Switch default-language to GHC2021 instead of Haskell2010

[Bodigrim]

Let's scrap 1000+ lines of language extensions.

Template B: This PR does not modify behaviour or interface

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[philderbeast]

This is possible now with the recent narrowing of the support window for GHC versions that build this project.

Since: 9.2.1
-- -XGHC2021

As discussed in #11723.

The last GHC 9.2 was released more than 3 years ago, which leaves us with GHC 9.4+.
-- #11966 (comment)

[philderbeast]

@Bodigrim could you please add a "Manual QA Note" to the description saying how we can validate locally with older versions of GHC?

cabal/.github/workflows/validate.yml

Lines 236 to 248 in eab682c

	validate-old-ghcs:
	name: Validate old ghcs ${{ matrix.extra-ghc }}
	# latest/latest doesn't have the old packages the old ghcs depend on
	runs-on: ubuntu-22.04
	needs: validate
	strategy:
	matrix:
	extra-ghc:
	["9.2.8", "9.0.2", "8.10.7", "8.8.4", "8.6.5", "8.4.4", "8.2.2", "8.0.2"]
	## GHC 7.10.3 does not install on ubuntu-latest with ghcup.
	## Older GHCs are not supported by ghcup in the first place.
	fail-fast: false

I tried quickly and failed to replicate the "validate-old-ghcs" step locally:

$ ghc --numeric-version
8.2.2

$ sh validate.sh -s build
Resolving dependencies...
Error: [Cabal-7107]
Could not resolve dependencies:
[__0] trying: Cabal-3.17.0.0 (user goal)
[__1] next goal: base (dependency of Cabal)
[__1] rejecting: base-4.10.1.0/installed-4.10.1.0 (conflict: Cabal => base>=4.17 && <5)
[__1] rejecting: base; 4.22.0.0, 4.21.2.0, 4.21.1.0, 4.21.0.0, 4.20.2.0, 4.20.1.0, 4.20.0.1, 4.20.0.0, 4.19.2.0, 4.19.1.0, 4.19.0.0, 4.18.3.0, 4.18.2.1, 4.18.2.0, 4.18.1.0, 4.18.0.0, 4.17.2.1, 4.17.2.0, 4.17.1.0, 4.17.0.0, 4.16.4.0, 4.16.3.0, 4.16.2.0, 4.16.1.0, 4.16.0.0, 4.15.1.0, 4.15.0.0, 4.14.3.0, 4.14.2.0, 4.14.1.0, 4.14.0.0, 4.13.0.0, 4.12.0.0, 4.11.1.0, 4.11.0.0, 4.10.1.0, 4.10.0.0, 4.9.1.0, 4.9.0.0, 4.8.2.0, 4.8.1.0, 4.8.0.0, 4.7.0.2, 4.7.0.1, 4.7.0.0, 4.6.0.1, 4.6.0.0, 4.5.1.0, 4.5.0.0, 4.4.1.0, 4.4.0.0, 4.3.1.0, 4.3.0.0, 4.2.0.2, 4.2.0.1, 4.2.0.0, 4.1.0.0, 4.0.0.0, 3.0.3.2, 3.0.3.1 (constraint from non-reinstallable package requires installed instance)
[__1] fail (backjumping, conflict set: Cabal, base)
After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: Cabal, base

$ cabal build all --enable-tests --enable-benchmarks --project-file=cabal.validate.project --with-compiler=ghc-8.2.2
Resolving dependencies...
Error: [Cabal-7107]
Could not resolve dependencies:
[__0] trying: Cabal-3.17.0.0 (user goal)
[__1] next goal: base (dependency of Cabal)
[__1] rejecting: base-4.10.1.0/installed-4.10.1.0 (conflict: Cabal => base>=4.17 && <5)
[__1] rejecting: base; 4.22.0.0, 4.21.2.0, 4.21.1.0, 4.21.0.0, 4.20.2.0, 4.20.1.0, 4.20.0.1, 4.20.0.0, 4.19.2.0, 4.19.1.0, 4.19.0.0, 4.18.3.0, 4.18.2.1, 4.18.2.0, 4.18.1.0, 4.18.0.0, 4.17.2.1, 4.17.2.0, 4.17.1.0, 4.17.0.0, 4.16.4.0, 4.16.3.0, 4.16.2.0, 4.16.1.0, 4.16.0.0, 4.15.1.0, 4.15.0.0, 4.14.3.0, 4.14.2.0, 4.14.1.0, 4.14.0.0, 4.13.0.0, 4.12.0.0, 4.11.1.0, 4.11.0.0, 4.10.1.0, 4.10.0.0, 4.9.1.0, 4.9.0.0, 4.8.2.0, 4.8.1.0, 4.8.0.0, 4.7.0.2, 4.7.0.1, 4.7.0.0, 4.6.0.1, 4.6.0.0, 4.5.1.0, 4.5.0.0, 4.4.1.0, 4.4.0.0, 4.3.1.0, 4.3.0.0, 4.2.0.2, 4.2.0.1, 4.2.0.0, 4.1.0.0, 4.0.0.0, 3.0.3.2, 3.0.3.1 (constraint from non-reinstallable package requires installed instance)
[__1] fail (backjumping, conflict set: Cabal, base)
After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: Cabal, base

