add flake check for format search path features

checks/fixtures/formats/a/foo.nix

@@ -0,0 +1 @@
+{}

checks/fixtures/formats/b/bar.nix

@@ -0,0 +1 @@
+{}

checks/fixtures/formats/c/baz.nix

@@ -0,0 +1 @@
+{}

checks/fixtures/formats/d/quux.nix

@@ -0,0 +1 @@
+{}

checks/test-format-search-path.nix

@@ -0,0 +1,78 @@
+{
+  nixpkgs,
+  self,
+  system,
+}: let
+  inherit
+    (self.packages.${system})
+    nixos-generate
+    ;
+
+  pkgs = nixpkgs.legacyPackages.${system};
+in
+  pkgs.runCommand "test-format-search-path" {} ''
+    rc=$?
+
+    fail() {
+      rc="$?"
+
+      if (( "$#" > 0 )); then
+        printf 1>&2 -- "$@"
+      else
+        printf 1>&2 -- 'unknown error\n'
+      fi
+    }
+
+    run() {
+      ${nixos-generate}/bin/nixos-generate "$@"
+    }
+
+    showFormatSearchPath() {
+      run --show-format-search-path "$@"
+    }
+
+    list() {
+      run --list "$@"
+    }
+
+    fixtures="${self}/checks/fixtures/formats"
+    builtin="${nixos-generate}/share/nixos-generator/formats"
+
+    path="$(showFormatSearchPath)" || fail 'error running nixos-generate\n'
+
+    expected="$builtin"
+
+    [[ "$path" == "$expected" ]] \
+      || fail 'expected format search path to contain:\n%s\ngot:\n%s\n' "$expected" "$path"
+
+    export NIXOS_GENERATORS_FORMAT_SEARCH_PATH="''${fixtures}/c:''${fixtures}/d"
+
+    path="$(showFormatSearchPath --format-search-path "''${fixtures}/b" --format-search-path "''${fixtures}/a")" \
+      || fail 'error running nixos-generate\n'
+
+    expected="\
+    ''${fixtures}/a
+    ''${fixtures}/b
+    ''${fixtures}/c
+    ''${fixtures}/d
+    $builtin"
+
+    [[ "$path" == "$expected" ]] \
+      || fail 'expected format search path to contain:\n%s\ngot:\n%s\n' "$expected" "$path"
+
+    declare -A formats
+    while read -r format; do
+      formats["$format"]=1
+    done < <(list --format-search-path "''${fixtures}/b" --format-search-path "''${fixtures}/a")
+
+    for format in foo bar baz quux; do
+      [[ -n "''${formats["$format"]:-}" ]] \
+        || fail 'expected formats to include %s\n' "$format"
+    done
+
+    if (( rc == 0 )); then
+      touch "$out"
+    fi
+
+    exit "$rc"
+  ''

flake.nix

@@ -151,6 +151,9 @@
                 test-customize-format = import ./checks/test-customize-format.nix {
                   inherit nixpkgs system;
                 };
+                test-format-search-path = import ./checks/test-format-search-path.nix {
+                  inherit nixpkgs self system;
+                };
               in
                 lib.mapAttrs makeLazyDrv (
                   {
@@ -159,7 +162,7 @@
                       nixos-generate
                       ;
 
-                    inherit test-customize-format;
+                    inherit test-customize-format test-format-search-path;
 
                     is-formatted = import ./checks/is-formatted.nix {
                       pkgs = nixpkgs.legacyPackages.${system};