Bug 1965599 - Allow serialization of Variant with non default constructible types. r=ipc-reviewers,nika

This will be generally useful for IPC serialization, and specifically
for MediaDataEncoder remoting.

Differential Revision: https://phabricator.services.mozilla.com/D248676

ipc/glue/IPCMessageUtilsSpecializations.h

@@ -536,20 +536,23 @@ struct ParamTraits<mozilla::Variant<Ts...>> {
   struct VariantReader {
     using Next = VariantReader<N - 1>;
 
-    static bool Read(MessageReader* reader, Tag tag, paramType* result) {
-      // Since the VariantReader specializations start at N , we need to
-      // subtract one to look at N - 1, the first valid tag.  This means our
-      // comparisons are off by 1.  If we get to N = 0 then we have failed to
-      // find a match to the tag.
-      if (tag == N - 1) {
-        // Recall, even though the template parameter is N, we are
-        // actually interested in the N - 1 tag.
-        // Default construct our field within the result outparameter and
-        // directly deserialize into the variant. Note that this means that
-        // every type in Ts needs to be default constructible
-        return ReadParam(reader, &result->template emplace<N - 1>());
+    // Since the VariantReader specializations start at N , we need to
+    // subtract one to look at N - 1, the first valid tag.  This means our
+    // comparisons are off by 1.  If we get to N = 0 then we have failed to
+    // find a match to the tag.
+    static constexpr size_t Idx = N - 1;
+    using T = typename mozilla::detail::Nth<Idx, Ts...>::Type;
+
+    static ReadResult<paramType> Read(MessageReader* reader, Tag tag) {
+      if (tag == Idx) {
+        auto p = ReadParam<T>(reader);
+        if (p) {
+          return ReadResult<paramType>(
+              std::in_place, mozilla::VariantIndex<Idx>{}, std::move(*p));
+        }
+        return {};
       } else {
-        return Next::Read(reader, tag, result);
+        return Next::Read(reader, tag);
       }
     }
 
@@ -560,17 +563,17 @@ struct ParamTraits<mozilla::Variant<Ts...>> {
   // a matching tag.
   template <typename dummy>
   struct VariantReader<0, dummy> {
-    static bool Read(MessageReader* reader, Tag tag, paramType* result) {
-      return false;
+    static ReadResult<paramType> Read(MessageReader* reader, Tag tag) {
+      return {};
     }
   };
 
-  static bool Read(MessageReader* reader, paramType* result) {
+  static ReadResult<paramType> Read(MessageReader* reader) {
     Tag tag;
     if (ReadParam(reader, &tag)) {
-      return VariantReader<sizeof...(Ts)>::Read(reader, tag, result);
+      return VariantReader<sizeof...(Ts)>::Read(reader, tag);
     }
-    return false;
+    return {};
   }
 };