I think the answer is that in a case when you need that speed, you hoist the stack allocation & zeroing and unsafe that buffer in the loop if need be. Test well. I am a huge Rust fan but also it is actually possible to write correct unsafe code.

If I am interacting with from IO space, I would much rather write the interaction code myself for the machine at hand than farm it out to an array of third party crates. ::shrug::

getting the machinery to let it properly be hoisted smoothly and safely would be nice, but it isn't required.

personally I think rust macros are very painful and the "worst feature", but that's speaking as someone who did a fair bit of Common Lisp.