core/pkgs/build-support/trivial-builders/default.nix

{
  lib,
  config,
  stdenv,
  stdenvNoCC,
  jq,
  lndir,
  runtimeShell,
  shellcheck-minimal,
}:

let
  inherit (lib) optionalAttrs warn;
in

rec {

  # Docs in doc/build-helpers/trivial-build-helpers.chapter.md
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-runCommand
  runCommand =
    name: env:
    runCommandWith {
      stdenv = stdenvNoCC;
      runLocal = false;
      inherit name;
      derivationArgs = env;
    };
  # Docs in doc/build-helpers/trivial-build-helpers.chapter.md
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-runCommandLocal
  runCommandLocal =
    name: env:
    runCommandWith {
      stdenv = stdenvNoCC;
      runLocal = true;
      inherit name;
      derivationArgs = env;
    };
  # Docs in doc/build-helpers/trivial-build-helpers.chapter.md
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-runCommandCC
  runCommandCC =
    name: env:
    runCommandWith {
      stdenv = stdenv;
      runLocal = false;
      inherit name;
      derivationArgs = env;
    };
  # `runCommandCCLocal` left out on purpose.
  # We shouldn’t force the user to have a cc in scope.

  # TODO: Move documentation for runCommandWith to the Nixpkgs manual
  /*
    Generalized version of the `runCommand`-variants
    which does customized behavior via a single
    attribute set passed as the first argument
    instead of having a lot of variants like
    `runCommand*`. Additionally it allows changing
    the used `stdenv` freely and has a more explicit
    approach to changing the arguments passed to
    `stdenv.mkDerivation`.
  */
  runCommandWith =
    let
      # prevent infinite recursion for the default stdenv value
      defaultStdenv = stdenv;
    in
    {
      # which stdenv to use, defaults to a stdenv with a C compiler, pkgs.stdenv
      stdenv ? defaultStdenv,
      # whether to build this derivation locally instead of substituting
      runLocal ? false,
      # extra arguments to pass to stdenv.mkDerivation
      derivationArgs ? { },
      # name of the resulting derivation
      name,
    # TODO(@Artturin): enable strictDeps always
    }:
    buildCommand:
    stdenv.mkDerivation (
      {
        enableParallelBuilding = true;
        inherit buildCommand name;
        passAsFile = [ "buildCommand" ] ++ (derivationArgs.passAsFile or [ ]);
      }
      // lib.optionalAttrs (!derivationArgs ? meta) {
        pos =
          let
            args = builtins.attrNames derivationArgs;
          in
          if builtins.length args > 0 then
            builtins.unsafeGetAttrPos (builtins.head args) derivationArgs
          else
            null;
      }
      // (lib.optionalAttrs runLocal {
        preferLocalBuild = true;
        allowSubstitutes = false;
      })
      // builtins.removeAttrs derivationArgs [ "passAsFile" ]
    );

  # Docs in doc/build-helpers/trivial-build-helpers.chapter.md
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-writeTextFile
  writeTextFile =
    {
      name,
      text,
      executable ? false,
      destination ? "",
      checkPhase ? "",
      meta ? { },
      allowSubstitutes ? false,
      preferLocalBuild ? true,
      derivationArgs ? { },
    }:
    let
      matches = builtins.match "/bin/([^/]+)" destination;
    in
    runCommand name
      (
        {
          inherit
            text
            executable
            checkPhase
            allowSubstitutes
            preferLocalBuild
            ;
          passAsFile = [ "text" ] ++ derivationArgs.passAsFile or [ ];
          meta =
            lib.optionalAttrs (executable && matches != null) { mainProgram = lib.head matches; }
            // meta
            // derivationArgs.meta or { };
        }
        // removeAttrs derivationArgs [
          "passAsFile"
          "meta"
        ]
      )
      ''
        target=$out${lib.escapeShellArg destination}
        mkdir -p "$(dirname "$target")"

        if [ -e "$textPath" ]; then
          mv "$textPath" "$target"
        else
          echo -n "$text" > "$target"
        fi

        if [ -n "$executable" ]; then
          chmod +x "$target"
        fi

        eval "$checkPhase"
      '';

  # See doc/build-helpers/trivial-build-helpers.chapter.md
  # or https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-text-writing
  writeText = name: text: writeTextFile { inherit name text; };

