Nix Flake Check Command and Evaluation Rules

R""( # Examples * Evaluate the flake in the current directory, and build its checks: ```console # nix flake check ``` * Verify that the `patchelf` flake evaluates, but don't build its checks: ```console # nix flake check --no-build github:NixOS/patchelf ``` # Description This command verifies that the flake specified by flake reference *flake-url* can be evaluated successfully (as detailed below), and that the derivations specified by the flake's `checks` output can be built successfully. If the `keep-going` option is set to `true`, Nix will keep evaluating as much as it can and report the errors as it encounters them. Otherwise it will stop at the first error. # Evaluation checks The following flake output attributes must be derivations: * `checks.`*system*`.`*name* * `defaultPackage.`*system* * `devShell.`*system* * `devShells.`*system*`.`*name* * `nixosConfigurations.`*name*`.config.system.build.toplevel` * `packages.`*system*`.`*name* The following flake output attributes must be [app definitions](./nix3-run.md): * `apps.`*system*`.`*name* * `defaultApp.`*system* The following flake output attributes must be [template definitions](./nix3-flake-init.md): * `defaultTemplate` * `templates.`*name* The following flake output attributes must be *Nixpkgs overlays*: * `overlay` * `overlays.`*name* The following flake output attributes must be *NixOS modules*: * `nixosModule` * `nixosModules.`*name* The following flake output attributes must be [bundlers](./nix3-bundle.md): * `bundlers.`*name* * `defaultBundler` In addition, the `hydraJobs` output is evaluated in the same way as Hydra's `hydra-eval-jobs` (i.e. as a arbitrarily deeply nested attribute set of derivations). Similarly, the `legacyPackages`.*system* output is evaluated like `nix-env --query --available `. )""

This document describes the `nix flake check` command, which verifies that a specified Nix flake can be successfully evaluated and that its `checks` output derivations can be built. It provides examples of its use, including an option to skip building checks (`--no-build`) and to continue evaluation despite errors (`keep-going`). The command performs rigorous evaluation checks on various flake output attributes, ensuring they conform to expected types such as derivations, app definitions, template definitions, Nixpkgs overlays, NixOS modules, and bundlers, along with special handling for `hydraJobs` and `legacyPackages`.