[Serialization] Raise an error (to recover from) when reading an unsafe decl

Intro UnsafeDeserializationError and instanciate it when attempting to
deserialize a decl marked as unsafe by a DESERIALIZATION_SAFETY record.

Author: xymus

lib/Serialization/Deserialization.cpp

@@ -155,6 +155,8 @@ const char DeclAttributesDidNotMatch::ID = '\0';
 void DeclAttributesDidNotMatch::anchor() {}
 const char InvalidRecordKindError::ID = '\0';
 void InvalidRecordKindError::anchor() {}
+const char UnsafeDeserializationError::ID = '\0';
+void UnsafeDeserializationError::anchor() {}
 
 /// Skips a single record in the bitstream.
 ///
@@ -5414,6 +5416,14 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
     } else if (recordID == decls_block::DESERIALIZATION_SAFETY) {
       IdentifierID declID;
       decls_block::DeserializationSafetyLayout::readRecord(scratch, declID);
+
+      if (MF.getResilienceStrategy() == ResilienceStrategy::Resilient &&
+          MF.getContext().LangOpts.EnableDeserializationSafety) {
+        auto name = MF.getIdentifier(declID);
+        LLVM_DEBUG(llvm::dbgs() << "Skipping unsafe deserialization: '"
+                                << name << "'\n");
+        return llvm::make_error<UnsafeDeserializationError>(name);
+      }
     } else {
       return llvm::Error::success();
     }

lib/Serialization/DeserializationErrors.h

@@ -494,6 +494,27 @@ class InvalidRecordKindError :
   }
 };
 
+// Decl was not deserialized because it's an internal detail maked as unsafe
+// at serialization.
+class UnsafeDeserializationError : public llvm::ErrorInfo<UnsafeDeserializationError, DeclDeserializationError> {
+  friend ErrorInfo;
+  static const char ID;
+  void anchor() override;
+
+public:
+  UnsafeDeserializationError(Identifier name) {
+    this->name = name;
+  }
+
+  void log(raw_ostream &OS) const override {
+    OS << "Decl '" << name << "' is unsafe to deserialize";
+  }
+
+  std::error_code convertToErrorCode() const override {
+    return llvm::inconvertibleErrorCode();
+  }
+};
+
 LLVM_NODISCARD
 static inline std::unique_ptr<llvm::ErrorInfoBase>
 takeErrorInfo(llvm::Error error) {