Use stock deriving strategy on generated Show instances for Uniques

[cdparks]

Hey we ran into this at Freckle trying to turn on -Wmissing-deriving-strategies with ghc 8.8.3. Currently, we're seeing warnings when compiling Entity declarations with a Unique, e.g.:

mkPersist sqlSettings [persistUpperCase|
  Thing
    foo Text
    UniqueThingFoo foo
    deriving Show Eq
|]

⇒

  <snip>/persistent/persistent-template/test/main.hs:44:1: warning: [-Wmissing-deriving-strategies]
      No deriving strategy specified. Did you want stock, newtype, or anyclass?
     |
  44 | mkPersist sqlSettings [persistUpperCase|
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...

Specifying the stock strategy for the Unique's generated Show instance silences this warning:

 data instance Unique Thing = UniqueThingFoo Text
-  deriving Show
+  deriving stock Show

Before submitting your PR, check that you've:

• Bumped the version number

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[parsonsmatt]

Current plan is to get rid of Show actually - but I'd really like to make that customizable. Perhaps we need a mpsUniqueDerives field?

[parsonsmatt]

We can definitely get this done for the persistent-2.10 branch to fix the immediate issue.

[cdparks]

get this done for the persistent-2.10 branch

Adding mpsUniqueDerives or this PR?

[parsonsmatt]

This PR, for sure, mpsUniqueDerives is a new field and thus a breaking change, so would need to go in for 2.11.

[parsonsmatt]

hmmm, I think there's gotta be a better way to just apply that one commit to the old branch than setting the base branch, which tries to get everything in

[cdparks]

Are you trying to just pick faa44fd16 and make persistent-2.10 the base? I can do some creative rebasing and force-push to my branch if you just want the code change minus version/changelog stuff

[cdparks]

(and by creative rebasing, I just mean dropping the top commit, and modifying the first one to not bump a version, so I guess not especially creative)

[parsonsmatt]

I think I just want to cherry pick that commit onto the persistent-2.10 branch

[cdparks]

Cool, let me know if you need me to fiddle with anything.

[chessai]

Don't forget Eq!

+1

[cdparks]

@parsonsmatt is this subsumed by #1127? If so, I'm fine to close this

[parsonsmatt]

If you'd like to make a backported release with this patch, we can do that. But otherwise, yeah, since we're not generated Show instances anymore we don't need this. Sorry for not getting back to it 😅

[cdparks]

Not a problem, we usually upgrade to the most recent version of persistent pretty quickly