User defined effects

[AriMKatz]

From Keno on Slack:

Yes, user defined effects would be useful, but that's a whole separate design space

Opening this as a placeholder /for future discussion based on Keno's feedback about the idea. Like #43852 but not hardcoded.

Could be useful to pass assumptions/ invariants to custom compiler plugin passes (such as for loop transformations and generally knowing which passes are valid when etc).