test(perSystem): make sure it is memoized

dev/flake-module.nix

@@ -42,6 +42,10 @@
       let tests = import ./tests/eval-tests.nix { flake-parts = self; };
       in tests.runTests pkgs.emptyFile // { internals = tests; };
 
+    checks.perSystem-memoize = pkgs.callPackage ./tests/perSystem-memoize.nix {
+      flake-parts = self;
+    };
+
     nix-unit.tests = import ./tests/nix-unit.nix { flake-parts = self; };
 
     # nix-unit evaluates the flake, which triggers the dev partition via

dev/tests/perSystem-memoize.nix

@@ -0,0 +1,32 @@
+# Test that perSystem evaluation is memoized: calling withSystem twice for the
+# same undeclared system should only evaluate perSystem once.
+# Run with:
+#   nix build .#checks.x86_64-linux.perSystem-memoize
+
+{ flake-parts, runCommand, nix }:
+runCommand "perSystem-memoize" { nativeBuildInputs = [ nix ]; } ''
+  export HOME="$(realpath .)"
+  unset NIX_STORE
+  export NIX_STORE_DIR=${builtins.storeDir}
+  export NIX_REMOTE="$HOME/storedata"
+
+  nix eval \
+    --extra-experimental-features 'nix-command flakes' \
+    --no-write-lock-file \
+    --offline \
+    --substituters "" \
+    --override-input flake-parts ${flake-parts} \
+    --override-input flake-parts/nixpkgs-lib ${flake-parts.inputs.nixpkgs-lib} \
+    "${./perSystem-memoize}#result" \
+    2>eval-stderr
+
+  count=$(grep -c "Evaluating perSystem for foo" eval-stderr || true)
+  cat eval-stderr >&2
+  echo "Trace count: $count"
+  if [ "$count" -ne 1 ]; then
+    echo "FAIL: expected perSystem to be evaluated exactly once, but it was evaluated $count times"
+    exit 1
+  fi
+  echo "PASS: perSystem evaluated exactly once"
+  touch $out
+''

dev/tests/perSystem-memoize/flake.nix

@@ -0,0 +1,15 @@
+{
+  inputs.flake-parts.url = "github:hercules-ci/flake-parts";
+  outputs = inputs:
+    inputs.flake-parts.lib.mkFlake { inherit inputs; } ({ withSystem, ... }: {
+      systems = [ ];
+      perSystem = { system, ... }:
+        builtins.trace "Evaluating perSystem for ${system}" { };
+      flake.result =
+        let
+          a = withSystem "foo" ({ config, ... }: null);
+          b = withSystem "foo" ({ config, ... }: "ok");
+        in
+        builtins.seq a b;
+    });
+}