error: assigning to `&T` is undefined behavior, consider using an `UnsafeCell`
   --> eco/stm32l4xx-hal/src/spi.rs:271:34
    |
271 |                           unsafe { ptr::write_volatile(&self.spi.dr as *const _ as *mut u8, byte) }
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
371 | / hal! {
372 | |     SPI2: (spi2, spi2_slave, pclk1),
373 | | }
    | |_- in this macro invocation
    |
    = note: for more information, visit <https://doc.rust-lang.org/book/ch15-05-interior-mutability.html>
    = note: even for types with interior mutability, the only legal way to obtain a mutable pointer from a shared reference is through `UnsafeCell::get`
    = note: this error originates in the macro `hal` (in Nightly builds, run with -Z macro-backtrace for more info)