{
  description = "The purely functional package manager";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11-small";
    nixpkgs-regression.url = "github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2";
    pre-commit-hooks = {
      # Can go back to `cachix/git-hooks.nix` when this is merged:
      # https://github.com/cachix/git-hooks.nix/pull/401
      url = "github:9999years/git-hooks.nix/add-default-pre-commit-hooks";
      inputs = {
        flake-compat.follows = "flake-compat";
        nixpkgs.follows = "nixpkgs";
        nixpkgs-stable.follows = "nixpkgs";
      };
    };
    flake-compat = { url = "github:edolstra/flake-compat"; flake = false; };
  };

  outputs = { self, nixpkgs, nixpkgs-regression, pre-commit-hooks, flake-compat }:

    let
      inherit (nixpkgs) lib;
      inherit (lib) fileset;

      officialRelease = true;

      # Set to true to build the release notes for the next release.
      buildUnreleasedNotes = false;

      version = lib.fileContents ./.version + versionSuffix;
      versionSuffix =
        if officialRelease
        then ""
        else "pre${builtins.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}_${self.shortRev or "dirty"}";

      linux32BitSystems = [ "i686-linux" ];
      linux64BitSystems = [ "x86_64-linux" "aarch64-linux" ];
      linuxSystems = linux32BitSystems ++ linux64BitSystems;
      darwinSystems = [ "x86_64-darwin" "aarch64-darwin" ];
      systems = linuxSystems ++ darwinSystems;

      crossSystems = [
        "armv6l-linux" "armv7l-linux"
        "x86_64-freebsd13" "x86_64-netbsd"
      ];

      stdenvs = [ "gccStdenv" "clangStdenv" "stdenv" "libcxxStdenv" "ccacheStdenv" ];

      forAllSystems = lib.genAttrs systems;

      forAllCrossSystems = lib.genAttrs crossSystems;

      forAllStdenvs = f:
        lib.listToAttrs
          (map
            (stdenvName: {
              name = "${stdenvName}Packages";
              value = f stdenvName;
            })
            stdenvs);

      # Memoize nixpkgs for different platforms for efficiency.
      nixpkgsFor = forAllSystems
        (system: let
          make-pkgs = crossSystem: stdenv: import nixpkgs {
            localSystem = {
              inherit system;
            };
            crossSystem = if crossSystem == null then null else {
              system = crossSystem;
            } // lib.optionalAttrs (crossSystem == "x86_64-freebsd13") {
              useLLVM = true;
            };
            overlays = [
              (overlayFor (p: p.${stdenv}))
            ];

            config.permittedInsecurePackages = [ "nix-2.13.6" ];
          };
          stdenvs = forAllStdenvs (make-pkgs null);
          native = stdenvs.stdenvPackages;
        in {
          inherit stdenvs native;
          static = native.pkgsStatic;
          cross = forAllCrossSystems (crossSystem: make-pkgs crossSystem "stdenv");
        });

      testNixVersions = pkgs: client: daemon: let
        nix = pkgs.callPackage ./package.nix {
          pname =
            "nix-tests"
            + lib.optionalString
            (lib.versionAtLeast daemon.version "2.4pre20211005" &&
            lib.versionAtLeast client.version "2.4pre20211005")
            "-${client.version}-against-${daemon.version}";

            inherit fileset;
        };
      in nix.overrideAttrs (prevAttrs: {
        NIX_DAEMON_PACKAGE = daemon;
        NIX_CLIENT_PACKAGE = client;

        dontBuild = true;
        doInstallCheck = true;

        configureFlags = prevAttrs.configureFlags ++ [
          # We don't need the actual build here.
          "--disable-build"
        ];

        installPhase = ''
          mkdir -p $out
        '';

        installCheckPhase = lib.optionalString pkgs.stdenv.hostPlatform.isDarwin ''
          export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES
        '' + ''
          mkdir -p src/nix-channel
          make installcheck -j$NIX_BUILD_CORES -l$NIX_BUILD_CORES
        '';
      });

      binaryTarball = nix: pkgs:
        let
          inherit (pkgs) buildPackages;
          installerClosureInfo = buildPackages.closureInfo { rootPaths = [ nix ]; };
        in

        buildPackages.runCommand "nix-binary-tarball-${version}"
          { #nativeBuildInputs = lib.optional (system != "aarch64-linux") shellcheck;
            meta.description = "Distribution-independent Nix bootstrap binaries for ${pkgs.system}";
          }
          ''
            cp ${installerClosureInfo}/registration $TMPDIR/reginfo

