Don't require UsbBus to be Sync

[dlaw]

Current behavior:

• UsbBus is Sync --> the resulting UsbDevice is Sync.
• UsbBus is not Sync --> compiler error due to unsatisfied trait bound

Proposed new behavior:

• UsbBus is Sync --> the resulting UsbDevice is Sync.
• UsbBus is not Sync --> the resulting UsbDevice is not Sync

In other words, it is safe to allow a non-Sync UsbBus because Rust will NOT derive Sync for the UsbDevice in this case.

Why is this useful?

I have a hard-real-time firmware application which uses the USB interface to provide secondary (non-real-time) diagnostics. I poll the UsbDevice in my main loop, while all my other real-time stuff is handled in various interrupts. I do not use USB interrupts and I never access the UsbDevice from an interrupt context.

Unfortunately, making the UsbBus Sync requires the use of critical sections which damage the real-time interrupt timing. Eliminating the critical sections is totally safe in this situation, and solves my timing issues, but doing so means that the UsbBus cannot safely be marked as Sync.

• For more context see https://github.com/stm32-rs/stm32-usbd/issues/32
• For an implemented example of the UsbBus changes for non-Sync usage, see https://github.com/dlaw/stm32-usbd-no-cs
• If accepted, this PR would allow me to eliminate the following highly unsound impl Sync: https://github.com/dlaw/stm32-usbd-no-cs/commit/7df1814aacca9c82c3d325276f290bdd5afa5696#diff-b7dd6c6dc06c9c5da00a0c048df395e5fed282392d70a222b1d0e34706d9baf3R26

[eldruin]

Thank you for the PR. Maybe somebody like @ryan-summers or so can comment on this?