unique_by

No two files matching select may share the value of key (a path template; tokens {path}/{dir}/{basename}/{stem}/{ext}/{parent_name}). Catches basename collisions across subdirectories. With case_insensitive: true the key is folded to lowercase before grouping, so README.md and readme.md collide — the case-insensitive-filesystem hazard (Windows / macOS).

- id: unique-basenames
  kind: unique_by
  select: "src/**/*.rs"
  key: "{stem}"
  level: warning

Options

Option	Type	Required	Default	Description
case_insensitive	boolean		false	Fold the key to lowercase before grouping, so keys that collide only under case-folding count as duplicates — the case-insensitive-filesystem hazard (Windows / macOS).
key	string		{basename}	Path-template producing a key per matched file. Default: {basename}.
select	string	yes		Glob selecting the files to deduplicate.

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.