a better solution would be to add a method called something like ulock that does a combined lock and unwrap.

That's exactly what's done above using an extension trait! You can mutex_val.ulock() with it!

Now that I think about it, I don’t like how unwrap can signal either “I know this can’t fail”, “the possible error states are too rare to care about” or “I can’t be bothered with real error handing right now”.

That's why you're told (clippy does that i think) to use expect instead, so you can signal "whatever string" you want to signal precisely.