  # See doc/build-helpers/trivial-build-helpers.chapter.md
  # or https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-text-writing
  writeTextDir =
    path: text:
    writeTextFile {
      inherit text;
      name = builtins.baseNameOf path;
      destination = "/${path}";
    };

  # See doc/build-helpers/trivial-build-helpers.chapter.md
  # or https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-text-writing
  writeScript =
    name: text:
    writeTextFile {
      inherit name text;
      executable = true;
    };

  # See doc/build-helpers/trivial-build-helpers.chapter.md
  # or https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-text-writing
  writeScriptBin =
    name: text:
    writeTextFile {
      inherit name text;
      executable = true;
      destination = "/bin/${name}";
    };

  # See doc/build-helpers/trivial-build-helpers.chapter.md
  # or https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-text-writing
  writeShellScript =
    name: text:
    writeTextFile {
      inherit name;
      executable = true;
      text = ''
        #!${runtimeShell}
        ${text}
      '';
      checkPhase = ''
        ${stdenv.shellDryRun} "$target"
      '';
    };

  # See doc/build-helpers/trivial-build-helpers.chapter.md
  # or https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-text-writing
  writeShellScriptBin =
    name: text:
    writeTextFile {
      inherit name;
      executable = true;
      destination = "/bin/${name}";
      text = ''
        #!${runtimeShell}
        ${text}
      '';
      checkPhase = ''
        ${stdenv.shellDryRun} "$target"
      '';
      meta.mainProgram = name;
    };

  # TODO: move parameter documentation to the Nixpkgs manual
  # See doc/build-helpers/trivial-build-helpers.chapter.md
  # or https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-writeShellApplication
  writeShellApplication =
    {
      /*
         The name of the script to write.

         Type: String
      */
      name,
      /*
         The shell script's text, not including a shebang.

         Type: String
      */
      text,
      /*
         Inputs to add to the shell script's `$PATH` at runtime.

         Type: [String|Derivation]
      */
      runtimeInputs ? [ ],
      /*
         Extra environment variables to set at runtime.

         Type: AttrSet
      */
      runtimeEnv ? null,
      /*
         `stdenv.mkDerivation`'s `meta` argument.

         Type: AttrSet
      */
      meta ? { },
      /*
         The `checkPhase` to run. Defaults to `shellcheck` on supported
         platforms and `bash -n`.

         The script path will be given as `$target` in the `checkPhase`.

         Type: String
      */
      checkPhase ? null,
      /*
         Checks to exclude when running `shellcheck`, e.g. `[ "SC2016" ]`.

         See <https://www.shellcheck.net/wiki/> for a list of checks.

         Type: [String]
      */
      excludeShellChecks ? [ ],
      /*
         Extra command-line flags to pass to ShellCheck.

         Type: [String]
      */
      extraShellCheckFlags ? [ ],
      /*
         Bash options to activate with `set -o` at the start of the script.

         Defaults to `[ "errexit" "nounset" "pipefail" ]`.

         Type: [String]
      */
      bashOptions ? [
        "errexit"
        "nounset"
        "pipefail"
      ],
      /*
        Extra arguments to pass to `stdenv.mkDerivation`.

        :::{.caution}
        Certain derivation attributes are used internally,
        overriding those could cause problems.
        :::

        Type: AttrSet
      */
      derivationArgs ? { },
    }:
    writeTextFile {
      inherit name meta derivationArgs;
      executable = true;
      destination = "/bin/${name}";
      allowSubstitutes = true;
      preferLocalBuild = false;
      text =
        ''
          #!${runtimeShell}
          ${lib.concatMapStringsSep "\n" (option: "set -o ${option}") bashOptions}
        ''
        + lib.optionalString (runtimeEnv != null) (
          lib.concatStrings (
            lib.mapAttrsToList (name: value: ''
              ${lib.toShellVar name value}
              export ${name}
            '') runtimeEnv
          )
        )
        + lib.optionalString (runtimeInputs != [ ]) ''

          export PATH="${lib.makeBinPath runtimeInputs}:$PATH"
        ''
        + ''

