There is also the issue where you have a pointer in the arena pointing to something on the heap. The GC will gladly kill the object on the heap as it has no idea something is still pointing to it.
Why wouldn't this just use generics to allocate a big slice of the type so that the GC can know whether or not the arena may contain pointers to the heap?
You can do that but you end up with something other than what the author wrote. The use cases are different. Arenas, at least in principle, should generally be able to allocate anything, not just a single type. If you have a single type you would do the equivalent thing in any language, not use arenas.
Generics in go, as they're implemented today, sadly have a fair bit of performance overhead. I haven't tried but my assumption would be that go generics are not fast enough to make an effective arena allocator. I'd be thrilled if someone could prove me wrong though!
The performance overhead is when you're calling a method on the generic type--Go has to lookup the specific implementation in a dictionary. Pretty sure that doesn't apply for straight-up container use cases like this one.