            dir=nix-${version}-${pkgs.system}
            fn=$out/$dir.tar.xz
            mkdir -p $out/nix-support
            echo "file binary-dist $fn" >> $out/nix-support/hydra-build-products
            tar cvfJ $fn \
              --owner=0 --group=0 --mode=u+rw,uga+r \
              --mtime='1970-01-01' \
              --absolute-names \
              --hard-dereference \
              --transform "s,$TMPDIR/reginfo,$dir/.reginfo," \
              --transform "s,$NIX_STORE,$dir/store,S" \
              $TMPDIR/reginfo \
              $(cat ${installerClosureInfo}/store-paths)
          '';

      overlayFor = getStdenv: final: prev:
        let
          currentStdenv = getStdenv final;
          comDeps = with final; commonDeps {
            inherit pkgs;
            inherit (currentStdenv.hostPlatform) isStatic;
          };
        in {
          nixStable = prev.nix;

          # Forward from the previous stage as we don’t want it to pick the lowdown override
          nixUnstable = prev.nixUnstable;

          build-release-notes =
            final.buildPackages.callPackage ./maintainers/build-release-notes.nix { };
          clangbuildanalyzer = final.buildPackages.callPackage ./misc/clangbuildanalyzer.nix { };
          boehmgc-nix = (final.boehmgc.override {
            enableLargeConfig = true;
          }).overrideAttrs (o: {
            patches = (o.patches or [ ]) ++ [
              ./boehmgc-coroutine-sp-fallback.diff

              # https://github.com/ivmai/bdwgc/pull/586
              ./boehmgc-traceable_allocator-public.diff
            ];
          });

          default-busybox-sandbox-shell = final.busybox.override {
            useMusl = true;
            enableStatic = true;
            enableMinimal = true;
            extraConfig = ''
              CONFIG_FEATURE_FANCY_ECHO y
              CONFIG_FEATURE_SH_MATH y
              CONFIG_FEATURE_SH_MATH_64 y

              CONFIG_ASH y
              CONFIG_ASH_OPTIMIZE_FOR_SIZE y

              CONFIG_ASH_ALIAS y
              CONFIG_ASH_BASH_COMPAT y
              CONFIG_ASH_CMDCMD y
              CONFIG_ASH_ECHO y
              CONFIG_ASH_GETOPTS y
              CONFIG_ASH_INTERNAL_GLOB y
              CONFIG_ASH_JOB_CONTROL y
              CONFIG_ASH_PRINTF y
              CONFIG_ASH_TEST y
            '';
          };

          nix = final.callPackage ./package.nix {
            inherit versionSuffix fileset;
            stdenv = currentStdenv;
            boehmgc = final.boehmgc-nix;
            busybox-sandbox-shell = final.busybox-sandbox-shell or final.default-busybox-sandbox-shell;
          };
        };

    in {
      # A Nixpkgs overlay that overrides the 'nix' and
      # 'nix.perl-bindings' packages.
      overlays.default = overlayFor (p: p.stdenv);

      hydraJobs = {

        # Binary package for various platforms.
        build = forAllSystems (system: self.packages.${system}.nix);

        # FIXME(Qyriad): remove this when the migration to Meson has been completed.
        # NOTE: mesonBuildClang depends on mesonBuild depends on build to avoid OOMs
        # on aarch64 builders caused by too many parallel compiler/linker processes.
        mesonBuild = forAllSystems (system:
          (self.packages.${system}.nix.override {
            buildWithMeson = true;
          }).overrideAttrs (prev: {
            buildInputs = prev.buildInputs ++ [ self.packages.${system}.nix ];
          }));
        mesonBuildClang = forAllSystems (system:
          (nixpkgsFor.${system}.stdenvs.clangStdenvPackages.nix.override {
              buildWithMeson = true;
          }).overrideAttrs (prev: {
            buildInputs = prev.buildInputs ++ [ self.hydraJobs.mesonBuild.${system} ];
          })
        );

        # Perl bindings for various platforms.
        perlBindings = forAllSystems (system: nixpkgsFor.${system}.native.nix.perl-bindings);

        # Binary tarball for various platforms, containing a Nix store
        # with the closure of 'nix' package.
        binaryTarball = forAllSystems (system: binaryTarball nixpkgsFor.${system}.native.nix nixpkgsFor.${system}.native);

        # docker image with Nix inside
        dockerImage = lib.genAttrs linux64BitSystems (system: self.packages.${system}.dockerImage);

