treefmt: add configuration wrappers

- `treefmt.withConfig` builds a treefmt package wrapped with a config
- `treefmt.buildConfig` builds a treefmt.toml file using a freeform module

pkgs/by-name/tr/treefmt/build-config.nix

@@ -0,0 +1,29 @@
+{
+  lib,
+  formats,
+}:
+module:
+let
+  settingsFormat = formats.toml { };
+  configuration = lib.evalModules {
+    modules = [
+      {
+        _file = ./build-config.nix;
+        freeformType = settingsFormat.type;
+      }
+      {
+        # Wrap user's modules with a default file location
+        _file = "<treefmt.buildConfig args>";
+        imports = lib.toList module;
+      }
+    ];
+  };
+  settingsFile = settingsFormat.generate "treefmt.toml" configuration.config;
+in
+settingsFile.overrideAttrs {
+  passthru = {
+    format = settingsFormat;
+    settings = configuration.config;
+    inherit (configuration) _module options type;
+  };
+}

pkgs/by-name/tr/treefmt/package.nix

@@ -1,6 +1,8 @@
 {
   lib,
   buildGoModule,
+  callPackage,
+  callPackages,
   fetchFromGitHub,
 }:
 buildGoModule rec {
@@ -27,6 +29,38 @@ buildGoModule rec {
     "-X github.com/numtide/treefmt/v2/build.Version=v${version}"
   ];
 
+  passthru = {
+    /**
+      Wrap treefmt, configured  using structured settings.
+
+      # Type
+
+      ```
+      AttrSet -> Derivation
+      ```
+
+      # Inputs
+
+      - `name`: `String` (default `"treefmt-configured"`)
+      - `settings`: `Module` (default `{ }`)
+      - `runtimeInputs`: `[Derivation]` (default `[ ]`)
+    */
+    withConfig = callPackage ./with-config.nix { };
+
+    /**
+      Build a treefmt config file from structured settings.
+
+      # Type
+
+      ```
+      Module -> Derivation
+      ```
+    */
+    buildConfig = callPackage ./build-config.nix { };
+
+    tests = callPackages ./tests.nix { };
+  };
+
   meta = {
     description = "one CLI to format the code tree";
     homepage = "https://github.com/numtide/treefmt";

pkgs/by-name/tr/treefmt/tests.nix

@@ -0,0 +1,110 @@
+{
+  lib,
+  runCommand,
+  testers,
+  treefmt,
+  nixfmt-rfc-style,
+}:
+let
+  inherit (treefmt) buildConfig withConfig;
+
+  testEqualContents =
+    args:
+    testers.testEqualContents (
+      args
+      // lib.optionalAttrs (builtins.isString args.expected) {
+        expected = builtins.toFile "expected" args.expected;
+      }
+    );
+
+  nixfmtExampleConfig = {
+    on-unmatched = "info";
+    tree-root-file = ".git/index";
+
+    formatter.nixfmt = {
+      command = "nixfmt";
+      includes = [ "*.nix" ];
+    };
+  };
+
+  nixfmtExamplePackage = withConfig {
+    settings = nixfmtExampleConfig;
+    runtimeInputs = [ nixfmt-rfc-style ];
+  };
+in
+{
+  buildConfigEmpty = testEqualContents {
+    assertion = "`buildConfig { }` builds an empty config file";
+    actual = buildConfig { };
+    expected = "";
+  };
+
+  buildConfigExample = testEqualContents {
+    assertion = "`buildConfig` builds the example config";
+    actual = buildConfig nixfmtExampleConfig;
+    expected = ''
+      on-unmatched = "info"
+      tree-root-file = ".git/index"
+      [formatter.nixfmt]
+      command = "nixfmt"
+      includes = ["*.nix"]
+    '';
+  };
+
+  buildConfigModules = testEqualContents {
+    assertion = "`buildConfig` evaluates modules to build a config";
+    actual = buildConfig [
+      nixfmtExampleConfig
+      { tree-root-file = lib.mkForce "overridden"; }
+    ];
+    expected = ''
+      on-unmatched = "info"
+      tree-root-file = "overridden"
+      [formatter.nixfmt]
+      command = "nixfmt"
+      includes = ["*.nix"]
+    '';
+  };
+
+  runNixfmtExample =
+    runCommand "run-nixfmt-example"
+      {
+        nativeBuildInputs = [ nixfmtExamplePackage ];
+        passAsFile = [
+          "input"
+          "expected"
+        ];
+        input = ''
+          {
+            foo="bar";
+            attrs={};
+            list=[];
+          }
+        '';
+        expected = ''
+          {
+            foo = "bar";
+            attrs = { };
+            list = [ ];
+          }
+        '';
+      }
+      ''
+        export XDG_CACHE_HOME=$(mktemp -d)
+        # The example config assumes the tree root has a .git/index file
+        mkdir .git && touch .git/index
+
+        # Copy the input file, then format it using the wrapped treefmt
+        cp $inputPath input.nix
+        treefmt
+
+        # Assert that input.nix now matches expected
+        if diff -u $expectedPath input.nix; then
+          touch $out
+        else
+          echo
+          echo "treefmt did not format input.nix as expected"
+          exit 1
+        fi
+      '';
+}

pkgs/by-name/tr/treefmt/with-config.nix

@@ -0,0 +1,31 @@
+{
+  lib,
+  runCommand,
+  treefmt,
+  makeBinaryWrapper,
+}:
+{
+  name ? "treefmt-with-config",
+  settings ? { },
+  runtimeInputs ? [ ],
+}:
+runCommand name
+  {
+    nativeBuildInputs = [ makeBinaryWrapper ];
+    treefmtExe = lib.getExe treefmt;
+    binPath = lib.makeBinPath runtimeInputs;
+    configFile = treefmt.buildConfig {
+      # Wrap user's modules with a default file location
+      _file = "<treefmt.withConfig settings arg>";
+      imports = lib.toList settings;
+    };
+    inherit (treefmt) meta version;
+  }
+  ''
+    mkdir -p $out/bin
+    makeWrapper \
+      $treefmtExe \
+      $out/bin/treefmt \
+      --prefix PATH : "$binPath" \
+      --add-flags "--config-file $configFile"
+  ''