devshell

My dev shell for every project, with my git hooks, packaged as a Nix flake. A project takes it as an input instead of spelling the setup out, and gets:

• a dev shell with interactive bash, the commands behind the hooks below, prek and treefmt, plus its own tools;
• nix fmt and a formatting check in nix flake check, through treefmt-nix: mdformat, nixfmt, taplo, and shfmt for .envrc.

Use

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    devshell = {
      url = "github:fdnt7/devshell";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs =
    { self, nixpkgs, devshell }:
    devshell.lib.mkOutputs { inherit self nixpkgs; } (pkgs: {
      packages = [ pkgs.cargo pkgs.rustc ];
    });
}

The function's result may also set treefmt, a treefmt-nix module added to the defaults, and anything mkShell takes, such as env. Pass systems beside self and nixpkgs for more than x86_64-linux.

Pick the hooks in prek.toml:

default_install_hook_types = ["pre-commit", "commit-msg"]
default_stages = ["pre-commit"]

# Their commands come from the dev shell.
[[repos]]
repo = "https://github.com/fdnt7/devshell"
rev = "<commit>"
hooks = [
  { id = "ascii-only" },
  { id = "cog-verify" },
  { id = "commit-msg-length" },
  { id = "no-co-authors" },
  { id = "treefmt" },
  { id = "typos" },
  { id = "typos-commit-msg" },
]

Add .envrc (use flake), and typos.toml or ascii-only.toml when the project needs exceptions. Then, once: direnv allow and prek install.

The flake input decides which version of the commands runs, and rev only which hooks are listed. nix flake update devshell brings in changes made here; prek autoupdate is needed only when a hook is added or renamed.

Hooks

Hook	Stage	What it checks
ascii-only	pre-commit	No non-ASCII characters, except those an allowlist opts in
cog-verify	commit-msg	A Conventional Commits message, by cog.toml if there is one
commit-msg-length	commit-msg	A subject of at most 50 characters, a blank line after it, and a body wrapped at 72
no-co-authors	commit-msg	No Co-authored-by trailers, for repositories with one author
treefmt	pre-commit	No file that the repository's treefmt would change
typos	pre-commit	No spelling mistakes, by typos.toml, reported but not fixed
typos-commit-msg	commit-msg	The same, for the commit message

ascii-only

Opt characters in with an ascii-only.toml at the repository root, pasting each one as it is:

allowed = [
  "•", # bullet: in a tool's output quoted somewhere
  "\u00A0", # a character that does not show can be an escape
]

Each entry must be a single non-ASCII character. The config itself is exempt from the check. To keep it elsewhere, pass its path:

hooks = [{ id = "ascii-only", args = ["--config", "path/to/ascii-only.toml"] }]

cog-verify

cocogitto reads its configuration, such as extra commit types, from cog.toml, so this flake packages it as nixpkgs has it. cocogitto has no official hook to use instead.

commit-msg-length

A body line may run past 72 characters when it can't be wrapped:

• a single word, such as a URL or a store path
• a trailer in the last paragraph, such as Refs: <url>

Comments and anything below the scissors line of git commit --verbose are not checked, since git strips them.

treefmt

The command is the repository's own treefmt wrapper, which treefmt-nix builds from the default formatters in lib/treefmt.nix and the repository's own additions. treefmt has no official hook to use instead.

typos

typos reads all its configuration from typos.toml, so this flake packages it as nixpkgs has it. The official hooks install typos with Python, Docker or Cargo and fix mistakes in place; this one uses the typos on PATH and only reports them. It checks the whole repository rather than the staged files, so the excludes in typos.toml apply.

typos-commit-msg

The same typos, on the message git passes the hook. That file sits under .git/, so typos still finds the repository's typos.toml, but only its [default] rules apply: the message has no file type. Commit messages tend to quote hashes, which typos reads as words; ignore them there, for example:

[default]
extend-ignore-re = [
  # Short and full commit hashes, like "ba2846c"
  '\b[0-9a-f]{7,40}\b',
]