add tooling for Nix files (#62)

Define a `flake_package_dev_deps` extension that obtains development
dependencies from nixpkgs. This uses the existing `flake.nix` in
`//extensions`. The dev dependencies include:
* `nixd`
* `nixfmt`
* `nixfmt-tree`

`nixd` (and `nixfmt`) must be fetched explicitly with
```
bazel fetch @nixd
```
as these are only used as part of the Zed project settings.

This PR also adds `gnused` as a package, as it is used extensively by
shell runfiles:

https://github.com/bazelbuild/rules_shell/blob/main/shell/runfiles/runfiles.bash#L184

Change-Id: Ibb8c58fbf876a2eb3c612b3985f45840be7f7d79

Author: oliverlee

.zed/settings.json

@@ -7,16 +7,17 @@
   "file_scan_exclusions": [".cache/"],
   "languages": {
     "Nix": {
-      "language_servers": [ "nil", "!nixd" ],
+      "language_servers": ["nixd", "!nil"],
       "formatter": {
-             "external": {
-               "command": "alejandra",
-               "arguments": ["--quiet", "--"]
-             }
-           }
+        "external": {
+          // initialize with
+          // bazel fetch @nixfmt
+          "command": "./external/+flake_package_dev_deps+nixfmt/bin/nixfmt",
+          "arguments": ["--quiet", "--"]
+        }
+      }
     },
     "Starlark": {
-      "enable_language_server": true,
       "language_servers": ["starpls"],
       "formatter": {
         "external": {
@@ -52,6 +53,21 @@
         ]
       }
     },
+    "nixd": {
+      "binary": {
+        // initialize with
+        // bazel fetch @nixd
+        "path": "./external/+flake_package_dev_deps+nixd/bin/nixd"
+      },
+      "settings": {
+        "nixpkgs": {
+          "expr": "import (builtins.getFlake \"./tools/nix\").inputs.nixpkgs {}"
+        },
+        "diagnostic": {
+          "suppress": ["sema-unused-def-lambda-noarg-formal"]
+        }
+      }
+    },
     "starpls": {
       "binary": {
         "arguments": [

MODULE.bazel

@@ -183,3 +183,15 @@ python = use_extension(
 )
 python.defaults(python_version = "3.11")
 python.toolchain(python_version = "3.11")
+
+flake_package_dev_deps = use_extension(
+    "//extensions:flake_package_deps.bzl",
+    "flake_package_dev_deps",
+    dev_dependency = True,
+)
+use_repo(
+    flake_package_dev_deps,
+    "nixd",
+    "nixfmt",
+    "nixfmt-tree",
+)

MODULE.bazel.lock

@@ -5,19 +5,33 @@
     nixpkgs.url = "github:nixos/nixpkgs/nixos-25.05";
   };
 
-  outputs = {
-    self,
-    nixpkgs,
-  }: let
-    systems = ["x86_64-linux" "aarch64-darwin"];
-    forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f system);
-  in {
-    packages = forAllSystems (
-      system: let
-        pkgs = nixpkgs.legacyPackages.${system};
-      in {
-        inherit (pkgs) gdb qemu;
-      }
-    );
-  };
+  outputs =
+    {
+      self,
+      nixpkgs,
+    }:
+    let
+      systems = [
+        "x86_64-linux"
+        "aarch64-darwin"
+      ];
+      forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f system);
+    in
+    {
+      packages = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          inherit (pkgs)
+            gdb
+            qemu
+            nixd
+            nixfmt-tree
+            ;
+          nixfmt = pkgs.nixfmt-rfc-style;
+        }
+      );
+    };
 }

extensions/flake.nix

@@ -22,6 +22,7 @@ exports_files(["flake.nix", "flake.lock"])
 """,
         executable = False,
     )
+    rctx.repo_metadata(reproducible = True)
 
 _flake_copy = repository_rule(
     implementation = _flake_copy_impl,
@@ -34,41 +35,69 @@ _flake_copy = repository_rule(
     ),
 )
 
