You're still forcing a mindset onto the language that isn't present. It's not 'a split stack' or any other particular solution. You pass in an allocator and it

You're still forcing a mindset onto the language that isn't present. It's not 'a split stack' or any other particular solution. You pass in an allocator and it does it whichever way your chose allocator picks. The goal is to explore sections of design space you're completely ruling out from before you even start.