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.

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

> 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);
      }
  }