          ${text}
        '';

      checkPhase =
        # GHC (=> shellcheck) isn't supported on some platforms (such as risc-v)
        # but we still want to use writeShellApplication on those platforms
        let
          shellcheckSupported = lib.meta.availableOn stdenv.buildPlatform shellcheck-minimal.compiler;
          excludeFlags = lib.optionals (excludeShellChecks != [ ]) [
            "--exclude"
            (lib.concatStringsSep "," excludeShellChecks)
          ];
          shellcheckCommand = lib.optionalString shellcheckSupported ''
            # use shellcheck which does not include docs
            # pandoc takes long to build and documentation isn't needed for just running the cli
            ${lib.getExe shellcheck-minimal} ${
              lib.escapeShellArgs (excludeFlags ++ extraShellCheckFlags)
            } "$target"
          '';
        in
        if checkPhase == null then
          ''
            runHook preCheck
            ${stdenv.shellDryRun} "$target"
            ${shellcheckCommand}
            runHook postCheck
          ''
        else
          checkPhase;
    };

  # Create a C binary
  # TODO: add to writers? pkgs/build-support/writers
  writeCBin =
    pname: code:
    runCommandCC pname
      {
        inherit pname code;
        executable = true;
        passAsFile = [ "code" ];
        # Pointless to do this on a remote machine.
        preferLocalBuild = true;
        allowSubstitutes = false;
        meta = {
          mainProgram = pname;
        };
      }
      ''
        n=$out/bin/${pname}
        mkdir -p "$(dirname "$n")"
        mv "$codePath" code.c
        $CC -x c code.c -o "$n"
      '';

  # TODO: deduplicate with documentation in doc/build-helpers/trivial-build-helpers.chapter.md
  #       see also https://github.com/NixOS/nixpkgs/pull/249721
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-concatText
  /*
    concat a list of files to the nix store.
    The contents of files are added to the file in the store.

    Example:

    # Writes my-file to /nix/store/<store path>
    concatTextFile {
      name = "my-file";
      files = [ drv1 "${drv2}/path/to/file" ];
    }

    See also the `concatText` helper function below.

    # Writes executable my-file to /nix/store/<store path>/bin/my-file
    concatTextFile {
      name = "my-file";
      files = [ drv1 "${drv2}/path/to/file" ];
      executable = true;
      destination = "/bin/my-file";
    }
  */
  concatTextFile =
    {
      name, # the name of the derivation
      files,
      executable ? false, # run chmod +x ?
      destination ? "", # relative path appended to $out eg "/bin/foo"
      checkPhase ? "", # syntax checks, e.g. for scripts
      meta ? { },
    }:
    runCommandLocal name
      {
        inherit
          files
          executable
          checkPhase
          meta
          destination
          ;
      }
      ''
        file=$out$destination
        mkdir -p "$(dirname "$file")"
        cat $files > "$file"

        if [ -n "$executable" ]; then
          chmod +x "$file"
        fi

        eval "$checkPhase"
      '';

  # TODO: deduplicate with documentation in doc/build-helpers/trivial-build-helpers.chapter.md
  #       see also https://github.com/NixOS/nixpkgs/pull/249721
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-concatText
  /*
    Writes a text file to nix store with no optional parameters available.

    Example:

    # Writes contents of files to /nix/store/<store path>
    concatText "my-file" [ file1 file2 ]
  */
  concatText = name: files: concatTextFile { inherit name files; };

  # TODO: deduplicate with documentation in doc/build-helpers/trivial-build-helpers.chapter.md
  #       see also https://github.com/NixOS/nixpkgs/pull/249721
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-concatText
  /*
    Writes a text file to nix store with and mark it as executable.

    Example:
    # Writes contents of files to /nix/store/<store path>
    concatScript "my-file" [ file1 file2 ]
  */
  concatScript =
    name: files:
    concatTextFile {
      inherit name files;
      executable = true;
    };

  /*
    TODO: Deduplicate this documentation.
    More docs in doc/build-helpers/trivial-build-helpers.chapter.md
    See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-symlinkJoin

    Create a forest of symlinks to the files in `paths`.

    This creates a single derivation that replicates the directory structure
    of all the input paths.

    BEWARE: it may not "work right" when the passed paths contain symlinks to directories.

