public extension PureYAML.Validation {
    /// The caseless namespace of built-in type-directed checks, referenceable by name.
    enum BuiltinValidation {
        /// Reports each duplicate key within a single mapping. The type-directed counterpart of the
        /// legacy `Rule.mappingKeysAreUnique`; fires on `Model.Mapping` values directly rather than
        /// switching on a `Model.Value`.
        public static var mappingKeysAreUnique: Validation<PureYAML.Model.Mapping, PureYAML.Model.Value> {
            Validation(description: "Mapping keys are unique") { context in
                var seen = Set<PureYAML.Model.Key>()
                var errors: [ValidationError] = []
                for pair in context.subject.pairs where !seen.insert(pair.keyNode).inserted {
                    errors.append(ValidationError(
                        severity: .error,
                        reason: "Duplicate mapping key '\(pair.keyNode.description)'",
                        path: context.path.appending(pair.keyNode.pathComponent),
                    ))
                }
                return errors
            }
        }
    }
}

extension PureYAML.Model.Key {
    /// The path component a duplicate-key error points at: a plain `.key` for a string key, a
    /// `.complexKey` for a sequence or mapping key (which has no string spelling).
    var pathComponent: PureYAML.Validation.Path.Component {
        switch self {
        case let .string(value):
            .key(value)
        case .mapping, .sequence:
            .complexKey(self)
        }
    }
}