[samatman]

It scales to complex examples as well. Retained memory would be handled with its own allocator: for a large data structure like an LRU cache, one would initialize it with a pointer to the allocator, and use that internally to manage the memory.

Blocking (or rather, non-blocking, which is clearly what you meant) IO is a different story. Zig had an async system, but it had problems and got removed a couple point releases ago. There's libxev[0] for evented programs, from Mitchell Hashimoto. It's not mature yet but it offers a good solution to single-threaded concurrency and non-blocking IO.

I don't think Zig is the best choice for multithreaded programs, however, unless they're carefully engineered to share little to no memory (using message passing, for instance). You'd have to take care of locking and atomic ops manually, and unlike memory bugs, Zig doesn't have a lot of built-in support for catching problems with that.

A language with manual memory allocation isn't going to be the language of choice for writing web servers, for pretty obvious reasons. But for an application like squeezing the best performance out of a resource-constrained environment, the tradeoffs start to make sense.

[0]: https://github.com/mitchellh/libxev