizer/Inputs/cross-module/c-module.c

@@ -0,0 +1,7 @@
+
+#include "c-module.h"
+
+long privateCFunc() {
+  return 123;
+}
+

test/SILOptimizer/Inputs/cross-module/c-module.h

@@ -0,0 +1,3 @@
+
+long privateCFunc();
+

test/SILOptimizer/Inputs/cross-module-objc.swift renamed to test/SILOptimizer/Inputs/cross-module/cross-module-objc.swift

@@ -1,5 +1,6 @@
 import Submodule
 @_implementationOnly import PrivateSubmodule
+@_implementationOnly import PrivateCModule
 
 private enum PE<T> {
   case A
@@ -268,11 +269,19 @@ public func callUnrelated<T>(_ t: T) -> T {
   return t
 }
 
-public func callImplementationOnly<T>(_ t: T) -> T {
+public func callImplementationOnlyType<T>(_ t: T) -> T {
   let p = PrivateStr(i: 27)
   print(p.test())
   return t
 }
 
+public func callImplementationOnlyFunc<T>(_ t: T) -> Int {
+  return privateFunc()
+}
+
+public func callCImplementationOnly<T>(_ t: T) -> Int {
+  return Int(privateCFunc())
+}
+
 
 public let globalLet = 529387

test/SILOptimizer/Inputs/cross-module.swift renamed to test/SILOptimizer/Inputs/cross-module/cross-module.swift

@@ -12,3 +12,6 @@ public struct PrivateStr {
   }
 }
 
+public func privateFunc() -> Int {
+  return 40
+}

test/SILOptimizer/Inputs/cross-private-submodule.swift renamed to test/SILOptimizer/Inputs/cross-module/cross-private-submodule.swift

@@ -0,0 +1,3 @@
+module PrivateCModule {
+  header "c-module.h"
+}