    Example:

    # adds symlinks of hello to current build.
    symlinkJoin { name = "myhello"; paths = [ pkgs.hello ]; }

    # adds symlinks of hello and stack to current build and prints "links added"
    symlinkJoin { name = "myexample"; paths = [ pkgs.hello pkgs.stack ]; postBuild = "echo links added"; }

    This creates a derivation with a directory structure like the following:

    /nix/store/sglsr5g079a5235hy29da3mq3hv8sjmm-myexample
    |-- bin
    |   |-- hello -> /nix/store/qy93dp4a3rqyn2mz63fbxjg228hffwyw-hello-2.10/bin/hello
    |   `-- stack -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/bin/stack
    `-- share
        |-- bash-completion
        |   `-- completions
        |       `-- stack -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/share/bash-completion/completions/stack
        |-- fish
        |   `-- vendor_completions.d
        |       `-- stack.fish -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1/share/fish/vendor_completions.d/stack.fish
    ...

    symlinkJoin and linkFarm are similar functions, but they output
    derivations with different structure.

    symlinkJoin is used to create a derivation with a familiar directory
    structure (top-level bin/, share/, etc), but with all actual files being symlinks to
    the files in the input derivations.

    symlinkJoin is used many places in nixpkgs to create a single derivation
    that appears to contain binaries, libraries, documentation, etc from
    multiple input derivations.

    linkFarm is instead used to create a simple derivation with symlinks to
    other derivations.  A derivation created with linkFarm is often used in CI
    as a easy way to build multiple derivations at once.
  */
  symlinkJoin =
    args_@{
      name,
      paths,
      preferLocalBuild ? true,
      allowSubstitutes ? false,
      postBuild ? "",
      ...
    }:
    let
      args =
        removeAttrs args_ [
          "name"
          "postBuild"
        ]
        // {
          inherit preferLocalBuild allowSubstitutes;
          passAsFile = [ "paths" ];
        }; # pass the defaults
    in
    runCommand name args ''
      mkdir -p $out
      for i in $(cat $pathsPath); do
        ${lndir}/bin/lndir -silent $i $out
      done
      ${postBuild}
    '';

  # TODO: move linkFarm docs to the Nixpkgs manual
  /*
    Quickly create a set of symlinks to derivations.

    This creates a simple derivation with symlinks to all inputs.

    entries can be a list of attribute sets like

    [ { name = "name" ; path = "/nix/store/..."; } ]

    or an attribute set name -> path like:

    { name = "/nix/store/..."; other = "/nix/store/..."; }

    Example:

    # Symlinks hello and stack paths in store to current $out/hello-test and
    # $out/foobar.
    linkFarm "myexample" [ { name = "hello-test"; path = pkgs.hello; } { name = "foobar"; path = pkgs.stack; } ]

    This creates a derivation with a directory structure like the following:

    /nix/store/qc5728m4sa344mbks99r3q05mymwm4rw-myexample
    |-- foobar -> /nix/store/6lzdpxshx78281vy056lbk553ijsdr44-stack-2.1.3.1
    `-- hello-test -> /nix/store/qy93dp4a3rqyn2mz63fbxjg228hffwyw-hello-2.10

    See the note on symlinkJoin for the difference between linkFarm and symlinkJoin.
  */
  linkFarm =
    name: entries:
    let
      entries' =
        if (lib.isAttrs entries) then
          entries
        # We do this foldl to have last-wins semantics in case of repeated entries
        else if (lib.isList entries) then
          lib.foldl (a: b: a // { "${b.name}" = b.path; }) { } entries
        else
          throw "linkFarm entries must be either attrs or a list!";

      linkCommands = lib.mapAttrsToList (name: path: ''
        mkdir -p "$(dirname ${lib.escapeShellArg "${name}"})"
        ln -s ${lib.escapeShellArg "${path}"} ${lib.escapeShellArg "${name}"}
      '') entries';
    in
    runCommand name
      {
        preferLocalBuild = true;
        allowSubstitutes = false;
        passthru.entries = entries';
      }
      ''
        mkdir -p $out
        cd $out
        ${lib.concatStrings linkCommands}
      '';

  # TODO: move linkFarmFromDrvs docs to the Nixpkgs manual
  /*
    Easily create a linkFarm from a set of derivations.

    This calls linkFarm with a list of entries created from the list of input
    derivations.  It turns each input derivation into an attribute set
    like { name = drv.name ; path = drv }, and passes this to linkFarm.

