Don't require UsbBus to be Sync.

If the UsbBus is not Sync, it can still be used to make a UsbDevice,
but that UsbDevice will not be Sync.  This is quite limiting (as it
mostly means that the USB device can't be accessed from interrupt
contexts), but it is entirely safe from the Rust safety perspective.

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 * [breaking] The control pipe is now provided in the `UsbDeviceBuilder` API to allow for user-provided control
 pipes. This makes it so that control pipes have configurable sizing.
+* Don't require UsbBus to be Sync. If a UsbBus is not Sync, it can still be used to make a UsbDevice, but that UsbDevice will not be Sync (ensuring soundness).
 
 ## [0.3.2] - 2024-03-06
 

src/bus.rs

@@ -17,7 +17,7 @@ use portable_atomic::{AtomicPtr, Ordering};
 /// take place before [`enable`](UsbBus::enable) is called. After the bus is enabled, in practice
 /// most access won't mutate the object itself but only endpoint-specific registers and buffers, the
 /// access to which is mostly arbitrated by endpoint handles.
-pub trait UsbBus: Sync + Sized {
+pub trait UsbBus: Sized {
     /// Allocates an endpoint and specified endpoint parameters. This method is called by the device
     /// and class implementations to allocate endpoints, and can only be called before
     /// [`enable`](UsbBus::enable) is called.