I'm a bit confused:

The mutex crate itself contains the following explanation of the relationship between mutex and mutex-traits:

The mutex-traits crate should be used by library crates that want to be generic over different ways of exclusive access.

The mutex crate should be used by applications that need to select which implementation is appropriate for their use case.

As embedded-hal-bus is a library, shouldn't it depend on mutex-traits?

But then, I don't get how code like MutexTraitsDevice implementations should be written without using the mutex crate. So this is more likely a documentation issue in mutex and mutex-traits (or just me being particularly slow today) than an actual issue with this pull request.

The mutex-traits is used in the library that wants to provide a mutex implementation. For example a RTOS library. embedded-hal-bus should use mutex crate.