Persist generated bounds to dummy file

.config/project/github-ci.nix

@@ -5,6 +5,7 @@
   ...
 }: let
   planName = "plan-\${{ runner.os }}-\${{ matrix.ghc }}\${{ matrix.bounds }}";
+  cabalFilePattern = "planned-bounds.cabal";
 in {
   services.github.workflow."build.yml".text = lib.generators.toYAML {} {
     name = "CI";
@@ -96,20 +97,25 @@ in {
           }
           {run = "cabal install cabal-plan-bounds";}
           {
-            name = "Load plans";
+            name = "download Cabal plans";
             uses = "actions/download-artifact@v4";
             "with" = {
               path = "plans";
               pattern = "plan-*";
               merge-multiple = true;
             };
           }
-          {run = "find plans/";}
-          {run = "find . -name '*.cabal' -exec cabal-plan-bounds plans/*.json --cabal {} \\;";}
-          {run = "git diff **/*.cabal";}
           {
-            name = "Fail if .cabal file was changed";
-            run = "git diff-files --quiet **/*.cabal || exit 1";
+            name = "Cabal plans considered in generated bounds";
+            run = "find plans/";
+          }
+          {
+            name = "update generated bounds";
+            run = "find . -name '${cabalFilePattern}' -exec cabal-plan-bounds plans/*.json --cabal {} \\;";
+          }
+          {
+            name = "check if bounds have changed";
+            run = "git diff --exit-code ${cabalFilePattern}";
           }
         ];
       };

planned-bounds.cabal

@@ -0,0 +1,35 @@
+cabal-version: 3.4
+
+name: planned-bounds
+version: 0.1.0.0
+synopsis: Dependency bounds as determined by `cabal-plan-bounds`.
+description: This is a non-built package intended to catch changes in our
+             dependency bounds. The actual packages use slightly different
+             bounds in some cases (always accompanied by a comment), and a
+             clearer syntax (see cabal-plan-bounds#18). This allows us to still
+             have a check that fails and forces us to review changes without
+             having to match `cabal-plan-bounds`’ results exactly.
+
+library
+  build-depends:
+    Cabal,
+    QuickCheck,
+    base,
+    bifunctors,
+    cabal-doctest,
+    comonad,
+    containers,
+    deriving-compat,
+    doctest,
+    either,
+    foldable1-classes-compat,
+    free,
+    hedgehog,
+    kan-extensions,
+    lens,
+    profunctors,
+    splitmix,
+    strict,
+    template-haskell,
+    th-abstraction,
+    transformers