[Serialization] Proof-of-concept: drop overriding methods if the base is missing

include/swift/AST/DiagnosticsSema.def

@@ -549,6 +549,11 @@ ERROR(serialization_target_too_new_repl,none,
       "module file's minimum deployment target is %0 v%1.%2%select{|.%3}3: %4",
       (StringRef, unsigned, unsigned, unsigned, StringRef))
 
+ERROR(serialization_fatal,Fatal,
+      "fatal error encountered while reading from module '%0'; "
+      "please file a bug report with your project and the crash log",
+      (StringRef))
+
 ERROR(reserved_member_name,none,
       "type member may not be named %0, since it would conflict with the"
       " 'foo.%1' expression", (DeclName, StringRef))

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">;
 

include/swift/Serialization/ModuleFile.h

@@ -26,6 +26,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/TinyPtrVector.h"
 #include "llvm/Bitcode/BitstreamReader.h"
+#include "llvm/Support/Error.h"
 #include "llvm/Support/MemoryBuffer.h"
 
 namespace llvm {
@@ -435,6 +436,10 @@ class ModuleFile : public LazyMemberLoader {
     return getStatus();
   }
 
+  /// Emits one last diagnostic, logs the error, and then aborts for the stack
+  /// trace.
+  void fatal(llvm::Error error) LLVM_ATTRIBUTE_NORETURN;
+
   ASTContext &getContext() const {
     assert(FileContext && "no associated context yet");
     return FileContext->getParentModule()->getASTContext();
@@ -542,12 +547,14 @@ class ModuleFile : public LazyMemberLoader {
   /// because it reads from the cursor, it is not possible to reset the cursor
   /// after reading. Nothing should ever follow an XREF record except
   /// XREF_PATH_PIECE records.
-  Decl *resolveCrossReference(ModuleDecl *M, uint32_t pathLen);
+  llvm::Expected<Decl *> resolveCrossReference(ModuleDecl *M, uint32_t pathLen);
 
   /// Populates TopLevelIDs for name lookup.
   void buildTopLevelDeclMap();
 
-  void configureStorage(AbstractStorageDecl *storage, unsigned rawStorageKind,
+  /// Sets the accessors for \p storage based on \p rawStorageKind.
+  void configureStorage(AbstractStorageDecl *storage,
+                        unsigned rawStorageKind,
                         serialization::DeclID getter,
                         serialization::DeclID setter,
                         serialization::DeclID materializeForSet,
@@ -743,8 +750,13 @@ class ModuleFile : public LazyMemberLoader {
   }
 
   /// Returns the type with the given ID, deserializing it if needed.
+  ///
+  /// \sa getTypeChecked
   Type getType(serialization::TypeID TID);
 
+  /// Returns the type with the given ID, deserializing it if needed.
+  llvm::Expected<Type> getTypeChecked(serialization::TypeID TID);
+
   /// Returns the identifier with the given ID, deserializing it if needed.
   Identifier getIdentifier(serialization::IdentifierID IID);
 
@@ -754,9 +766,21 @@ class ModuleFile : public LazyMemberLoader {
   /// \param ForcedContext Optional override for the decl context of certain
   ///                      kinds of decls, used to avoid re-entrant
   ///                      deserialization.
+  ///
+  /// \sa getDeclChecked
   Decl *getDecl(serialization::DeclID DID,
                 Optional<DeclContext *> ForcedContext = None);
 
+  /// Returns the decl with the given ID, deserializing it if needed.
+  ///
+  /// \param DID The ID for the decl within this module.
+  /// \param ForcedContext Optional override for the decl context of certain
+  ///                      kinds of decls, used to avoid re-entrant
+  ///                      deserialization.
+  llvm::Expected<Decl *>
+  getDeclChecked(serialization::DeclID DID,
+                 Optional<DeclContext *> ForcedContext = None);
+
   /// Returns the decl context with the given ID, deserializing it if needed.
   DeclContext *getDeclContext(serialization::DeclContextID DID);
 

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);