Add an in place std::optional bitsery extension

To avoid unnecessarily recreating the object during deserialization.
This has also been upstreamed to
https://github.com/fraillt/bitsery/pull/78.

src/common/bitsery/ext/in-place-optional.h

@@ -0,0 +1,91 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020-2021 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#pragma once
+
+#include <bitsery/ext/std_optional.h>
+
+namespace bitsery {
+namespace ext {
+
+/**
+ * A temporary replacement for `bitsery::ext::InPlaceOptional` to avoid
+ * reinitializing the object we're deserializing into if it already holds a
+ * value. This follows the same idea as our `InPLaceVariant`.
+ *
+ * This is copied almost verbatim from `bitsery::ext::InPlaceOptional` (we can't
+ * access the private member, so we couldn't override just the deserialization
+ * method).
+ */
+class InPlaceOptional {
+   public:
+    /**
+     * Works with std::optional types
+     * @param alignBeforeData only makes sense when bit-packing enabled, by
+     * default aligns after writing/reading bool state of optional
+     */
+    explicit InPlaceOptional(bool alignBeforeData = true)
+        : _alignBeforeData{alignBeforeData} {}
+
+    template <typename Ser, typename T, typename Fnc>
+    void serialize(Ser& ser, const std::optional<T>& obj, Fnc&& fnc) const {
+        ser.boolValue(static_cast<bool>(obj));
+        if (_alignBeforeData)
+            ser.adapter().align();
+        if (obj)
+            fnc(ser, const_cast<T&>(*obj));
+    }
+
+    template <typename Des, typename T, typename Fnc>
+    void deserialize(Des& des, std::optional<T>& obj, Fnc&& fnc) const {
+        bool exists{};
+        des.boolValue(exists);
+        if (_alignBeforeData)
+            des.adapter().align();
+        if (exists) {
+            // Reinitializing nontrivial types may be expensive
+            // especially when they reference heap data, so if `obj`
+            // already holds a value then we'll deserialize into the
+            // existing object
+            if constexpr (!std::is_trivial_v<T>) {
+                if (obj) {
+                    fnc(des, *obj);
+                    return;
+                }
+            }
+
+            obj = ::bitsery::Access::create<T>();
+            fnc(des, *obj);
+        } else {
+            obj = std::nullopt;
+        }
+    }
+
+   private:
+    bool _alignBeforeData;
+};
+}  // namespace ext
+
+namespace traits {
+template <typename T>
+struct ExtensionTraits<ext::InPlaceOptional, std::optional<T>> {
+    using TValue = T;
+    static constexpr bool SupportValueOverload = true;
+    static constexpr bool SupportObjectOverload = true;
+    static constexpr bool SupportLambdaOverload = true;
+};
+}  // namespace traits
+}  // namespace bitsery