    Example:

    # Symlinks the hello, gcc, and ghc derivations in $out
    linkFarmFromDrvs "myexample" [ pkgs.hello pkgs.gcc pkgs.ghc ]

    This creates a derivation with a directory structure like the following:

    /nix/store/m3s6wkjy9c3wy830201bqsb91nk2yj8c-myexample
    |-- gcc-wrapper-9.2.0 -> /nix/store/fqhjxf9ii4w4gqcsx59fyw2vvj91486a-gcc-wrapper-9.2.0
    |-- ghc-8.6.5 -> /nix/store/gnf3s07bglhbbk4y6m76sbh42siym0s6-ghc-8.6.5
    `-- hello-2.10 -> /nix/store/k0ll91c4npk4lg8lqhx00glg2m735g74-hello-2.10
  */
  linkFarmFromDrvs =
    name: drvs:
    let
      mkEntryFromDrv = drv: {
        name = drv.name;
        path = drv;
      };
    in
    linkFarm name (map mkEntryFromDrv drvs);

  # TODO: move onlyBin docs to the Nixpkgs manual
  /*
    Produce a derivation that links to the target derivation's `/bin`,
    and *only* `/bin`.

    This is useful when your favourite package doesn't have a separate
    bin output and other contents of the package's output (e.g. setup
    hooks) cause trouble when used in your environment.
  */
  onlyBin =
    drv:
    runCommand "${drv.name}-only-bin" { } ''
      mkdir -p $out
      ln -s ${lib.getBin drv}/bin $out/bin
    '';

  # Docs in doc/builders/special/makesetuphook.section.md
  # See https://nixos.org/manual/nixpkgs/unstable/#sec-pkgs.makeSetupHook
  makeSetupHook =
    {
      name ? lib.warn "calling makeSetupHook without passing a name is deprecated." "hook",
      deps ? [ ],
      # hooks go in nativeBuildInput so these will be nativeBuildInput
      propagatedBuildInputs ? [ ],
      # these will be buildInputs
      depsTargetTargetPropagated ? [ ],
      meta ? { },
      passthru ? { },
      substitutions ? { },
    }:
    script:
    runCommand name
      (
        substitutions
        // {
          # TODO(@Artturin:) substitutions should be inside the env attrset
          # but users are likely passing non-substitution arguments through substitutions
          # turn off __structuredAttrs to unbreak substituteAll
          __structuredAttrs = false;
          inherit meta;
          inherit depsTargetTargetPropagated;
          propagatedBuildInputs =
            # remove list conditionals before 23.11
            lib.warnIf (!lib.isList deps)
              "'deps' argument to makeSetupHook must be a list. content of deps: ${toString deps}"
              (
                lib.warnIf (deps != [ ])
                  "'deps' argument to makeSetupHook is deprecated and will be removed in release 23.11., Please use propagatedBuildInputs instead. content of deps: ${toString deps}"
                  propagatedBuildInputs
                ++ (if lib.isList deps then deps else [ deps ])
              );
          strictDeps = true;
          # TODO 2023-01, no backport: simplify to inherit passthru;
          passthru =
            passthru
            // optionalAttrs (substitutions ? passthru) (
              warn "makeSetupHook (name = ${lib.strings.escapeNixString name}): `substitutions.passthru` is deprecated. Please set `passthru` directly." substitutions.passthru
            );
        }
      )
      (
        ''
          mkdir -p $out/nix-support
          cp ${script} $out/nix-support/setup-hook
          recordPropagatedDependencies
        ''
        + lib.optionalString (substitutions != { }) ''
          substituteAll ${script} $out/nix-support/setup-hook
        ''
      );

  # Docs in doc/build-helpers/trivial-build-helpers.chapter.md
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-writeReferencesToFile
  # TODO: Convert to throw after Nixpkgs 24.05 branch-off.
  writeReferencesToFile =
    (if config.allowAliases then lib.warn else throw)
      "writeReferencesToFile is deprecated in favour of writeClosure"
      (path: writeClosure [ path ]);

  # Docs in doc/build-helpers/trivial-build-helpers.chapter.md
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-writeClosure
  writeClosure =
    paths:
    runCommand "runtime-deps"
      {
        # Get the cleaner exportReferencesGraph interface
        __structuredAttrs = true;
        exportReferencesGraph.graph = paths;
        nativeBuildInputs = [ jq ];
      }
      ''
        jq -r ".graph | map(.path) | sort | .[]" "$NIX_ATTRS_JSON_FILE" > "$out"
      '';

