[DI] Simplify immutability checks on _storage base

Author: xedin

lib/SILOptimizer/Mandatory/DIMemoryUseCollector.cpp

@@ -468,25 +468,30 @@ DIMemoryObjectInfo::getPathStringToElement(unsigned Element,
 
 /// If the specified value is a 'let' property in an initializer, return true.
 bool DIMemoryObjectInfo::isElementLetProperty(unsigned Element) const {
-  NullablePtr<NominalTypeDecl> NTD;
-
   // If this is an element of a `_storage` tuple, we need to
   // check the `$Storage` to determine whether underlying storage
   // backing element is immutable.
   if (auto *storageVar = getAsTypeWrapperLocalStorageVar()) {
     auto *wrappedType = cast<NominalTypeDecl>(
         storageVar->getDeclContext()->getInnermostTypeContext());
     assert(wrappedType && "_storage reference without type wrapper");
-    NTD = wrappedType->getTypeWrapperStorageDecl();
-  } else {
-    // If we aren't representing 'self' in a non-delegating initializer, then we
-    // can't have 'let' properties.
-    if (!isNonDelegatingInit())
-      return IsLet;
-
-    NTD = MemorySILType.getNominalOrBoundGenericNominal();
+
+    auto storageVarType = storageVar->getInterfaceType()->getAs<TupleType>();
+    assert(Element < storageVarType->getNumElements());
+    auto propertyName = storageVarType->getElement(Element).getName();
+
+    auto *storageDecl = wrappedType->getTypeWrapperStorageDecl();
+    auto *property = storageDecl->lookupDirect(propertyName).front();
+    return cast<VarDecl>(property)->isLet();
   }
 
+  // If we aren't representing 'self' in a non-delegating initializer, then we
+  // can't have 'let' properties.
+  if (!isNonDelegatingInit())
+    return IsLet;
+
+  auto NTD = MemorySILType.getNominalOrBoundGenericNominal();
+
   if (!NTD) {
     // Otherwise, we miscounted elements?
     assert(Element == 0 && "Element count problem");
@@ -496,7 +501,7 @@ bool DIMemoryObjectInfo::isElementLetProperty(unsigned Element) const {
   auto &Module = MemoryInst->getModule();
 
   auto expansionContext = TypeExpansionContext(*MemoryInst->getFunction());
-  for (auto *VD : NTD.get()->getStoredProperties()) {
+  for (auto *VD : NTD->getStoredProperties()) {
     auto FieldType = MemorySILType.getFieldType(VD, Module, expansionContext);
     unsigned NumFieldElements =
         getElementCountRec(expansionContext, Module, FieldType, false);