Add -emit-public-type-metadata-accessors to work around metadata linkage bug.

IRGen can introduce calls to type metadata accessors for types that
should not be visible to the current translate, which can manifest in
linker errors within a module (for references to private types when
whole module optimization is disabled) or across modules (for
references to private/internal types in another module). Introduce a
new compiler flag `-emit-public-type-metadata-accessors` that emits
all type metadata accessors with public linkage, to work around the
problem in affected projects. This flag is intended to go away once we
have a proper solution.

This bug has been around in Swift "forever", but compiling the
overlays using -enable-resilience has exacerbated the problem and
caused regressions. This is a short-term fix to
rdar://problem/40229755 while we work on the correct long-term fix.

Author: DougGregor

include/swift/Basic/LangOptions.h

@@ -223,6 +223,9 @@ namespace swift {
     /// Enables key path resilience.
     bool EnableKeyPathResilience = false;
 
+    /// Enables public emission of private metadata accessors.
+    bool EmitPublicTypeMetadataAccessors = false;
+
     /// If set to true, the diagnosis engine can assume the emitted diagnostics
     /// will be used in editor. This usually leads to more aggressive fixit.
     bool DiagnosticsEditorMode = false;

include/swift/Option/Options.td

@@ -355,6 +355,11 @@ def warn_swift3_objc_inference : Flag<["-"], "warn-swift3-objc-inference">,
   Alias<warn_swift3_objc_inference_complete>,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild, HelpHidden]>;
 
+def emit_public_type_metadata_accessors :
+  Flag<["-"], "emit-public-type-metadata-accessors">,
+  Flags<[FrontendOption]>,
+  HelpText<"Emit all type metadata accessors as public">;
+
 def Rpass_EQ : Joined<["-"], "Rpass=">,
   Flags<[FrontendOption]>,
   HelpText<"Report performed transformations by optimization passes whose "

lib/Driver/ToolChains.cpp

@@ -183,6 +183,8 @@ static void addCommonFrontendArgs(const ToolChain &TC, const OutputInfo &OI,
                        options::OPT_warn_swift3_objc_inference_minimal,
                        options::OPT_warn_swift3_objc_inference_complete);
   inputArgs.AddLastArg(arguments, options::OPT_typo_correction_limit);
+  inputArgs.AddLastArg(arguments,
+                       options::OPT_emit_public_type_metadata_accessors);
   inputArgs.AddLastArg(arguments, options::OPT_enable_app_extension);
   inputArgs.AddLastArg(arguments, options::OPT_enable_testing);
   inputArgs.AddLastArg(arguments, options::OPT_g_Group);

lib/Frontend/CompilerInvocation.cpp

@@ -309,6 +309,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  Opts.EmitPublicTypeMetadataAccessors =
+    Args.hasArg(OPT_emit_public_type_metadata_accessors);
+
   Opts.EnableNSKeyedArchiverDiagnostics =
       Args.hasFlag(OPT_enable_nskeyedarchiver_diagnostics,
                    OPT_disable_nskeyedarchiver_diagnostics,

lib/IRGen/MetadataRequest.cpp

@@ -589,6 +589,17 @@ bool irgen::isTypeMetadataAccessTrivial(IRGenModule &IGM, CanType type) {
   return false;
 }
 
+/// Determine whether we should promote the type metadata access function
+/// for the given nominal type to "public".
+static bool promoteMetadataAccessFunctionToPublic(
+                                              const NominalTypeDecl *nominal) {
+  ASTContext &ctx = nominal->getASTContext();
+
+  // When -emit-public-type-metadata-accessors is provided, promote all
+  // of the metadata access functions to public.
+  return ctx.LangOpts.EmitPublicTypeMetadataAccessors;
+}
+
 /// Return the standard access strategy for getting a non-dependent
 /// type metadata object.
 MetadataAccessStrategy irgen::getTypeMetadataAccessStrategy(CanType type) {
@@ -621,8 +632,12 @@ MetadataAccessStrategy irgen::getTypeMetadataAccessStrategy(CanType type) {
     case FormalLinkage::PublicUnique:
       return MetadataAccessStrategy::PublicUniqueAccessor;
     case FormalLinkage::HiddenUnique:
+      if (promoteMetadataAccessFunctionToPublic(nominal))
+        return MetadataAccessStrategy::PublicUniqueAccessor;
       return MetadataAccessStrategy::HiddenUniqueAccessor;
     case FormalLinkage::Private:
+      if (promoteMetadataAccessFunctionToPublic(nominal))
+        return MetadataAccessStrategy::PublicUniqueAccessor;
       return MetadataAccessStrategy::PrivateAccessor;
 
     case FormalLinkage::PublicNonUnique:

test/IRGen/Inputs/emit_public_type_metadata_accessors_other.swift

@@ -0,0 +1,3 @@
+func foo() -> Any {
+  return Wrapper()
+}

test/IRGen/emit_public_type_metadata_accessors.swift

@@ -0,0 +1,17 @@
+// RUN: %swift -module-name test -target x86_64-apple-macosx10.9  -emit-ir -parse-stdlib -emit-public-type-metadata-accessors -primary-file %s %S/Inputs/emit_public_type_metadata_accessors_other.swift | %FileCheck --check-prefix=CHECK-PUBLIC %s
+
+// RUN: %swift -module-name test -target x86_64-apple-macosx10.9  -emit-ir -parse-stdlib -primary-file %s %S/Inputs/emit_public_type_metadata_accessors_other.swift | %FileCheck --check-prefix=CHECK-NONPUBLIC %s
+
+private class C { }
+
+// CHECK-PUBLIC: define swiftcc %swift.metadata_response @"$S4test3Foo33_DEC9477CC6E8E6E7A9CE422B1DBE7EA4LLVMa"
+// CHECK-NONPUBLIC: define internal swiftcc %swift.metadata_response @"$S4test3Foo33_DEC9477CC6E8E6E7A9CE422B1DBE7EA4LLVMa"
+private struct Foo {
+  private let c: C = C()
+}
+
+public struct Wrapper {
+  private let foo: Foo = Foo()
+}
+
+