Create a migration scalafix

[djspiewak]

I have no idea how scalafix stuff works, but given it's limitations, I do suspect it can handle most of the 2 -> 3 migration in many cases. At least it's worth a shot!

[fthomas]

I've a basic skeleton for the migration scalafix ready at series/3.x...fthomas:topic/scalafix which now needs to be filled with patches.

I'm aware of the following changes, that could be added to the scalafix:

• Bracket -> MonadCancel
• Bracket#guarantee(a)(b) -> MonadCancel#guarantee(a, b)

Which other changes should it rewrite?

[djspiewak]

Ooooooh huzzah!

Other random changes:

• async -> async_ in many cases (and cancelableF -> async, as well as the other variations of it)
• suspend -> defer
• F.uncancelable(fa) -> F.uncancelable(_ => fa)
• join -> joinAndEmbed (actually joinWithNever - correction by @kubukoz)
• ExitCase -> Outcome
• Some package name changes (e.g. Semaphore was in concurrent and is now in std; Ref and Deferred are in cats.effect)
• Resource#parZip -> both
• Delete ContextShift entirely
• Delete Blocker entirely
• Any concrete-type implicit constraints (such as Concurrent[IO]) can be deleted entirely because they're now universally available
• blocker.delay(...) needs to be Sync[F].blocking(...), though that might not be possible, depending on whether or not you can figure out the type of blocker
• It's too bad it's probably impossible to help people out with ExecutionContexts they've been passing around in lieu of the new executionContext effect
• ContextShift[F].shift -> Spawn[F].cede. This is a slightly weird one because shift also does other things, but I'd wager that 99% of the time when people use it, they mean what cede now does
• async(...).guarantee(shift) can just be async(...)
• There is some restructuring and renaming within the Outcome algebra vis-a-vis ExitCase. Completed vs Succeeded(_), for example
• I don't think there's any way to automatically migrate continual

I think those are the biggest differences that can be automatically migrated? I'll probably think of some more things later.

[fthomas]

Thanks Daniel, this will keep me busy for a while. rolling up my sleeves

[djspiewak]

Thank you so much for tackling this!