> The real reason was the “Infectious API” problem. To get performance benefits, you can’t just create an arena locally; you have to pass it down the call stack so functions can allocate inside it. This forces a rewrite of function signatures.

Sorry, but it doesn't seem that difficult (famous last words). Add a new implicit parameter to all objects just like "this" called "thisArena". When a call to any allocation is made, pass "thisArena" implicitly, unless something else passed explicitly.

That way the arena is viral all the way down and you can create sub-arenas. It also does not require actually passing the arena as parameter.

You don't even need to rewrite any new code, just recompile it.