[feature] Support defmt without flashing via a new --promise-hardware-unchanged

[dzfranklin]

I'm debugging a finicky peripheral so I often want to ask ("If I hard power cycle everything does it start working"), and it feels wasteful to reflash each time. My understanding is probe-run requires a reflash for defmt because you might see subtly wrong logs if the elf on your harddrive didn't match the one flashed.

It's easy to change the elf on your harddrive without noticing (i.e. an editor configured to check on save), but I'm guessing most people would know if they reflashed their hardware or plugged in different hardware.

I'm proposing every time probe-run flashes it copies the elf to {target}/probe_run/last_flash/{binary_name}.

If you run a project with defmt logs without flashing and provide --promise-hardware-unchanged probe-run would use the cached elf.

Another option would be to remove the check. cargo-embed doesn't require reflashing to see defmt logs, and I heard from someone on the probe-rs matrix that in practice it's pretty obvious if you used the wrong elf.

[Urhengulas]

Hi @danielzfranklin, Thank you for opening the issue!

I see your point and I also would agree that it should be obvious if you are using the wrong elf, if you are experienced with defmt. My only concern is that people just getting started with defmt (and maybe embedded development overall) could have a hard time figuring the problem out. But then it is the question why they would decide to not re-flash the elf.

My intuition is to ease the restrictions, by allowing to use --no-flash even if using defmt, but emitting a warning that this might cause problems.

I will get a second opinion, but what do you think about it?

[dzfranklin]

That sounds great.