> If I write a Rust program which relies on this for a constant, it works fine, because it is constant, but if I try to compile it with Rust 1.68 (from earlier this year) it won't work.
How would it not work? If your program compiles and runs with 1.69, then surely that's because it never tries to modify the value. But if your program never tries to modify the value, how would it fail with 1.68?
In this case, the fn `std::net::SocketAddr::new` was made a const fn recently--but was a regular fn before.
>that's because it never tries to modify the value
That's something different. If you want variables that are read-only, you don't need any extra keyword. It's the normal case:
let x = 5;
If you want a mutable variable, you add a `mut` keyword.
let mut x = 5;
But if you want a compile-time constant, you use `const` instead.
const x: u32 = 5;
The latter will be evaluated by the compiler at compile time (so interpreted) and substituted BEFORE the user runs the program. This is new-ish stuff and is slowly worming its way into the standard library.
* Modulo soundness bugs being fixed. I've personally encountered 2 instances of that since 1.0, and they all occurred before 2017.
That's a bold claim for a tool that doesn't even have a Wikipedia entry.
Edit: or a subreddit.
I would implore you to open your mind up to resources that aren't popular social media because that's where the real gold/value is in development/engineering.
I'm not claiming to be "one of the absolutely most valuable and greatest tools available" though. (Well, maybe the "greatest tool" part ;-) And if someone else claimed that about me, I'd think that either there was something very wrong with them, or that one of my parents had started using alt accounts I didn't know about.
Considering some of the software tools that do have dedicated wikipedia pages and subreddits, and that I'd never heard of godbolt before, I don't think it's out of line to be skeptical of such a hyperbolic claim.