This is silly.

If you make code that requires allocation and does not have an allocator passed into it, there is literally nothing stopping you from allocating anyway. Zig does not stop you from, for example, heap allocating without the usage of an allocator (Zig does not know what heap allocation even is). You can directly use the std page_allocator (and others) wherever you want, too.

There are very few good reasons to do this (e.g. spawning a thread requires particular allocation; using an allocator might not be sound depending on the OS).

All you have to do is document that it allocates memory and that's fine; there is no split.