[MoveOnlyAddressChecker] Fix empty struct repr.

An empty struct without a deinit gets a single bit which is used to
track the struct's liveness.  Previously, an empty struct with a deinit
also only got a single bit.  Consequently, when discarding the struct
(i.e. dropping the deinit), there was no bit left to represent the
struct.  This resulted in a failure to track liveness for the value.

rdar://126863003

Author: nate-chandler

lib/SIL/Utils/FieldSensitivePrunedLiveness.cpp

@@ -71,14 +71,14 @@ TypeSubElementCount::TypeSubElementCount(SILType type, SILModule &mod,
           type.getFieldType(fieldDecl, mod, context), mod, context);
     number = numElements;
 
+    // If we do not have any elements, just set our size to 1.
+    if (number == 0)
+      number = 1;
     if (type.isValueTypeWithDeinit()) {
       // 'self' has its own liveness represented as an additional field at the
       // end of the structure.
       ++number;
     }
-    // If we do not have any elements, just set our size to 1.
-    if (number == 0)
-      number = 1;
 
     return;
   }
@@ -450,6 +450,9 @@ void TypeTreeLeafTypeRange::constructFilteredProjections(
       callback(newValue, TypeTreeLeafTypeRange(start, next), NeedsDestroy);
       start = next;
     }
+    if (start == 0) {
+      ++start;
+    }
     if (type.isValueTypeWithDeinit()) {
       // 'self' has its own liveness
       ++start;

test/SILOptimizer/moveonly_addresschecker_unmaximized.sil

@@ -203,3 +203,36 @@ bb0(%s_addr : $*S):
   %retval = tuple ()
   return %retval : $()
 }
+
+sil @getter : $@convention(thin) (@guaranteed M) -> @owned String
+sil @die : $@convention(thin) (@guaranteed @noescape @callee_guaranteed () -> @owned String) -> Never
+
+// CHECK-LABEL: sil [ossa] @partial_apply_of_borrow_of_deinitless_empty_struct : {{.*}} {
+// CHECK:       bb0([[M_IN:%[^,]+]] :
+// CHECK:         [[STACK:%[^,]+]] = alloc_stack $M
+// CHECK:         store [[M_IN]] to [init] [[STACK]]
+// CHECK:         [[ADDR:%[^,]+]] = drop_deinit [[STACK]]
+// CHECK:         [[MB:%[^,]+]] = load_borrow [[ADDR]]
+// CHECK:         [[GETTER:%[^,]+]] = function_ref @getter
+// CHECK:         [[PA:%[^,]+]] = partial_apply [callee_guaranteed] [on_stack] [[GETTER]]([[MB]])
+// CHECK:         [[DIE:%[^,]+]] = function_ref @die
+// CHECK:         apply [[DIE]]([[PA]])
+// CHECK:         destroy_value [[PA]]
+// CHECK:         end_borrow [[MB]]
+// CHECK:         unreachable
+// CHECK-LABEL: } // end sil function 'partial_apply_of_borrow_of_deinitless_empty_struct'
+sil [ossa] @partial_apply_of_borrow_of_deinitless_empty_struct : $@convention(method) (@owned M) -> () {
+bb0(%m_in : @owned $M):
+  %stack = alloc_stack $M
+  %addr1 = mark_unresolved_non_copyable_value [consumable_and_assignable] %stack : $*M
+  store %m_in to [init] %addr1 : $*M
+  %nodeinit = drop_deinit %addr1 : $*M
+  %addr = mark_unresolved_non_copyable_value [no_consume_or_assign] %nodeinit : $*M
+  %m = load [copy] %addr : $*M
+  %mb = begin_borrow %m : $M
+  %getter = function_ref @getter : $@convention(thin) (@guaranteed M) -> @owned String
+  %pa = partial_apply [callee_guaranteed] [on_stack] %getter(%mb) : $@convention(thin) (@guaranteed M) -> @owned String
+  %die = function_ref @die : $@convention(thin) (@guaranteed @noescape @callee_guaranteed () -> @owned String) -> Never
+  apply %die(%pa) : $@convention(thin) (@guaranteed @noescape @callee_guaranteed () -> @owned String) -> Never
+  unreachable
+}

validation-test/SILOptimizer/gh68328.swift

@@ -7,7 +7,7 @@
 // REQUIRES: executable_test
 
 struct Example: ~Copyable {
-    private var failureString: String = "Goodbye."
+    private var failureString: String { "Goodbye." }
     deinit { fatalError("FATAL ERROR: \(failureString)") }
 }