[Bodigrim]

@philderbeast this PR does not change the support range of GHCs, the bound base>=4.17 && <5 remained unchanged. Validation against older versions of GHC is performed by CI.

validate-old-ghcs CI job runs sh validate.sh -s build with env.GHC_FOR_RELEASE (which is 9.10.3 at the moment) and only then executes sh validate.sh --lib-only -s lib-suite-extras --extra-hc "${{ env.EXTRA_GHC }}", where env.EXTRA_GHC points to versions from 8.0 to 9.2. Does it clarify?

cabal/.github/workflows/validate.yml

Lines 236 to 302 in eab682c

	validate-old-ghcs:
	name: Validate old ghcs ${{ matrix.extra-ghc }}
	# latest/latest doesn't have the old packages the old ghcs depend on
	runs-on: ubuntu-22.04
	needs: validate
	strategy:
	matrix:
	extra-ghc:
	["9.2.8", "9.0.2", "8.10.7", "8.8.4", "8.6.5", "8.4.4", "8.2.2", "8.0.2"]
	## GHC 7.10.3 does not install on ubuntu-latest with ghcup.
	## Older GHCs are not supported by ghcup in the first place.
	fail-fast: false
	steps:
	- uses: actions/checkout@v6
	- name: Install prerequisites for old GHCs
	run: |
	sudo apt-get update
	sudo apt-get install libncurses5 libtinfo5
	- name: Install extra compiler
	run: ghcup install ghc ${{ matrix.extra-ghc }}
	- name: GHCup logs
	if: always()
	run: cat /usr/local/.ghcup/logs/*
	- name: Install primary compiler
	uses: haskell-actions/setup@v2
	id: setup-haskell
	with:
	ghc-version: ${{ env.GHC_FOR_RELEASE }}
	cabal-version: latest
	- name: GHC versions
	run: |
	ghc --version
	"ghc-${{ matrix.extra-ghc }}" --version
	# runner.os isn't sufficient for binary compatible caches
	- name: Get runner OS/version for cache keys
	id: get-osver
	run: echo "osver=$ImageOS" >> "$GITHUB_OUTPUT"
	# As we are reusing the cached build dir from the previous step
	# the generated artifacts are available here,
	# including the cabal executable and the test suite
	- uses: actions/cache@v5
	with:
	path: |
	${{ steps.setup-haskell.outputs.cabal-store }}
	dist-*
	key: ${{ steps.get-osver.outputs.osver }}-${{ env.GHC_FOR_RELEASE }}-${{ github.sha }}
	restore-keys: ${{ steps.get-osver.outputs.osver }}-${{ env.GHC_FOR_RELEASE }}-
	- name: Validate build
	id: build
	run: sh validate.sh ${{ env.COMMON_FLAGS }} -s build
	- name: "Validate lib-suite-extras --extra-hc ghc-${{ matrix.extra-ghc }}"
	env:
	EXTRA_GHC: ghc-${{ matrix.extra-ghc }}
	run: sh validate.sh ${{ env.COMMON_FLAGS }} --lib-only -s lib-suite-extras --extra-hc "${{ env.EXTRA_GHC }}"
	# See the comment above about running all tests but still failing if one
	# of them does; it also applies here.

[philderbeast]

Thanks @Bodigrim. With that prompting, I was able to run a few local validations.

$ ghc --numeric-version
9.14.1
$ ./validate.sh --extra-hc=ghc-8.2.2

$ ghc --numeric-version
9.4.8
$ ./validate.sh --extra-hc=ghc-9.14.1

Installing ghc-7.10.3 with ghcup install ghc 7.10.3 --install-targets "install STRIP_CMD=true":

$ ghc --numeric-version
9.14.1
$ ./validate.sh --extra-hc=ghc-7.10.3

[philderbeast]

Impressive diff, +61, -1,118 in 378 files.

[Bodigrim]

This can use another review please ;)