defmt: implements Format for cells

[DBLouis]

This patch implement the Format trait for Cell<T> and RefCell<T>

[Urhengulas]

Hi @DBLouis, Thank you for your PR! Could you please move your code to defmt/src/impls/core_/cell.rs and add that as a new mod?

[Urhengulas]

Thank you!

Secondly, I'd propose to stay close to the representations in std.

This code:

use std::cell::{Cell, RefCell};

fn main() {
    let a = Cell::new(5);
    println!("{:?}", a);

    let b = RefCell::new(5);
    println!("{:?}", &b);

    let _c = b.try_borrow_mut().unwrap();
    println!("{:?}", &b);

    let d = b.try_borrow_mut().unwrap_err();
    println!("{:?}", d);
}

Produces that output:

Cell { value: 5 }
RefCell { value: 5 }
RefCell { value: <borrowed> }
BorrowMutError

(I couldn't figure out how to trigger a BorrowError, but it should be just "BorrowError")

Do you want to quickly change that? Else I can also do it.

[DBLouis]

You can trigger a BorrowError by borrowing mutably first then attempting to borrow immutably Playground

[Urhengulas]

Thank you!

bors r+

[bors]

Build succeeded:

• ci