chore: add a more ergonomic (and cleaner) devshell

- project a pre-commit hook for contributors
- add an editorconfig

.editorconfig

@@ -0,0 +1,29 @@
+# Editor configuration, see http://editorconfig.org
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+charset = utf-8
+indent_style = space
+indent_size = 2
+
+# Ignore diffs/patches
+[*.{diff,patch}]
+end_of_line = unset
+insert_final_newline = unset
+trim_trailing_whitespace = unset
+indent_size = unset
+
+[*.md]
+max_line_length = off
+trim_trailing_whitespace = false
+
+[tests/cases/**]
+end_of_line = unset
+insert_final_newline = unset
+trim_trailing_whitespace = unset
+charset = unset
+indent_style = unset
+indent_size = unset

README.md

@@ -116,6 +116,21 @@ Let's get Alejandra on our systems:
 - [nixpkgs-fmt](https://github.com/nix-community/nixpkgs-fmt)
 - [nixfmt](https://github.com/serokell/nixfmt)
 
+## Contributing
+
+#### With `direnv`
+
+```console
+$ direnv allow
+```
+
+#### Without `direnv`
+
+```console
+$ nix develop -c $SHELL
+$ menu
+```
+
 ## Footnotes
 
 [^benchmark-specs]:

flake.lock

@@ -38,6 +38,21 @@
         "type": "github"
       }
     },
+    "devshell": {
+      "locked": {
+        "lastModified": 1643393796,
+        "narHash": "sha256-+wb2OFWJjXUZw3HQezACc9Lj/5uuhNpUtrjDiIYw8H4=",
+        "owner": "numtide",
+        "repo": "devshell",
+        "rev": "fff3dc6e4538f6df85ee3027f13cc7730b23f61d",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "devshell",
+        "type": "github"
+      }
+    },
     "fenix": {
       "inputs": {
         "nixpkgs": [
@@ -111,6 +126,7 @@
     "root": {
       "inputs": {
         "alejandra": "alejandra",
+        "devshell": "devshell",
         "fenix": "fenix",
         "flakeCompat": "flakeCompat",
         "flakeUtils": "flakeUtils",

flake.nix

@@ -20,6 +20,7 @@
     treefmt.url = "github:numtide/treefmt";
     treefmt.inputs.flake-utils.follows = "flakeUtils";
     treefmt.inputs.nixpkgs.follows = "nixpkgs";
+    devshell.url = "github:numtide/devshell";
   };
   outputs =
     inputs:
@@ -31,6 +32,7 @@
           nixpkgs = import inputs.nixpkgs { inherit system; };
           cargoToml = builtins.fromTOML ( builtins.readFile ./Cargo.toml );
           treefmt = inputs.treefmt.defaultPackage.${ system };
+          devshell = inputs.devshell.legacyPackages.${ system };
           fenix = inputs.fenix.packages.${ system };
           fenixPlatform = nixpkgs.makeRustPlatform { inherit ( fenix.latest ) cargo rustc; };
         in
@@ -60,11 +62,16 @@
                 };
               };
           devShell =
-            nixpkgs.mkShell
+            devshell.mkShell
               {
+                imports = [ "${ devshell.extraModulesDir }/git/hooks.nix" ];
+                git.hooks = {
+                  enable = true;
+                  pre-commit.text = builtins.readFile ./pre-commit.sh;
+                };
+                name = "alejandra";
                 packages = [
                   fenix.rust-analyzer
-                  fenix.latest.cargo
                   fenix.latest.clippy
                   fenix.latest.rust-src
                   fenix.latest.rustc
@@ -75,7 +82,21 @@
                   nixpkgs.nodePackages.prettier
                   nixpkgs.nodePackages.prettier-plugin-toml
                   nixpkgs.shfmt
-                  treefmt
+                ];
+                commands = [
+                  {
+                    package = fenix.latest.cargo;
+                    name = "cargo";
+                    help = "the cargo tool";
+                  }
+                  {
+                    package = treefmt;
+                    category = "formatters";
+                  }
+                  {
+                    package = nixpkgs.editorconfig-checker;
+                    category = "formatters";
+                  }
                 ];
               };
         }

pre-commit.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+
+if git rev-parse --verify HEAD >/dev/null 2>&1; then
+  against=HEAD
+else
+  # Initial commit: diff against an empty tree object
+  against=$(${git}/bin/git hash-object -t tree /dev/null)
+fi
+
+diff="git diff-index --name-only --cached $against --diff-filter d"
+all_files=($($diff))
+
+# Format the entire tree.
+treefmt
+
+# check editorconfig
+editorconfig-checker -- "${all_files[@]}"
+if [[ $? != '0' ]]; then
+  printf "%b\n" \
+    "\nCode is not aligned with .editorconfig" \
+    "Review the output and commit your fixes" >&2
+  exit 1
+fi
+
+