Add a staging flag -enable-experimental-deserialization-recovery.

No promises that this is stable yet!

Author: jrose-apple

include/swift/Basic/LangOptions.h

@@ -175,6 +175,12 @@ namespace swift {
     /// new enough?
     bool EnableTargetOSChecking = true;
 
+    /// Whether to attempt to recover from missing cross-references and other
+    /// errors when deserializing from a Swift module.
+    ///
+    /// This is a staging flag; eventually it will be on by default.
+    bool EnableDeserializationRecovery = false;
+
     /// Should we use \c ASTScope-based resolution for unqualified name lookup?
     bool EnableASTScopeLookup = false;
 

include/swift/Option/FrontendOptions.td

@@ -260,6 +260,10 @@ def enable_experimental_property_behaviors :
   Flag<["-"], "enable-experimental-property-behaviors">,
   HelpText<"Enable experimental property behaviors">;
 
+def enable_experimental_deserialization_recovery :
+  Flag<["-"], "enable-experimental-deserialization-recovery">,
+  HelpText<"Attempt to recover from missing xrefs (etc) in swiftmodules">;
+
 def enable_cow_existentials : Flag<["-"], "enable-cow-existentials">,
   HelpText<"Enable the copy-on-write existential implementation">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -874,6 +874,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableClassResilience |=
     Args.hasArg(OPT_enable_class_resilience);
 
+  Opts.EnableDeserializationRecovery |=
+    Args.hasArg(OPT_enable_experimental_deserialization_recovery);
+
   Opts.DisableAvailabilityChecking |=
       Args.hasArg(OPT_disable_availability_checking);
 

tools/swift-ide-test/swift-ide-test.cpp

@@ -318,6 +318,12 @@ EnableSwift3ObjCInference(
                   llvm::cl::desc("Enable Swift 3's @objc inference rules"),
                   llvm::cl::init(false));
 
+static llvm::cl::opt<bool>
+EnableDeserializationRecovery(
+    "enable-experimental-deserialization-recovery",
+    llvm::cl::desc("Attempt to recover from missing xrefs (etc) in swiftmodules"),
+    llvm::cl::init(false));
+
 static llvm::cl::opt<bool>
 DisableObjCAttrRequiresFoundationModule(
     "disable-objc-attr-requires-foundation-module",
@@ -2995,6 +3001,8 @@ int main(int argc, char *argv[]) {
   InitInvok.getLangOptions().EnableSwift3ObjCInference =
     options::EnableSwift3ObjCInference ||
     InitInvok.getLangOptions().isSwiftVersion3();
+  InitInvok.getLangOptions().EnableDeserializationRecovery |=
+    options::EnableDeserializationRecovery;
   InitInvok.getClangImporterOptions().ImportForwardDeclarations |=
     options::ObjCForwardDeclarations;
   InitInvok.getClangImporterOptions().InferImportAsMember |=