-def _native_binary_build_file_content(binary):
+def _native_binary_build_file_content(binary, *, package = None):
     return """
 load("@bazel_skylib//rules:native_binary.bzl", "native_binary")
 
 native_binary(
-    name = "{binary}",
+    name = "{package}",
     out = "{binary}",
     src = "bin/{binary}",
     visibility = ["//visibility:public"],
 )
-""".format(binary = binary)
+""".format(
+        binary = binary,
+        package = package or binary,
+    )
 
-def _flake_package_deps_impl(_mctx):
+def _provide_binary(pkgs):
     _flake_copy(
         name = "flake_copy",
         flake_file = "//extensions:flake.nix",
         flake_lock_file = "//extensions:flake.lock",
     )
 
-    nixpkgs_flake_package(
-        name = "qemu-system-arm",
-        nix_flake_file = "@flake_copy//:flake.nix",
-        nix_flake_lock_file = "@flake_copy//:flake.lock",
-        package = "qemu",
-        build_file_content = _native_binary_build_file_content("qemu-system-arm"),
-    )
+    for package in pkgs:
+        if type(package) != "dict":
+            package = {"name": package}
 
-    nixpkgs_flake_package(
-        name = "gdb",
-        nix_flake_file = "@flake_copy//:flake.nix",
-        nix_flake_lock_file = "@flake_copy//:flake.lock",
-        package = "gdb",
-        build_file_content = _native_binary_build_file_content("gdb"),
-    )
+        defaults = {
+            "nix_flake_file": "@flake_copy//:flake.nix",
+            "nix_flake_lock_file": "@flake_copy//:flake.lock",
+            "package": package["name"],
+            "build_file_content": _native_binary_build_file_content(package["name"]),
+        }
+
+        nixpkgs_flake_package(
+            **(defaults | package)
+        )
+
+def _flake_package_deps_impl(_mctx):
+    _provide_binary([
+        "gdb",
+        {
+            "name": "qemu-system-arm",
+            "package": "qemu",
+        },
+    ])
 
 flake_package_deps = module_extension(
     implementation = _flake_package_deps_impl,
 )
+
+def _flake_package_dev_deps_impl(_mctx):
+    _provide_binary([
+        "nixd",
+        "nixfmt",
+        {
+            "name": "nixfmt-tree",
+            "build_file_content": _native_binary_build_file_content(
+                package = "nixfmt-tree",
+                binary = "treefmt",
+            ),
+        },
+    ])
+
+flake_package_dev_deps = module_extension(
+    implementation = _flake_package_dev_deps_impl,
+)

extensions/flake_package_deps.bzl

@@ -1,7 +1,8 @@
 load("@buildifier_prebuilt//:rules.bzl", "buildifier")
 load("@hedron_compile_commands//:refresh_compile_commands.bzl", "refresh_compile_commands")
+load("@local_workspace_directories//:defs.bzl", "BAZEL_WORKSPACE_ROOT")
 load("@platforms//host:constraints.bzl", "HOST_CONSTRAINTS")
-load("@rules_multirun//:defs.bzl", "multirun")
+load("@rules_multirun//:defs.bzl", "command", "multirun")
 load("@rules_shell//shell:sh_binary.bzl", "sh_binary")
 
 buildifier(
@@ -16,6 +17,29 @@ buildifier(
     mode = "fix",
 )
 
+command(
+    name = "nixfmt.check",
+    args = [
+        "--ci",
+        "--tree-root",
+        BAZEL_WORKSPACE_ROOT,
+        "--working-dir",
+        BAZEL_WORKSPACE_ROOT,
+    ],
+    command = "@nixfmt-tree",
+)
+
+command(
+    name = "nixfmt",
+    args = [
+        "--tree-root",
+        BAZEL_WORKSPACE_ROOT,
+        "--working-dir",
+        BAZEL_WORKSPACE_ROOT,
+    ],
+    command = "@nixfmt-tree",
+)
+
 genrule(
     name = "gen-clang-format",
     outs = ["clang-format.bash"],
@@ -52,6 +76,7 @@ multirun(
     commands = [
         ":buildifier.check",
         ":clang-format.check",
+        ":nixfmt.check",
     ],
 )
 
@@ -60,6 +85,7 @@ multirun(
     commands = [
         ":buildifier",
         ":clang-format",
+        ":nixfmt",
     ],
 )