Discourage use of static mut in singleton

[EvansJahja]

The code in this part about Singleton no longer works as of Rust 2024

https://docs.rust-embedded.org/book/peripherals/singletons.html

static mut PERIPHERALS: Peripherals = Peripherals {
    serial: Some(SerialPort),
};

giving error instead of warning "mutable reference to mutable static"

[BartMassey]

"error: creating a mutable reference to mutable static is discouraged"? Yeah, we should stick #allow(static_mut_refs)] somewhere. A pull request would be welcome. Thanks much for reporting this.

[EvansJahja]

I'm a beginner myself and not sure about the best practice.

Would using static (without mut) followed by &raw mut better? I think this is the preferrable syntax and also gives opportunity to explain.

Other would be UnsafeCell or UnsafeSyncCell.

I could try opening a PR and maybe others can point out if my understanding is sound or not

[BartMassey]

Honestly, there's nothing wrong with the code as it stands as far as I know. That said, those other possibilities are there too.

Feel free to post some code either in a PR or just attach it here and we'll be happy to look at it.

[EvansJahja]

Pushed #390 just now. Any feedback would be appreciated.