Outperforming Rust with Functional Programming

> Here, we do something that is not possible to do in Rust or C - we safely stack-allocate the intermediate value,

There's no heap allocation in the Rust nor C code.

I haven't looked into the codegen to see what's different here.

tl;dr: the C and Rust versions had signed integers rather than unsigned like the ATS version, and if you make that change they end up more similar.

probably going to sound stupid but what part of this was functional?

I'm a little lost as well. I only see recursion and while loops.