        # API docs for Nix's unstable internal C++ interfaces.
        internal-api-docs = let
          nixpkgs = nixpkgsFor.x86_64-linux.native;
          inherit (nixpkgs) pkgs;

          nix = pkgs.callPackage ./package.nix {
            inherit versionSuffix fileset officialRelease buildUnreleasedNotes;
            inherit (pkgs) build-release-notes;
            internalApiDocs = true;
            boehmgc = pkgs.boehmgc-nix;
            busybox-sandbox-shell = pkgs.busybox-sandbox-shell;
          };
        in
          nix.overrideAttrs (prev: {
            # This Hydra job is just for the internal API docs.
            # We don't need the build artifacts here.
            dontBuild = true;
            doCheck = false;
            doInstallCheck = false;
          });

        # System tests.
        tests = import ./tests/nixos { inherit lib nixpkgs nixpkgsFor; } // {

          # Make sure that nix-env still produces the exact same result
          # on a particular version of Nixpkgs.
          evalNixpkgs =
            with nixpkgsFor.x86_64-linux.native;
            runCommand "eval-nixos" { buildInputs = [ nix ]; }
              ''
                type -p nix-env
                # Note: we're filtering out nixos-install-tools because https://github.com/NixOS/nixpkgs/pull/153594#issuecomment-1020530593.
                time nix-env --store dummy:// -f ${nixpkgs-regression} -qaP --drv-path | sort | grep -v nixos-install-tools > packages
                [[ $(sha1sum < packages | cut -c1-40) = 402242fca90874112b34718b8199d844e8b03d12 ]]
                mkdir $out
              '';

          nixpkgsLibTests =
            forAllSystems (system:
              import (nixpkgs + "/lib/tests/release.nix")
                { pkgs = nixpkgsFor.${system}.native;
                  nixVersions = [ self.packages.${system}.nix ];
                }
            );
        };

        pre-commit = builtins.mapAttrs (system: pre-commit-lib: pre-commit-lib.run {
          src = self;
          hooks = {
            no-commit-to-branch = {
              enable = true;
              settings.branch = ["main"];
            };
            check-case-conflicts.enable = true;
            check-executables-have-shebangs = {
              enable = true;
              stages = [ "commit" ];
            };
            check-shebang-scripts-are-executable = {
              enable = true;
              stages = [ "commit" ];
            };
            check-symlinks = {
              enable = true;
              excludes = [ "^tests/functional/lang/symlink-resolution/broken$" ];
            };
            check-merge-conflicts.enable = true;
            end-of-file-fixer = {
              enable = true;
              excludes = [
                "\\.drv$"
                "^tests/functional/lang/"
              ];
            };
            mixed-line-endings = {
              enable = true;
              excludes = [ "^tests/functional/lang/" ];
            };
            # TODO: Once the test suite is nicer, clean up and start
            # enforcing trailing whitespace on tests that don't explicitly
            # check for it.
            trim-trailing-whitespace = {
              enable = true;
              stages = [ "commit" ];
              excludes = [ "^tests/functional/lang/" ];
            };
          };
        }) pre-commit-hooks.lib;
      };

      checks = forAllSystems (system: let
        rl-next-check = name: dir:
          let pkgs = nixpkgsFor.${system}.native;
          in pkgs.buildPackages.runCommand "test-${name}-release-notes" { } ''
          LANG=C.UTF-8 ${lib.getExe pkgs.build-release-notes} ${dir} >$out
        '';
      in {
        # FIXME(Qyriad): remove this when the migration to Meson has been completed.
        mesonBuild = self.hydraJobs.mesonBuild.${system};
        mesonBuildClang = self.hydraJobs.mesonBuildClang.${system};
        binaryTarball = self.hydraJobs.binaryTarball.${system};
        perlBindings = self.hydraJobs.perlBindings.${system};
        nixpkgsLibTests = self.hydraJobs.tests.nixpkgsLibTests.${system};
        rl-next = rl-next-check "rl-next" ./doc/manual/rl-next;
        rl-next-dev = rl-next-check "rl-next-dev" ./doc/manual/rl-next-dev;
        pre-commit = self.hydraJobs.pre-commit.${system};
      } // (lib.optionalAttrs (builtins.elem system linux64BitSystems)) {
        dockerImage = self.hydraJobs.dockerImage.${system};
      });

