This came up on IRC a while back, and I thought I'd write it down for the sake of posterity.

Signals should have an optional must_read and must_write parameter for linting purposes. This would be used to mark Signals intended for external use by other Elaboratables, and warn (error?) when they are not used correctly; for example, a FIFO's input would be marked must_write and its output would be marked must_read, because if the input was not written to, or the output not read from, the FIFO would be useless.

I would suggest semantics for these as must_read and must_write producing a warning if they are never used as the source, or destination of a connection between Signals (I don't know the nMigen term for this).

Perhaps one could go further and warn if an Elaboratable does not declare any of its Signals must_read or must_write, as their linting purpose is unhelpful if all signals are optional. This would likely be quite noisy on an established nMigen codebase however.