Use UnsafeCell instead of static-mut

[jonathanpallant]

The code at

defmt/firmware/defmt-rtt/src/lib.rs

Line 140 in 4db33ca

static mut BUFFER: [u8; BUF_SIZE] = [0; BUF_SIZE];

uses a static mut. It goes to some trouble to ensure that the handle() function cannot be called twice, but perhaps we can use the type system to guarantee that is the case. Or if not, maybe we should use an UnsafeCell.

Also, the buffer is placed in an uninit section but we don't use the MaybeUninit type for the buffer. If there's a reason, we should perhaps explain why.

[Urhengulas]

@japaric Can you shed some light on if there is a reason it should be implemented like it is right now?

[evading]

Also, the buffer is placed in an uninit section but we don't use the MaybeUninit type for the buffer. If there's a reason, we should perhaps explain why.

I was just about to file an issue about that. In the commit that introduced the uninit section in cortex-m-rt it says that MaybeUninit is the only correct way to use that section 1.