@mcc It looks like you're passing a reference to an array (`&[u8; N]`) where the signature expects a reference to a slice (`&[u8]`). Try adding an explicit `.as_slice()` to the array expression.
In general this shouldn't be necessary because `[u8; N]` implements `AsRef<[u8]>`. This is what `ByteStream`'s docs describe as the requirement for the type inside the stream, so… ¯\_(ツ)_/¯
(Caveats: I don't use Rocket, I have no idea what's happening inside that macro, and I can't see the call site.)