Add local tooling and CI wiring without bloating flake.nix

ci.nix

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+# We're doing things a bit differently because Nix doesn't let us
+# split out the dev dependencies and subflakes are broken, let alone "superflakes".
+# See dev/README.md
+let
+  flake = import ./dev;
+  inherit (flake.inputs.nixpkgs) lib;
+in {
+  inherit (flake) herculesCI;
+} // {
+  checks = lib.recurseIntoAttrs flake.checks.${builtins.currentSystem};
+}

dev/README.md

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+
+# Separate `tools` flake
+
+Wouldn't recommend this pattern normally, but I'm trying to keep
+deps low for `flake-modules-core` until we have split dev inputs
+that don't carry over to dependent lock files.

dev/default.nix

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+let
+  flake = builtins.getFlake (toString ./.);
+  fmc-lib = import ../lib.nix { inherit (flake.inputs.nixpkgs) lib; };
+  self = { inherit (flake) inputs; } // 
+    fmc-lib.evalFlakeModule
+      { inherit self; }
+      ./flake-module.nix;
+in
+  self.config.flake // { inherit (flake) inputs; }

dev/flake-module.nix

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+flakeModuleArgs@{ config, lib, ... }:
+
+{
+  imports = [
+  ];
+  systems = [ "x86_64-linux" "aarch64-darwin" ];
+  perSystem = system: { config, self', inputs', pkgs, ... }: {
+    _module.args.pkgs = inputs'.nixpkgs.legacyPackages;
+    devShells.default = pkgs.mkShell {
+      nativeBuildInputs = [ pkgs.nixpkgs-fmt ];
+    };
+    packages = {
+      inherit (pkgs.nixosOptionsDoc { inherit (flakeModuleArgs) options; })
+        optionsDocBook;
+      optionsMarkdown = pkgs.runCommand "options-markdown"
+        {
+          inherit (config.packages) optionsDocBook;
+          nativeBuildInputs = [ pkgs.pandoc ];
+        } ''
+        mkdir $out
+        pandoc \
+          --from docbook \
+          --to markdown \
+          --output $out/options.md \
+          $optionsDocBook
+      '';
+    };
+  };
+  flake = {
+    options.herculesCI = lib.mkOption { type = lib.types.raw; };
+    config.herculesCI = {
+      onPush.default.outputs = { 
+        inherit (config.flake) packages checks;
+      };
+    };
+
+    # for repl exploration / debug
+    config.config = config;
+    options.mySystem = lib.mkOption { default = config.allSystems.${builtins.currentSystem}; };
+  };
+}

dev/flake.lock

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+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1652425756,
+        "narHash": "sha256-3+5j3ZU43oC5PxRfGDQ6xs5sYnypS5GBhVP6uf7PVGM=",
+        "owner": "hercules-ci",
+        "repo": "nixpkgs",
+        "rev": "81a0a8be297a88f338ee0fd90c330cd94b4a55bd",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "ref": "functionTo-properly",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

dev/flake.nix

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+{
+  description = "Dependencies for development purposes";
+
+  inputs = {
+    # Flakes don't give us a good way to depend on .., so we don't.
+    # This has drastic consequences of course.
+    nixpkgs.url = "github:hercules-ci/nixpkgs/functionTo-properly";
+  };
+
+  outputs = { self, ... }:
+  {
+  };
+}

dev/repl.nix

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+# convenience for loading into nix repl
+let self = builtins.getFlake (toString ./.);
+in self // { inherit self; }

shell.nix

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+# non-idiomatic, see tools/README.md
+(import ./dev).devShells.${builtins.currentSystem}.default