[eru]

Yes, missing immutability is a big part of the problem in Go.

Given that Go eschewed generics for the longest time, I can sort of see why they left out immutability markers:

To keep your sanity, you'd want some functions to take (and return!) both mutable and immutable data, as the situation requires. But some other functions should only take mutable data or only immutable data.

Thus ideally you'd need some kind of 'generic' (im-)mutability handling in your type system.

(Rust's borrow checker is basically one way to really deal with this (im-)mutability genericity.

For example, a function to do binary search on a sorted array doesn't change the array; thus it could take either a mutable or immutable version. But if the array changes (via another thread) while the function is running, then you might get into trouble.)