command_idempotent

Run a user-declared formatter/checker in its --check (idempotence) mode once: exit 0 ⇒ the tree is formatter-clean (silent); non-zero ⇒ violation(s). The sibling of generated_file_fresh — that rule diffs a generator’s captured stdout against a committed file; this trusts a checker’s own --check exit code. alint never runs a mutating formatter and never writes the tree. With files_from (stdout/stderr) the tool’s own offender list is parsed into one violation per file (optional files_pattern regex, capture group 1 = path, for tools that wrap the path in a message like cargo fmt’s Diff in <path> at line N); without it, one violation for the whole invocation. A non-zero exit is never swallowed into a pass. Single-shot, opt-in. Trust-gated like command (see below): declarable only in your own top-level config.

- id: code-is-formatted
  kind: command_idempotent
  command: ["cargo", "fmt", "--all", "--", "--check"]
  workdir: "."
  files_from: stderr
  files_pattern: "Diff in (.+) at"
  level: error
  message: "run `cargo fmt` — code is not formatter-clean"

Options

Option	Type	Required	Default	Description
command	list of string	yes		Checker argv (no shell), run in its --check / idempotence mode; exit 0 = clean.
files_from	one of none | stdout | stderr		none	On failure, parse this stream into per-file violations (default: none = one violation for the whole invocation).
files_pattern	string		null	Regex whose capture group 1 is a file path, applied per output line (requires files_from; omit for bare-path listers).
timeout	integer (>= 1)		null	Checker timeout in seconds (default 120). On timeout the child is killed and one violation is emitted.
workdir	string		null	Checker cwd, relative to the lint root (default: lint root).

Plus the common level, id, and when fields. This rule analyses the whole repository, so it takes no paths. This table is generated from the JSON Schema; option types and defaults are authoritative.