SIL Deserializer: Assert that we don't deserialize during "lowered" stage.

Pre-IRGen lowering passes take SIL out of canonical form, making it potentially incompatible with the serialized canonical SIL.

Author: jckarter

lib/Serialization/DeserializeSIL.cpp

@@ -387,6 +387,19 @@ SILFunction *SILDeserializer::readSILFunction(DeclID FID,
                                               StringRef name,
                                               bool declarationOnly,
                                               bool errorIfEmptyBody) {
+  // We can't deserialize function bodies after IRGen lowering passes have
+  // happened since other definitions in the module will no longer be in
+  // canonical SIL form.
+  switch (SILMod.getStage()) {
+  case SILStage::Raw:
+  case SILStage::Canonical:
+    break;
+    
+  case SILStage::Lowered:
+    llvm_unreachable("cannot deserialize into a module that has entered "
+                     "Lowered stage");
+  }
+  
   if (FID == 0)
     return nullptr;
   assert(FID <= Funcs.size() && "invalid SILFunction ID");