Why does it arbitrarily choose length 5?
It's a dream once you understand it. You don't need to understand the math, just the general concept of type unification, and that type inference can flow in "both directions", as opposed to in e.g. C++ where `auto` inference only works in one direction.
Rust is not exactly HM type inference, but it's something close to it. http://smallcultfollowing.com/babysteps/blog/2014/07/09/an-e...
If you tried comparing it to something that wasn’t an array, or if you tried comparing it to multiple different-sized arrays, it would complain.
let array = core::array::from_fn(|i| i);
assert_eq!(array, [0, 1, 2, 3, 4, 5]);