maintainers/scripts: test for haskell configuration overlays

The added nix expression allows maintainers to check for regressions in
the configuration overlays employed by haskellPackages and friends. The
reasoning behind this is that, if we add an override for something, it
should also build. To test this fact, we extract all attributes touched
by a configuration and obtain all relevant derivations corresponding to
it which can then be thrown into nix-build --keep-going.

I've been using this expression to verify configuration-ghc-9.2.x.nix
for a week or so which works quite well. The amount of stale overrides
in other configuration makes it a bit more painful for other use cases
at the moment.

maintainers/scripts/haskell/test-configurations.nix

@@ -0,0 +1,113 @@
+/* Nix expression to test for regressions in the Haskell configuration overlays.
+
+   test-configurations.nix determines all attributes touched by given Haskell
+   configuration overlays (i. e. pkgs/development/haskell-modules/configuration-*.nix)
+   and builds all derivations (or at least a reasonable subset) affected by
+   these overrides.
+
+   By default, it checks `configuration-{common,nix,ghc-8.10.x}.nix`. You can
+   invoke it like this:
+
+     nix-build maintainers/scripts/haskell/test-configurations.nix --keep-going
+
+   It is possible to specify other configurations:
+
+     nix-build maintainers/scripts/haskell/test-configurations.nix \
+       --arg files '[ "configuration-ghc-9.0.x.nix" "configuration-ghc-9.2.x.nix" ]' \
+       --keep-going
+
+   You can also just supply a single string:
+
+     nix-build maintainers/scripts/haskell/test-configurations.nix \
+       --argstr files "configuration-arm.nix" --keep-going
+
+   You can even supply full paths which is handy, as it allows for tab-completing
+   the configurations:
+
+     nix-build maintainers/scripts/haskell/test-configurations.nix \
+       --argstr files pkgs/development/haskell-modules/configuration-arm.nix \
+       --keep-going
+
+   By default, derivation that fail to evaluate are skipped, unless they are
+   “just” marked as broken. You can check for other eval errors like this:
+
+     nix-build maintainers/scripts/haskell/test-configurations.nix \
+       --arg skipEvalErrors false --keep-going
+
+   You can also disable checking broken packages by passing a nixpkgs config:
+
+     nix-build maintainers/scripts/haskell/test-configurations.nix \
+       --arg config '{ allowBroken = false; }' --keep-going
+
+*/
+{ files ? [
+    "configuration-common.nix"
+    "configuration-nix.nix"
+    "configuration-ghc-8.10.x.nix"
+  ]
+, nixpkgsPath ? ../../..
+, config ? { allowBroken = true; }
+, skipEvalErrors ? true
+}:
+
+let
+  pkgs = import nixpkgsPath { inherit config; };
+  inherit (pkgs) lib;
+
+  # see usage explanation for the input format `files` allows
+  files' = builtins.map builtins.baseNameOf (
+    if !builtins.isList files then [ files ] else files
+  );
+
+  setsForFile = fileName:
+    let
+      # extract the unique part of the config's file name
+      configName = builtins.head (
+        builtins.match "configuration-(.+).nix" fileName
+      );
+      # match the major and minor version of the GHC the config is intended for, if any
+      configVersion = lib.concatStrings (
+        builtins.match "ghc-([0-9]+).([0-9]+).x" configName
+      );
+      # return all package sets under haskell.packages matching the version components
+      setsForVersion = builtins.map (name: pkgs.haskell.packages.${name}) (
+        builtins.filter (lib.hasPrefix "ghc${configVersion}") (
+          builtins.attrNames pkgs.haskell.packages
+        )
+      );
+
+      defaultSets = [ pkgs.haskellPackages ];
+    in {
+      # use plain haskellPackages for the version-agnostic files
+      # TODO(@sternenseemann): also consider currently selected versioned sets
+      "common" = defaultSets;
+      "nix" = defaultSets;
+      "arm" = defaultSets;
+      "darwin" = defaultSets;
+    }.${configName} or setsForVersion;
+
+  # evaluate a configuration and only return the attributes changed by it
+  overriddenAttrs = fileName: builtins.attrNames (
+    import (nixpkgsPath + "/pkgs/development/haskell-modules/${fileName}") {
+      haskellLib = pkgs.haskell.lib.compose;
+      inherit pkgs;
+    } {} {}
+  );
+
+  # list of derivations that are affected by overrides in the given configuration
+  # overlays. For common, nix, darwin etc. only the derivation from the default
+  # package set will be emitted.
+  packages = builtins.filter (v:
+    v != null && (skipEvalErrors -> (builtins.tryEval (v.outPath or v)).success)
+  ) (
+    lib.concatMap (fileName:
+      let
+        sets = setsForFile fileName;
+        attrs = overriddenAttrs fileName;
+      in
+        lib.concatMap (set: builtins.map (attr: set.${attr}) attrs) sets
+    ) files'
+  );
+in
+
+packages