{
  lib,
  runCommandLocal,
  pkgs,
}:
let
  # Format a list of errors with an error message and trailing newline
  describeErrors =
    desc: errors:
    lib.optionals (errors != [ ]) (lib.toList desc ++ lib.map (v: "- ${v}") errors ++ [ "" ]);

  # Build error messages for the given declared & generated names
  checkDeclarations =
    {
      # The plugin's name
      name,
      # A list of package paths declared in declarationFile
      # A path can be either a single string, or a list; as expected by `mkPackageOption`
      packages,
      # A list of names declared in declarationFile
      declared,
      # A list of names generated by generate-files
      generated,
      # The filename where names are declared (used in error messages)
      declarationFile,
    }:
    let
      missingFromPkgs = builtins.concatMap (
        loc: lib.optional (!lib.hasAttrByPath loc pkgs) (lib.concatStringsSep "." loc)
      ) (builtins.map lib.toList packages);
      undeclared = lib.filter (name: !(lib.elem name declared)) generated;
      uselesslyDeclared = lib.filter (name: !(lib.elem name generated)) declared;
    in
    describeErrors "${name}: The following are not found in `pkgs`, but are declared in ${declarationFile}:" missingFromPkgs
    ++ describeErrors "${name}: The following are not declared in ${declarationFile}:" undeclared
    ++ describeErrors "${name}: The following are not listed upstream, but are declared in ${declarationFile}:" uselesslyDeclared;

  # The error message provided to the derivation.
  # The test fails if this is non-empty.
  errors = lib.concatStringsSep "\n" (
    checkDeclarations (
      let
        inherit (import ../plugins/lsp/lsp-packages.nix) unpackaged packages customCmd;
      in
      {
        name = "lsp";
        declarationFile = "plugins/lsp/lsp-packages.nix";

        packages = builtins.attrValues packages;

        declared = unpackaged ++ lib.attrsets.attrNames (packages // customCmd);

        generated = lib.pipe ../generated/lspconfig-servers.json [
          lib.importJSON
          (builtins.map (lib.getAttr "name"))
          lib.lists.unique
        ];
      }
    )
    ++ checkDeclarations (
      let
        inherit (import ../plugins/by-name/none-ls/packages.nix lib) noPackage packaged;
      in
      {
        name = "none-ls";
        declarationFile = "plugins/by-name/none-ls/packages.nix";

        packages = builtins.filter (pkg: pkg != null) (builtins.attrValues packaged);

        declared = noPackage ++ lib.attrsets.attrNames packaged;

        generated = lib.pipe ../generated/none-ls.nix [
          import
          lib.attrsets.attrValues
          lib.lists.concatLists
          lib.lists.unique
        ];
      }
    )
    ++ checkDeclarations (
      let
        inherit (import ../plugins/by-name/efmls-configs/packages.nix lib) packaged unpackaged;
      in
      {
        name = "efmls";
        declarationFile = "plugins/by-name/efmls-configs/packages.nix";

        packages = builtins.attrValues packaged;

        declared = unpackaged ++ lib.attrsets.attrNames packaged;

        generated = lib.pipe ../generated/efmls-configs.nix [
          import
          lib.attrsets.attrValues
          (lib.map ({ linter, formatter }: linter.possible ++ formatter.possible))
          lib.lists.concatLists
          lib.lists.unique
        ];
      }
    )
  );
in
runCommandLocal "generated-sources-test"
  {
    __structuredAttrs = true;
    inherit errors;
  }
  ''
    if [ -n "$errors" ]; then
      echo -n "$errors"
      exit 1
    fi
    touch "$out"
  ''