Remove support for GHC < 9.0

[blackgnezdo]

Remove conditional source code for ghc < 9.0

[blackgnezdo]

@jinwoo WDYT? Time to take out this garbage? People who need the old ghc will have the old releases and we won't have to worry about #109 ever again.

[jinwoo]

Sorry we're still stuck with GHC 8.10 and I'd like to still keep it. But I don't work with Haskell anymore. @agrue What do you think?

[blackgnezdo]

Sorry we're still stuck with GHC 8.10 and I'd like to still keep it. But I don't work with Haskell anymore. @agrue What do you think?

I don't think it matters to you. You can keep using the old version that's already imported in //third_party.

Alternatively, if you want to be sure that we don't break it going forward and the new versions still work on 8.10, somebody should add this version to the build matrix.

[agrue]

It's a bit scary, because this is a dependency of proto-lens. So if proto-lens eventually depends on a new version of ghc-source-gen that only works with GHC 9+, we won't be able to upgrade to that version. And unfortunately the "you can stay on the old version" logic doesn't apply there, because the proto ecosystem keeps evolving (e.g. we just recently had to add support for "proto editions" because proto files we depend on started using those features).

So if we have to do a similar thing in the future, we'll be stuck unable to import the new proto-lens version with support for the new features. Unless we try to back-port those features onto the old internal version but... ugh.

I don't know how to weigh this against the needs of external users though. If some other maintainer wants to merge this I don't feel like I can stop you.

[blackgnezdo]

It's a bit scary, because this is a dependency of proto-lens. So if proto-lens eventually depends on a new version of ghc-source-gen that only works with GHC 9+, we won't be able to upgrade to that version. And unfortunately the "you can stay on the old version" logic doesn't apply there, because the proto ecosystem keeps evolving (e.g. we just recently had to add support for "proto editions" because proto files we depend on started using those features).

So if we have to do a similar thing in the future, we'll be stuck unable to import the new proto-lens version with support for the new features. Unless we try to back-port those features onto the old internal version but... ugh.

I have two points to offer here:

• ghc-source-gen last received API changing updates over 3 years ago (gasp, I was still a googler!)
• proto-lens can be the place to enforce the dependency on a 8.10 compatible version of this library (it even has 8.10 in its build matrix)

I don't know how to weigh this against the needs of external users though. If some other maintainer wants to merge this I don't feel like I can stop you.

I don't feel strongly enough to advocate for this really. This PR can simply stay open until something changes...

[agrue]

That makes sense to me. Okay, well if you ever do feel strongly about it, I can't really argue against it.