  # Docs in doc/build-helpers/trivial-build-helpers.chapter.md
  # See https://nixos.org/manual/nixpkgs/unstable/#trivial-builder-writeDirectReferencesToFile
  writeDirectReferencesToFile =
    path:
    runCommand "runtime-references"
      {
        exportReferencesGraph = [
          "graph"
          path
        ];
        inherit path;
      }
      ''
        touch ./references
        while read p; do
          read dummy
          read nrRefs
          if [[ $p == $path ]]; then
            for ((i = 0; i < nrRefs; i++)); do
              read ref;
              echo $ref >>./references
            done
          else
            for ((i = 0; i < nrRefs; i++)); do
              read ref;
            done
          fi
        done < graph
        sort ./references >$out
      '';

  # TODO: move writeStringReferencesToFile docs to the Nixpkgs manual
  /*
    Extract a string's references to derivations and paths (its
    context) and write them to a text file, removing the input string
    itself from the dependency graph. This is useful when you want to
    make a derivation depend on the string's references, but not its
    contents (to avoid unnecessary rebuilds, for example).

    Note that this only works as intended on Nix >= 2.3.
  */
  writeStringReferencesToFile =
    string:
    /*
       The basic operation this performs is to copy the string context
       from `string` to a second string and wrap that string in a
       derivation. However, that alone is not enough, since nothing in the
       string refers to the output paths of the derivations/paths in its
       context, meaning they'll be considered build-time dependencies and
       removed from the wrapper derivation's closure. Putting the
       necessary output paths in the new string is however not very
       straightforward - the attrset returned by `getContext` contains
       only references to derivations' .drv-paths, not their output
       paths. In order to "convert" them, we try to extract the
       corresponding paths from the original string using regex.
    */
    let
      # Taken from https://github.com/NixOS/nix/blob/130284b8508dad3c70e8160b15f3d62042fc730a/src/libutil/hash.cc#L84
      nixHashChars = "0123456789abcdfghijklmnpqrsvwxyz";
      context = builtins.getContext string;
      derivations = lib.filterAttrs (n: v: v ? outputs) context;
      # Objects copied from outside of the store, such as paths and
      # `builtins.fetch*`ed ones
      sources = lib.attrNames (lib.filterAttrs (n: v: v ? path) context);
      packages = lib.mapAttrs' (name: value: {
        inherit value;
        name = lib.head (builtins.match "${builtins.storeDir}/[${nixHashChars}]+-(.*)\.drv" name);
      }) derivations;
      # The syntax of output paths differs between outputs named `out`
      # and other, explicitly named ones. For explicitly named ones,
      # the output name is suffixed as `-name`, but `out` outputs
      # aren't suffixed at all, and thus aren't easily distinguished
      # from named output paths. Therefore, we find all the named ones
      # first so we can use them to remove false matches when looking
      # for `out` outputs (see the definition of `outputPaths`).
      namedOutputPaths = lib.flatten (
        lib.mapAttrsToList (
          name: value:
          (map (
            output:
            lib.filter lib.isList (
              builtins.split "(${builtins.storeDir}/[${nixHashChars}]+-${name}-${output})" string
            )
          ) (lib.remove "out" value.outputs))
        ) packages
      );
      # Only `out` outputs
      outputPaths = lib.flatten (
        lib.mapAttrsToList (
          name: value:
          if lib.elem "out" value.outputs then
            lib.filter (
              x:
              lib.isList x
              &&
                # If the matched path is in `namedOutputPaths`,
                # it's a partial match of an output path where
                # the output name isn't `out`
                lib.all (o: !lib.hasPrefix (lib.head x) o) namedOutputPaths
            ) (builtins.split "(${builtins.storeDir}/[${nixHashChars}]+-${name})" string)
          else
            [ ]
        ) packages
      );
      allPaths = lib.concatStringsSep "\n" (lib.unique (sources ++ namedOutputPaths ++ outputPaths));
      allPathsWithContext = builtins.appendContext allPaths context;
    in
    if builtins ? getContext then
      writeText "string-references" allPathsWithContext
    else
      writeDirectReferencesToFile (writeText "string-file" string);

