[move-only] When emitting an initializer for an empty struct, store an instance to make sure it is initialized.

I ran into this while fixing the parent commit when attempting to add the
interpreter test in this commit into the aforementioned parent commit.

rdar://107974302

Author: gottesmm

lib/SILGen/SILGenConstructor.cpp

@@ -631,7 +631,22 @@ void SILGenFunction::emitValueConstructor(ConstructorDecl *ctor) {
   if (!isDelegating) {
     auto *typeDC = ctor->getDeclContext();
     auto *nominal = typeDC->getSelfNominalTypeDecl();
-    emitMemberInitializers(ctor, selfDecl, nominal);
+
+    // If we have an empty move only struct, then we will not initialize it with
+    // any member initializers, breaking SIL. So in that case, just construct a
+    // SIL struct value and initialize the memory with that.
+    //
+    // DISCUSSION: This only happens with noncopyable types since the memory
+    // lifetime checker doesn't seem to process trivial locations. But empty
+    // move only structs are non-trivial, so we need to handle this here.
+    if (isa<StructDecl>(nominal) && nominal->isMoveOnly() &&
+        nominal->getStoredProperties().empty()) {
+      auto *si = B.createStruct(ctor, lowering.getLoweredType(), {});
+      B.emitStoreValueOperation(ctor, si, selfLV.getLValueAddress(),
+                                StoreOwnershipQualifier::Init);
+    } else {
+      emitMemberInitializers(ctor, selfDecl, nominal);
+    }
   }
 
   emitProfilerIncrement(ctor->getTypecheckedBody());

test/Interpreter/moveonly.swift

@@ -89,3 +89,17 @@ Tests.test("deinit not called in init when assigned") {
   }
   expectEqual(0, FD2.count)
 }
+
+Tests.test("empty struct") {
+  @_moveOnly
+  struct EmptyStruct {
+    func doSomething() {}
+    var value: Bool { false }
+  }
+
+  let e = EmptyStruct()
+  e.doSomething()
+  if e.value {
+    let _ = consume e
+  }
+}

test/SILGen/moveonly.swift

@@ -779,3 +779,27 @@ func checkMarkMustCheckOnCaptured(x: __owned FD) {
     func clodger<T>(_: () -> T) {}
     clodger({ consumeVal(x) })
 }
+
+//////////////////
+// Empty Struct //
+//////////////////
+
+@_moveOnly
+struct EmptyStruct {
+  // Make sure we explicitly initialize empty struct as appropriate despite the
+  // fact we do not have any fields.
+  //
+  // CHECK-LABEL: sil hidden [ossa] @$s8moveonly11EmptyStructVACycfC : $@convention(method) (@thin EmptyStruct.Type) -> @owned EmptyStruct {
+  // CHECK: [[BOX:%.*]] = alloc_box ${ var EmptyStruct }, var, name "self"
+  // CHECK: [[MARKED_UNINIT:%.*]] = mark_uninitialized [rootself] [[BOX]]
+  // CHECK: [[PROJECT:%.*]] = project_box [[MARKED_UNINIT]]
+  // CHECK: [[STRUCT:%.*]] = struct $EmptyStruct ()
+  // CHECK: store [[STRUCT]] to [init] [[PROJECT]]
+  // CHECK: [[MV_CHECK:%.*]] = mark_must_check [assignable_but_not_consumable] [[PROJECT]]
+  // CHECK: [[LOADED_VALUE:%.*]] = load [copy] [[MV_CHECK]]
+  // CHECK: destroy_value [[MARKED_UNINIT]]
+  // CHECK: return [[LOADED_VALUE]]
+  // CHECK: } // end sil function '$s8moveonly11EmptyStructVACycfC'
+  init() {
+  }
+}