serializedAs

struct Proxy
{
    import std.conv;
    static void serialize(ref Packer p, ref int val) { p.pack(to!string(val)); }
    static void deserialize(ref Unpacker u, ref int val) { string tmp; u.unpack(tmp); val = to!int(tmp); }
}
struct S
{
    // The Packer/Unpacker proxy handler is applied this field.
    @serializedAs!Proxy int num;
    string str;
}