      packages = forAllSystems (system: rec {
        inherit (nixpkgsFor.${system}.native) nix;
        default = nix;
      } // (lib.optionalAttrs (builtins.elem system linux64BitSystems) {
        nix-static = nixpkgsFor.${system}.static.nix;
        dockerImage =
          let
            pkgs = nixpkgsFor.${system}.native;
            image = import ./docker.nix { inherit pkgs; tag = version; };
          in
          pkgs.runCommand
            "docker-image-tarball-${version}"
            { meta.description = "Docker image with Nix for ${system}"; }
            ''
              mkdir -p $out/nix-support
              image=$out/image.tar.gz
              ln -s ${image} $image
              echo "file binary-dist $image" >> $out/nix-support/hydra-build-products
            '';
      } // builtins.listToAttrs (map
          (crossSystem: {
            name = "nix-${crossSystem}";
            value = nixpkgsFor.${system}.cross.${crossSystem}.nix;
          })
          crossSystems)
        // builtins.listToAttrs (map
          (stdenvName: {
            name = "nix-${stdenvName}";
            value = nixpkgsFor.${system}.stdenvs."${stdenvName}Packages".nix;
          })
          stdenvs)));

      devShells = let
        makeShell = pkgs: stdenv:
          let
            nix = pkgs.callPackage ./package.nix {
              inherit stdenv versionSuffix fileset;
              boehmgc = pkgs.boehmgc-nix;
              busybox-sandbox-shell = pkgs.busybox-sandbox-shell or pkgs.default-busybox-sandbox;
              forDevShell = true;
            };
            pre-commit = self.hydraJobs.pre-commit.${pkgs.system} or {};
          in
            (nix.override {
              buildUnreleasedNotes = true;
              officialRelease = false;
            }).overrideAttrs (prev: {
              # Required for clang-tidy checks
              buildInputs = prev.buildInputs
                ++ lib.optional (pre-commit ? enabledPackages) pre-commit.enabledPackages
                ++ lib.optionals (stdenv.cc.isClang) [ pkgs.llvmPackages.llvm pkgs.llvmPackages.clang-unwrapped.dev ];
              nativeBuildInputs = prev.nativeBuildInputs
                ++ lib.optional (stdenv.cc.isClang && !stdenv.buildPlatform.isDarwin) pkgs.buildPackages.bear
                # Required for clang-tidy checks
                ++ lib.optionals (stdenv.cc.isClang) [ pkgs.buildPackages.cmake pkgs.buildPackages.ninja pkgs.buildPackages.llvmPackages.llvm.dev ]
                ++ lib.optional
                  (stdenv.cc.isClang && stdenv.hostPlatform == stdenv.buildPlatform)
                  # for some reason that seems accidental and was changed in
                  # NixOS 24.05-pre, clang-tools is pinned to LLVM 14 when
                  # default LLVM is newer.
                  (pkgs.buildPackages.clang-tools.override { inherit (pkgs.buildPackages) llvmPackages; })
                ++ [
                  # FIXME(Qyriad): remove once the migration to Meson is complete.
                  pkgs.buildPackages.meson
                  pkgs.buildPackages.ninja

                  pkgs.buildPackages.clangbuildanalyzer
                ];

              src = null;

              installFlags = "sysconfdir=$(out)/etc";
              strictDeps = false;

              shellHook = ''
                PATH=$prefix/bin:$PATH
                unset PYTHONPATH
                export MANPATH=$out/share/man:$MANPATH

                # Make bash completion work.
                XDG_DATA_DIRS+=:$out/share

                ${lib.optionalString (pre-commit ? shellHook) pre-commit.shellHook}
              '';
            } // lib.optionalAttrs (stdenv.buildPlatform.isLinux && pkgs.glibcLocales != null) {
              # Required to make non-NixOS Linux not complain about missing locale files during configure in a dev shell
              LOCALE_ARCHIVE = "${lib.getLib pkgs.glibcLocales}/lib/locale/locale-archive";
            });
        in
        forAllSystems (system:
          let
            makeShells = prefix: pkgs:
              lib.mapAttrs'
              (k: v: lib.nameValuePair "${prefix}-${k}" v)
              (forAllStdenvs (stdenvName: makeShell pkgs pkgs.${stdenvName}));
          in
            (makeShells "native" nixpkgsFor.${system}.native) //
            (makeShells "static" nixpkgsFor.${system}.static) //
            (forAllCrossSystems (crossSystem: let pkgs = nixpkgsFor.${system}.cross.${crossSystem}; in makeShell pkgs pkgs.stdenv)) //
            {
              default = self.devShells.${system}.native-stdenvPackages;
            }
        );
  };
}