I pretty new to Rust and I’m wondering why global mutables are hard? At first gl...

umanwizard · 2025-12-04T23:19:48 1764890388

> I pretty new to Rust and I’m wondering why global mutables are hard?

They're not.

  fn main() {
      unsafe {
          COUNTER += 1;
          println!("COUNTER = {}", COUNTER);
      }
  
      unsafe {
          COUNTER += 10;
          println!("COUNTER = {}", COUNTER);
      }
  }

Global mutable variables are as easy in Rust as in any other language. Unlike other languages, Rust also provides better things that you can use instead.

raggi · 2025-12-05T00:49:58 1764895798

People always complain about unsafe, so I prefer to just show the safe version.

  use std::sync::Mutex;

  static LIST: Mutex<Vec<String>> = Mutex::new(Vec::new());

  fn main() -> Result<(), Box<dyn std::error::Error>> {

      LIST.lock()?.push("hello world".to_string());
      println!("{}", LIST.lock()?[0]);

      Ok(())
  }

hu3 · 2025-12-05T22:46:51 1764974811

This is completely different from the previous example.

It doesn't increment anything for starters. The example would be more convoluted if it did the same thing.

And strings in rust always delivers the WTFs I need o na Friday:

    "hello world".to_string()

raggi · 2025-12-06T02:18:17 1764987497

    use std::sync::Mutex;
    fn main() -> Result<(), Box<dyn std::error::Error>> {
        static PEDANTRY: Mutex<u64> = Mutex::new(0);
        *PEDANTRY.lock()? += 1;
        println!("{}", PEDANTRY.lock()?);
        Ok(())
    }

hu3 · 2025-12-06T21:36:29 1765056989

Still different. Yours only increments once. Doesn't pass basic QA.

And declaring a static variable inside a function, even if in main, smells.

raggi · 2025-12-06T22:23:41 1765059821

OP you keep comparing to doesn't even declare the variable. I'm done with you.

umanwizard · 2025-12-07T17:31:00 1765128660

OP here, not showing how to declare the variable was an oversight on my part.

  static mut COUNTER: u32 = 0;

(at top-level)

If on 2024 edition, you will additionally need

  #![allow(static_mut_refs)]

raggi · 2025-12-07T18:07:13 1765130833

Yup, your example was fine, as I said I just wanted to show a safe method too :-)

masklinn · 2025-12-05T05:38:51 1764913131

That is correct. Kinda. Refcell can not work because Rust considers globals to be shared by multiple threads so requires thread safety.

And that’s where a number of people blow a gasket.

masklinn · 2025-12-05T09:53:33 1764928413

A second component is that statics require const initializers, so for most of rust’s history if you wanted a non-trivial global it was either a lot of faffing about or using third party packages (lazy_static, once_cell).

Since 1.80 the vast majority of uses are a LazyLock away.

written-beyond · 2025-12-04T22:51:23 1764888683

I don't think it's specifically hard, it's more related to how it probably needed more plumbing in the language that authors thought would add to much baggage and let the community solve it. Like the whole async runtime debates