Vk: add a helper for connecting just one structure.

Without the extra overhead of Reference.

src/Magnum/Vk/Implementation/structureHelpers.h

@@ -39,24 +39,36 @@ namespace Magnum { namespace Vk { namespace Implementation {
    accident. Thus no "give me the first structure of this type" or "remove any
    structure of this type from the chain". */
 
-/* Meant to be used for connecting a longer chain of structures. Anything
-   that was connected to the `next` pointer before is reconnected to
-   `structure.pNext`; the `next` reference is rebound to the `structure.pNext`
-   field connected so it can be passed to another structureConnect() again. */
-template<class T> inline void structureConnect(Containers::Reference<void*>& next, T& structure, VkStructureType type) {
+/* Connecting one structure to a pNext chain. Anything that was connected to
+   the `next` pointer before is reconnected to `structure.pNext` */
+template<class T> inline void structureConnectOne(void*& next, T& structure, VkStructureType type) {
     void* const previousNext = next;
-    *next = &structure;
+    next = &structure;
     structure.sType = type;
     structure.pNext = previousNext;
-    next = structure.pNext;
 }
 
-template<class T> inline void structureConnect(Containers::Reference<const void*>& next, T& structure, VkStructureType type) {
+template<class T> inline void structureConnectOne(const void*& next, T& structure, VkStructureType type) {
     /* There's no better way as the pNext are either const void* or void*
        and it's a mess. For example VkDeviceCreateInfo has const void* but it
        can point to VkPhysicalDeviceFeatures2 which then has void* as it's
        primarily an output structure. So we'll just drop all const-correctness
        and operate on void*. */
+    structureConnectOne(const_cast<void*&>(next), structure, type);
+}
+
+/* Meant to be used for connecting a longer chain of structures. Anything
+   that was connected to the `next` pointer before is reconnected to
+   `structure.pNext`; the `next` reference is rebound to the `structure.pNext`
+   field connected so it can be passed to another structureConnect() again. */
+template<class T> inline void structureConnect(Containers::Reference<void*>& next, T& structure, VkStructureType type) {
+    structureConnectOne(*next, structure, type);
+    next = structure.pNext;
+}
+
+template<class T> inline void structureConnect(Containers::Reference<const void*>& next, T& structure, VkStructureType type) {
+    /* Same reasoning as in structureConnectOne(), we just drop all
+       const-correctness and operate on void*. */
     structureConnect(reinterpret_cast<Containers::Reference<void*>&>(next), structure, type);
 }
 

src/Magnum/Vk/Test/StructureHelpersTest.cpp

@@ -32,6 +32,7 @@ namespace Magnum { namespace Vk { namespace Test { namespace {
 struct StructureHelpersTest: TestSuite::Tester {
     explicit StructureHelpersTest();
 
+    template<class T> void connectOne();
     template<class T> void connect();
     template<class T> void find();
     template<class T> void disconnectChain();
@@ -39,6 +40,8 @@ struct StructureHelpersTest: TestSuite::Tester {
 
 StructureHelpersTest::StructureHelpersTest() {
     addTests<StructureHelpersTest>({
+        &StructureHelpersTest::connectOne<VkDeviceCreateInfo>,
+        &StructureHelpersTest::connectOne<VkPhysicalDeviceFeatures2>,
         &StructureHelpersTest::connect<VkDeviceCreateInfo>,
         &StructureHelpersTest::connect<VkPhysicalDeviceFeatures2>,
         &StructureHelpersTest::find<VkDeviceCreateInfo>,
@@ -55,6 +58,23 @@ template<> struct Type<void*> {
     static const char* name() { return "void*"; }
 };
 
+template<class T> void StructureHelpersTest::connectOne() {
+    typedef typename std::remove_reference<decltype(T{}.pNext)>::type NextType;
+    setTestCaseTemplateName(Type<NextType>::name());
+
+    VkPhysicalDeviceVariablePointersFeatures variableFeatures{};
+
+    T info{};
+    info.pNext = &variableFeatures;
+
+    VkPhysicalDeviceMultiviewFeatures multiviewFeatures{};
+    Implementation::structureConnectOne(info.pNext, multiviewFeatures, VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_MULTIVIEW_FEATURES);
+    CORRADE_COMPARE(info.pNext, &multiviewFeatures);
+    CORRADE_COMPARE(multiviewFeatures.sType, VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_MULTIVIEW_FEATURES);
+    /* The pre-existing next pointer should be preserved */
+    CORRADE_COMPARE(multiviewFeatures.pNext, &variableFeatures);
+}
+
 template<class T> void StructureHelpersTest::connect() {
     typedef typename std::remove_reference<decltype(T{}.pNext)>::type NextType;
     setTestCaseTemplateName(Type<NextType>::name());