[LifetimeCompletion] Don't destroy alloc_boxes.

A destroy_value of an alloc_box unconditionally destroys the value
within the box.  On unreachable paths, such memory may not be
initialized, so it cannot be destroyed unconditionally.  Moreover,
destroying values stored in memory is outside the purview of OSSA
lifetime completion.

Author: nate-chandler

lib/SIL/Utils/OSSALifetimeCompletion.cpp

@@ -62,6 +62,9 @@ static SILInstruction *endOSSALifetime(SILValue value, SILBuilder &builder) {
   auto loc =
     RegularLocation::getAutoGeneratedLocation(builder.getInsertionPointLoc());
   if (value->getOwnershipKind() == OwnershipKind::Owned) {
+    if (value->getType().is<SILBoxType>()) {
+      return builder.createDeallocBox(loc, value);
+    }
     return builder.createDestroyValue(loc, value);
   }
   return builder.createEndBorrow(loc, value);

test/SILOptimizer/ossa_lifetime_completion.sil

@@ -445,7 +445,7 @@ indirect enum IndirectEnumNontrivialPayload {
 // CHECK:         cond_br undef, {{bb[0-9]+}}, [[BASIC_BLOCK4:bb[0-9]+]]
 // CHECK:       [[BASIC_BLOCK4]]:
 // CHECK:         end_borrow [[C]]
-// CHECK:         destroy_value [[BOX]]
+// CHECK:         dealloc_box [[BOX]]
 // CHECK:         unreachable
 // CHECK-LABEL: } // end sil function 'project_box_owned'
 sil [ossa] @project_box_owned : $@convention(thin) (@owned IndirectEnumNontrivialPayload) -> () {
@@ -498,3 +498,18 @@ bb0:
   dealloc_stack %result_addr : $*()
   return %retval : $()
 }
+
+// CHECK-LABEL: begin running test {{.*}} on alloc_box: ossa-lifetime-completion
+// CHECK-LABEL: sil [ossa] @alloc_box : {{.*}} {
+// CHECK:         [[BOX:%[^,]+]] = alloc_box
+// CHECK:         dealloc_box [[BOX]]
+// CHECK-LABEL: } // end sil function 'alloc_box'
+// CHECK-LABEL: end running test {{.*}} on alloc_box: ossa-lifetime-completion
+sil [ossa] @alloc_box : $@convention(thin) (@owned C) -> () {
+entry(%instance : @owned $C):
+  %box = alloc_box ${ var C }
+  specify_test "ossa-lifetime-completion %box"
+  %addr = project_box %box : ${ var C }, 0
+  store %instance to [init] %addr : $*C
+  unreachable
+}