Warn GHC 9 Compatibility to LSP Client

[konn]

As I proposed in #1990, this PR makes ghcide to send Warning to LSP clients if GHC >= 9.0.1.
It sends the message right before the initialisation; if it's inappropriate, please fix me.

It also adds README to the notes on GHC 9 compatibility.

When invoked on GHC 9 projects, ghcide warns like this:

expanded:

[jneira]

We have something similar in hls-test-utils but more general, maybe it worths move the ghc version check code to Development.IDE.GHC.Compat and make it available for the entire codebase (hls-test-utils depends on ghcide):

haskell-language-server/hls-test-utils/src/Test/Hls/Util.hs

Lines 110 to 126 in a0def3c

	data GhcVersion
	= GHC810
	| GHC88
	| GHC86
	| GHC84
	deriving (Eq,Show)
	ghcVersion :: GhcVersion
	#if (defined(MIN_VERSION_GLASGOW_HASKELL) && (MIN_VERSION_GLASGOW_HASKELL(8,10,0,0)))
	ghcVersion = GHC810
	#elif (defined(MIN_VERSION_GLASGOW_HASKELL) && (MIN_VERSION_GLASGOW_HASKELL(8,8,0,0)))
	ghcVersion = GHC88
	#elif (defined(MIN_VERSION_GLASGOW_HASKELL) && (MIN_VERSION_GLASGOW_HASKELL(8,6,0,0)))
	ghcVersion = GHC86
	#elif (defined(MIN_VERSION_GLASGOW_HASKELL) && (MIN_VERSION_GLASGOW_HASKELL(8,4,0,0)))
	ghcVersion = GHC84
	#endif

[wz1000]

I think this is not worth the annoyance on every start up. I would consider it acceptable if it was limited to the first start.

[jneira]

I think this is not worth the annoyance on every start up. I would consider it acceptable if it was limited to the first start.

Is it possible using only the lsp spec? If it is not maybe the simpler thing is add the warning in the vscode editor, when it downlods hls for ghc-9.0.1. It should be done for each editor though.

[anka-213]

vscode is the only one that downloads the language server automatically, so it's probably not as important for the other editors.

[jneira]

maybe we could convert the alert in a log output, like the warning about the implicit cradle?

[konn]

maybe we could convert the alert in a log output, like the warning about the implicit cradle?

Hmm, log output cannot be seen unless a user actively opens and navigate it so it makes less sense, I think...
Well, if there is not much consensus on adding this warning, I would close this.

[jneira]

Hmm, log output cannot be seen unless a user actively opens and navigate it so it makes less sense, I think...
Well, if there is not much consensus on adding this warning, I would close this.

Well i can imagine a situation where users not using the automatic download of vscode get an error or detect a plugin is not enabled (and they don't know they are disabled for ghc 9) and examine and/or post the logs in an issue. The message will help them or us to detect quickly the cause.

[jneira]

@konn do you mind if i take over the pr to convert the alert in a log message? fine with closing it too if you think it is better

[konn]

Sorry for leaving this PR idle (I've been a bit busy these days...).

@konn do you mind i take over the pr to convert the alert in a los message?

Ah, please take this over! And thanks for promoting GhcVer things to project-level.

[jneira]

lgtm 😉, thanks for adding the warning