Why is immutability enforced in Rust unless otherwise specified with `mut`?

Question

Why is immutability forced in Rust, unless you specify mut? Is this a design choice for safety, do you consider this how it should be naturally in other languages?

I should probably clarify, I'm still a newbie at Rust. So is this a design choice related to another feature in the language?

Answer 1

The Rust-Book actually addresses this topic.

There is no single reason that bindings are immutable by default, but we can think about it through one of Rust’s primary focuses: safety. If you forget to say mut, the compiler will catch it, and let you know that you have mutated something you may not have intended to mutate. If bindings were mutable by default, the compiler would not be able to tell you this. If you did intend mutation, then the solution is quite easy: add mut.

There are other good reasons to avoid mutable state when possible, but they’re out of the scope of this guide. In general, you can often avoid explicit mutation, and so it is preferable in Rust. That said, sometimes, mutation is what you need, so it’s not verboten.

Basically it is the C++-Mantra that everything that you don't want to modify should be const, just properly done by reversing the rules. Also see this Stackoverflow article about C++.