Soundly alias and reference the serial for defmt

jamesmunns commented 5 months ago

The current approach is unsound for two reasons:

you hold on to the &mut T, then while it is live you dereference the aliasing pointer to perform defmt actions. This violates safety rules of exclusive references. If you would like to keep the handle around just to enforce the lifetime, you can use PhantomData<&'a mut T> instead, which will keep the lifetime check without keeping a second, aliased reference.
It is possible to safely mem::forget() the handle, ending the lifetime without running the destructor, and then allowing for aliased behavior.

Additionally, using the unsafe serial fn is UB any defmt behavior occurs while you are holding the reference.

I'm also not sure why the transmute is necessary to go from &mut dyn EraseWrite to *mut dyn EraseWrite.

gauteh commented 5 months ago

Ref, need blocking Write on rp-hal: https://github.com/rp-rs/rp-hal/issues/760 , should probably just upgrade to eh-1.

gauteh commented 4 months ago

Finally got the time to make some effort on this. Think it is getting closer now. The pi-pico example is going to fail until the blocking write trait is implemented, or we upgrade to eh-1 (#11).

gauteh / defmt-serial

Soundly alias and reference the serial for defmt #10