Description
Right now ui tests only target the output of the currently built rustc. As far as I can tell, there's no -Z flag to tell rustc "act as if you were a stable release". This means that when we customize the diagnostic output based on release channel, we cannot test what stable users will actually see. An example of this is this (as of now unmerged) change that on stable refers to unsized locals and unsized fn arguments with the same note telling the user that's not allowed, while on nightly it tells the user which feature flag is gating that being allowed, without the note stating it's not allowed. That kind of behavior is replicated in multiple diagnostics involving nightly features already. It's the same "problem" we'd have with editions if we didn't already have a stable flag to specify them.