What you get for free with the global allocator is a guarantee that you will use the same allocator for each operation on an allocation. That is the property that is claimed to be difficult to keep if you literally do the Zig thing and have e.g. ArrayListUnmanaged which requires you to pass in the allocator again to free the backing slice.
Ok so those are 'unsafe'. Plenty of safe (when statically checked) data structures in zig, like the managed array list, where the associated allocator is attached to the object.
