`static mut` transform allows aliasing `&mut` references

[jonas-schievink]

PoC:

#![feature(asm)]
#![no_std]
#![no_main]

extern crate cortex_m_rt as rt;
extern crate panic_halt;
extern crate cortex_m;
extern crate cortex_m_semihosting as semi;

extern crate stm32l0xx_hal;

use rt::{entry, exception};
use cortex_m::interrupt::{self, Mutex};
use core::cell::RefCell;
use semi::hprintln;

static STORAGE: Mutex<RefCell<Option<&'static mut u8>>> = Mutex::new(RefCell::new(None));

#[exception]
fn SVCall() {
    static mut STORED: bool = false;
    static mut STAT: u8 = 0;

    let stat: &'static mut u8 = STAT;

    if *STORED {
        let alias = interrupt::free(move |cs| {
            STORAGE.borrow(cs).borrow_mut().take().unwrap()
        });

        hprintln!("stat ={:p}", stat).unwrap();
        hprintln!("alias={:p}", alias).unwrap();
        loop {
            unsafe { asm!(""); }
        }
    } else {
        interrupt::free(move |cs| {
            *STORAGE.borrow(cs).borrow_mut() = Some(stat);
        });
        *STORED = true;
    }
}

#[entry]
fn you_died_of_dis_entry() -> ! {
    hprintln!("hello").unwrap();
    loop {
        unsafe { asm!("svc 0"); }
    }
}

Output:

hello
stat =0x20000009
alias=0x20000009

[jonas-schievink]

If I'm not mistaken this would be fixed by the solution proposed in #205 (comment)