[ownership] When lowering store_borrow, RAUW its result with its input dest.

A store_borrow is a manner to temporarily "borrow" a guaranteed value into
memory for purposes like reabstraction. To make this safer in OSSA, we treat a
store_borrow's result as an interior pointer into the stored guaranteed value,
causing all uses of that result to be validated as being within the lifetime of
the guaranteed value.

NOTE: This is not the complete store_borrow verification story. We also will
verify that the memory that is being store_borrowed into is not written to while
the store_borrow's result is live.

Author: gottesmm

Diff for: lib/SILOptimizer/Mandatory/OwnershipModelEliminator.cpp

@@ -255,7 +255,10 @@ bool OwnershipModelEliminatorVisitor::visitStoreBorrowInst(
                               StoreOwnershipQualifier::Unqualified);
   });
 
-  // Then remove the qualified store.
+  // Then remove the qualified store after RAUWing si with its dest. This
+  // ensures that any uses of the interior pointer result of the store_borrow
+  // are rewritten to be on the dest point.
+  si->replaceAllUsesWith(si->getDest());
   eraseInstruction(si);
   return true;
 }

Diff for: test/SILOptimizer/ownership_model_eliminator.sil

@@ -5,6 +5,7 @@ sil_stage raw
 import Builtin
 
 sil @use_native_object : $@convention(thin) (@owned Builtin.NativeObject) -> ()
+sil @use_native_object_inguaranteed : $@convention(thin) (@in_guaranteed Builtin.NativeObject) -> ()
 sil @use_int32 : $@convention(thin) (Builtin.Int32) -> ()
 
 enum Either<T, R> {
@@ -358,3 +359,23 @@ bb0(%0a : $PairOfInt):
   %0b = unchecked_value_cast %0a : $PairOfInt to $Builtin.Int64
   return %0b : $Builtin.Int64
 }
+
+// Make sure we RAUW the store_borrow's result with its dest while lowering.
+//
+// CHECK-LABEL: sil @lower_store_borrow_result_correctly : $@convention(thin) (@guaranteed Builtin.NativeObject) -> () {
+// CHECK: bb0([[ARG:%.*]] :
+// CHECK:   [[STACK:%.*]] = alloc_stack
+// CHECK:   store [[ARG]] to [[STACK]]
+// CHECK:   apply {{%.*}}([[STACK]])
+// CHECK: } // end sil function 'lower_store_borrow_result_correctly'
+sil [ossa] @lower_store_borrow_result_correctly : $@convention(thin) (@guaranteed Builtin.NativeObject) -> () {
+bb0(%0 : @guaranteed $Builtin.NativeObject):
+  // This is materializing %0 into memory to be passed as an in_guaranteed arg.
+  %1 = alloc_stack $Builtin.NativeObject
+  %result = store_borrow %0 to %1 : $*Builtin.NativeObject
+  %f = function_ref @use_native_object_inguaranteed : $@convention(thin) (@in_guaranteed Builtin.NativeObject) -> ()
+  apply %f(%result) : $@convention(thin) (@in_guaranteed Builtin.NativeObject) -> ()
+  dealloc_stack %1 : $*Builtin.NativeObject
+  %9999 = tuple()
+  return %9999 : $()
+}