  # Docs in doc/build-helpers/fetchers.chapter.md
  # See https://nixos.org/manual/nixpkgs/unstable/#requirefile
  requireFile =
    {
      name ? null,
      sha256 ? null,
      sha1 ? null,
      hash ? null,
      url ? null,
      message ? null,
      hashMode ? "flat",
    }:
    assert (message != null) || (url != null);
    assert (sha256 != null) || (sha1 != null) || (hash != null);
    assert (name != null) || (url != null);
    let
      msg =
        if message != null then
          message
        else
          ''
            Unfortunately, we cannot download file ${name_} automatically.
            Please go to ${url} to download it yourself, and add it to the Nix store
            using either
              nix-store --add-fixed ${hashAlgo} ${name_}
            or
              nix-prefetch-url --type ${hashAlgo} file:///path/to/${name_}
          '';
      hashAlgo =
        if hash != null then
          (builtins.head (lib.strings.splitString "-" hash))
        else if sha256 != null then
          "sha256"
        else
          "sha1";
      hashAlgo_ = if hash != null then "" else hashAlgo;
      hash_ =
        if hash != null then
          hash
        else if sha256 != null then
          sha256
        else
          sha1;
      name_ = if name == null then baseNameOf (toString url) else name;
    in
    stdenvNoCC.mkDerivation {
      name = name_;
      outputHashMode = hashMode;
      outputHashAlgo = hashAlgo_;
      outputHash = hash_;
      preferLocalBuild = true;
      allowSubstitutes = false;
      builder = writeScript "restrict-message" ''
        source ${stdenvNoCC}/setup
        cat <<_EOF_

        ***
        ${msg}
        ***

        _EOF_
        exit 1
      '';
    };

  # TODO: move copyPathToStore docs to the Nixpkgs manual
  /*
    Copy a path to the Nix store.
    Nix automatically copies files to the store before stringifying paths.
    If you need the store path of a file, ${copyPathToStore <path>} can be
    shortened to ${<path>}.
  */
  copyPathToStore = builtins.filterSource (p: t: true);

  # TODO: move copyPathsToStore docs to the Nixpkgs manual
  # Copy a list of paths to the Nix store.
  copyPathsToStore = builtins.map copyPathToStore;

  # TODO: move applyPatches docs to the Nixpkgs manual
  /*
    Applies a list of patches to a source directory.

    Example:

    # Patching nixpkgs:

    applyPatches {
      src = pkgs.path;
      patches = [
        (pkgs.fetchpatch {
          url = "https://github.com/NixOS/nixpkgs/commit/1f770d20550a413e508e081ddc08464e9d08ba3d.patch";
          sha256 = "1nlzx171y3r3jbk0qhvnl711kmdk57jlq4na8f8bs8wz2pbffymr";
        })
      ];
    }
  */
  applyPatches =
    {
      src,
      name ?
        (
          if builtins.typeOf src == "path" then
            builtins.baseNameOf src
          else if builtins.isAttrs src && builtins.hasAttr "name" src then
            src.name
          else
            throw "applyPatches: please supply a `name` argument because a default name can only be computed when the `src` is a path or is an attribute set with a `name` attribute."
        )
        + "-patched",
      patches ? [ ],
      prePatch ? "",
      postPatch ? "",
      ...
    }@args:
    if patches == [ ] && prePatch == "" && postPatch == "" then
      src # nothing to do, so use original src to avoid additional drv
    else
      stdenvNoCC.mkDerivation (
        {
          inherit
            name
            src
            patches
            prePatch
            postPatch
            ;
          preferLocalBuild = true;
          allowSubstitutes = false;
          phases = "unpackPhase patchPhase installPhase";
          installPhase = "cp -R ./ $out";
        }
        # Carry `meta` information from the underlying `src` if present.
        // (optionalAttrs (src ? meta) { inherit (src) meta; })
        // (removeAttrs args [
          "src"
          "name"
          "patches"
          "prePatch"
          "postPatch"
        ])
      );

  # TODO: move docs to Nixpkgs manual
  # An immutable file in the store with a length of 0 bytes.
  emptyFile = runCommand "empty-file" {
    outputHashAlgo = "sha256";
    outputHashMode = "recursive";
    outputHash = "0ip26j2h11n1kgkz36rl4akv694yz65hr72q4kv4b3lxcbi65b3p";
    preferLocalBuild = true;
  } "touch $out";

  # TODO: move docs to Nixpkgs manual
  # An immutable empty directory in the store.
  emptyDirectory = runCommand "empty-directory" {
    outputHashAlgo = "sha256";
    outputHashMode = "recursive";
    outputHash = "0sjjj9z1dhilhpc8pq4154czrb79z9cm044jvn75kxcjv6v5l2m5";
    preferLocalBuild = true;
  } "mkdir $out";
}