introduce treefmt.withConfig

This is useful if you want to configure treefmt with nix, and precisely
pass all the commands from nixpkgs.

Co-authored-by: Sridhar Ratnakumar <srid@srid.ca>

default.nix

@@ -13,6 +13,18 @@ let
 
   cargoToml = with builtins; (fromTOML (readFile ./Cargo.toml));
 
+  # Use the Nix module system to validate the treefmt config file format.
+  evalModule = config:
+    lib.evalModules {
+      modules = [
+        {
+          _module.args = { inherit nixpkgs lib treefmt; };
+        }
+        ./module-options.nix
+        config
+      ];
+    };
+
   # What is used when invoking `nix run github:numtide/treefmt`
   treefmt = rustPackages.rustPlatform.buildRustPackage {
     inherit (cargoToml.package) name version;
@@ -34,6 +46,12 @@ let
     cargoLock.lockFile = ./Cargo.lock;
 
     meta.description = "one CLI to format the code tree";
+
+    passthru.withConfig = config:
+      let
+        mod = evalModule config;
+      in
+      mod.config.build.wrapper;
   };
 
   # Add all the dependencies of treefmt, plus more build tools
@@ -68,7 +86,13 @@ let
   });
 in
 {
-  inherit treefmt devShell;
+  inherit treefmt devShell evalModule;
+
+  # module that generates and wraps the treefmt config with Nix
+  module = ./module-options.nix;
+
+  # reduce a bit of repetition
+  inherit (treefmt.passthru) withConfig;
 
   # A collection of packages for the project
   docs = nixpkgs.callPackage ./docs { };

flake.nix

@@ -18,9 +18,22 @@
           };
         in
         {
-          inherit packages;
+          # This contains a mix of packages, modules, ...
+          legacyPackages = packages;
 
           devShells.default = packages.devShell;
+
+          # In Nix 2.8 you can run `nix fmt` to format this whole repo.
+          #
+          # Because we load the treefmt.toml and don't define links to the
+          # packages in Nix, the formatter has to run inside of `nix develop`
+          # to have the various tools on the PATH.
+          #
+          # It also assumes that the project root has a flake.nix (override this by setting `projectRootFile`).
+          formatter = packages.treefmt.withConfig {
+            settings = nixpkgs.lib.importTOML ./treefmt.toml;
+            projectRootFile = "flake.nix";
+          };
         };
     };
 }

module-options.nix

@@ -0,0 +1,113 @@
+{ lib, nixpkgs, treefmt, ... }:
+let
+  # A new kind of option type that calls lib.getExe on derivations
+  exeType = lib.mkOptionType {
+    name = "exe";
+    description = "Path to executable";
+    check = (x: lib.isString x || builtins.isPath x || lib.isDerivation x);
+    merge = loc: defs:
+      let res = lib.mergeOneOption loc defs; in
+      if lib.isString res || builtins.isPath res then
+        "${res}"
+      else
+        lib.getExe res;
+  };
+
+  # The schema of the treefmt.toml data structure.
+  configSchema = with lib; {
+    excludes = mkOption {
+      description = "A global list of paths to exclude. Supports glob.";
+      type = types.listOf types.str;
+      default = [ ];
+      example = [ "./node_modules/**" ];
+    };
+
+    formatter = mkOption {
+      type = types.attrsOf (types.submodule [{
+        options = {
+          command = mkOption {
+            description = "Executable obeying the treefmt formatter spec";
+            type = exeType;
+          };
+
+          options = mkOption {
+            description = "List of arguments to pass to the command";
+            type = types.listOf types.str;
+            default = [ ];
+          };
+
+          includes = mkOption {
+            description = "List of files to include for formatting. Supports globbing.";
+            type = types.listOf types.str;
+          };
+
+          excludes = mkOption {
+            description = "List of files to exclude for formatting. Supports globbing. Takes precedence over the includes.";
+            type = types.listOf types.str;
+            default = [ ];
+          };
+        };
+      }]);
+      default = { };
+      description = "Set of formatters to use";
+    };
+  };
+
+  configFormat = nixpkgs.formats.toml { };
+in
+{
+  # Schema
+  options = {
+    settings = configSchema;
+
+    package = lib.mkOption {
+      description = "Package wrapped in the build.wrapper output";
+      type = lib.types.package;
+      default = treefmt;
+    };
+
+    projectRootFile = lib.mkOption {
+      description = ''
+        File to look for to determine the root of the project in the
+        build.wrapper.
+      '';
+      example = "flake.nix";
+    };
+
+    # Outputs
+    build = {
+      configFile = lib.mkOption {
+        description = ''
+          Contains the generated config file derived from the settings.
+        '';
+        type = lib.types.path;
+      };
+      wrapper = lib.mkOption {
+        description = ''
+          The treefmt package, wrapped with the config file.
+        '';
+        type = lib.types.package;
+      };
+    };
+  };
+  # Config
+  config.build = {
+    configFile = configFormat.generate "treefmt.toml" config.settings;
+
+    wrapper = nixpkgs.writeShellScriptBin "treefmt" ''
+      find_up() (
+        ancestors=()
+        while [[ ! -f "$1" ]]; do
+          ancestors+=("$PWD")
+          if [[ $PWD == / ]]; then
+            echo "ERROR: Unable to locate the projectRootFile ($1) in any of: ''${ancestors[*]@Q}" >&2
+            exit 1
+          fi
+          cd ..
+        done
+      )
+      tree_root=$(find_up "${config.projectRootFile}")
+      exec ${config.package}/bin/treefmt --config-file ${config.build.configFile} "$@" --tree-root "$tree_root"
+    '';
+  };
+};