Build GHC in GHC2021

[Oleg Grenrus]

I think Arnaud was talkin about ghc, i.e. compiler/, not about 
everything (incl libraries/base e.g.).

I tried, the PoC is at 
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/11743

I needed to do `NoPolyKinds` in few modules (could give kind signature 
too, but NoPolyKinds were quicker).
Especially that hits hs-boot files.

By doing `default-language: GHC2021`, and going through modules with 
BangPatterns (and removing that, and other GHC2021 extensions), the diff 
stats are nice: 91 files changed, 24 insertions(+), 272 deletions(-)

- Oleg

On 8.12.2023 13.33, Andreas Klebinger via ghc-devs wrote:
>
> On a whim tried enabling it by default but it failed first because 
> GeneralizedNewtypeDeriving is incompatible with safe haskell
> and once I disabled GeneralizedNewtypeDeriving it failed with some 
> error about missmatched kinds.
>
> I would welcome a patch doing this, but it's not a priority. 
> Especially since it doesn't seem to be as simple as changing
> the base language and removing some pragmas.
>
> Am 07/12/2023 um 14:29 schrieb Arnaud Spiwack:
>> Indeed. I didn't realise the ambiguity in my wording.
>>
>> I'd like for GHC to be built, with Hadrian, using GHC2021 as the base 
>> language.
>>
>>
>> On Thu, 7 Dec 2023 at 14:01, Tom Ellis 
>> <tom-lists-haskell-cafe-2023 at jaguarpaw.co.uk> wrote:
>>
>>     On Thu, Dec 07, 2023 at 12:53:02PM +0000, Richard Eisenberg wrote:
>>     > I think this is an excellent idea! So excellent, that we've
>>     already done it. :)
>>     >
>>     > When I try to compile with GHC 9.6.2 (what I have lying
>>     around), GHC2021 is in effect.
>>     >
>>     > Is there something different you were thinking of?
>>
>>     I think Arnaud meant that compilations of GHC's codebase itself
>>     should
>>     use the GHC2021 setting.
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>>
>>
>> -- 
>> Arnaud Spiwack
>> Director, Research at https://moduscreate.com and https://tweag.io.
>>
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