Firstly, an arena-based allocator is a custom allocator, which is something most programmers never need to touch and probably don't know even know about. It is

Firstly, an arena-based allocator is a custom allocator, which is something most programmers never need to touch and probably don't know even know about. It is a "low-level memory trick". Secondly, we were talking about how Zig makes allocation first-class. This is a prime use case: specifying your own custom arena-based allocator.