This creates the drill-down issue we see with React props where we have to pass objects around in the call chain just so that somewhere down the line we can use it.

React gets around this with the context hook and which you can access implicitly if it has been injected at a higher level.

Do you know if Zig supports something of the sort?

It doesn't and likely never will.

This has been a non-issue for years with Allocator. I fail to see why it will be a problem with IO.