Except the intermediate values are stack-allocated in Rust and C as well. This article claims that stack-allocated data is faster than heap-allocated as well, which is dubious without further clarification. malloc is indeed slower than pushing to the stack, and the stack will almost certainly be in cache, but this program is small enough that its working set will always be in cache so there should be minimal difference.
I’m curious what the modular function is doing in Rust - the C implementation just uses a simple (n % 2) == 0 while the Rust code does ((n % 2) + 2) % 2 and it’s not clear why.
(n % 2) == 0
((n % 2) + 2) % 2
((n % 2) + 2) % 2 properly handles negatives, though it shouldn’t matter here.