stm32: no way to tell whether operations on a UART in Blocking mode would block

[scgtrp]

Given a Uart(Rx)<'_, Blocking>, a reasonable thing one might want to do would be to poll it for data if there is any to read, but not block if there isn't.

There seems to be no way to do this? There is a nb_read that does that exact thing, but it's behind pub(crate), as is any clear way to access RXNE directly to check such a thing oneself.

Can we have e.g. impl ReadReady for non-async UARTs?