haskell.lib: remove controlPhases and ghcInfo

doc/release-notes/rl-2511.section.md

···
 
 - The GHCJS 8.10.7, exposed via `haskell.compiler.ghcjs` and `haskell.compiler.ghcjs810`, has been removed. Downstream users should migrate their projects to the new JavaScript backend of GHC proper which can be used via `pkgsCross.ghcjs` from Nixpkgs.
 
+- The `ghcInfo` and `controlPhases` functions have been removed from `haskell.lib.compose` and `haskell.lib`. They were unused and would return incorrect results.
+
+  - Instead of using `controlPhases`, the `doCheck` attribute of derivations can be accessed which is more accurate. `doBenchmark` can't be introspected at the moment.
+  - To detect cross compilation, the relevant platforms exposed by `stdenv` should be compared instead of using `ghcInfo`.
+  - Nixpkgs no longer packages any `ghcjs` compiler.
+  - To find a suitable `nativeGhc`, `buildHaskellPackages` should be used. `ghcInfo` would use `ghc.bootPkgs.ghc` if cross compiling,
+    and the given `ghc` otherwise. This approach is not recommended since it results in mismatched GHC versions.
+
 - `gnome-keyring` no longer ships with an SSH agent anymore because it has been deprecated upstream. You should use `gcr_4` instead, which provides the same features. More information on why this was done can be found on [the relevant GCR upstream PR](https://gitlab.gnome.org/GNOME/gcr/-/merge_requests/67).
 
 - `stdenv.mkDerivation` and other derivation builders that use it no longer allow the value of `env` to be anything but an attribute set, for the purpose of setting environment variables that are available to the [builder](https://nix.dev/manual/nix/latest/store/derivation/#builder) process. An environment variable called `env` can still be provided by means of `mkDerivation { env.env = ...; }`, though we recommend to use a more specific name than "env".

pkgs/development/haskell-modules/lib/compose.nix

···
   # nix-shell evaluation, return a nix-shell optimized environment.
   shellAware = p: if lib.inNixShell then p.env else p;
 
-  ghcInfo = ghc: rec {
-    isCross = (ghc.cross or null) != null;
-    isGhcjs = ghc.isGhcjs or false;
-    nativeGhc = if isCross || isGhcjs then ghc.bootPkgs.ghc else ghc;
-  };
-
-  ### mkDerivation helpers
-  # These allow external users of a haskell package to extract
-  # information about how it is built in the same way that the
-  # generic haskell builder does, by reusing the same functions.
-  # Each function here has the same interface as mkDerivation and thus
-  # can be called for a given package simply by overriding the
-  # mkDerivation argument it used. See getHaskellBuildInputs above for
-  # an example of this.
-
-  # Some information about which phases should be run.
-  controlPhases =
-    ghc:
-    let
-      inherit (ghcInfo ghc) isCross;
-    in
-    {
-      doCheck ? !isCross,
-      doBenchmark ? false,
-      ...
-    }:
-    {
-      inherit doCheck doBenchmark;
-    };
-
   # Utility to convert a directory full of `cabal2nix`-generated files into a
   # package override set
   #

pkgs/development/haskell-modules/lib/default.nix

···
   # nix-shell evaluation, return a nix-shell optimized environment.
   shellAware = compose.shellAware;
 
-  ghcInfo = compose.ghcInfo;
-
-  ### mkDerivation helpers
-  # These allow external users of a haskell package to extract
-  # information about how it is built in the same way that the
-  # generic haskell builder does, by reusing the same functions.
-  # Each function here has the same interface as mkDerivation and thus
-  # can be called for a given package simply by overriding the
-  # mkDerivation argument it used. See getHaskellBuildInputs above for
-  # an example of this.
-
-  # Some information about which phases should be run.
-  controlPhases = compose.controlPhases;
-
   # Utility to convert a directory full of `cabal2nix`-generated files into a
   # package override set
   #