[Linter] Proposal: `prefer_wildcard_variables` lint rule

[ricardodalarme]

Proposal for New Dart Lint Rule

TLDR

Add a lint rule to prefer wildcard variables (_) over multiple underscores (_, __, ___) in unused parameters.

Motivation

The upcoming Dart 3.7.0 release introduces support for wildcard variables (_) that can be reused for multiple unused parameters. Previously, developers commonly relied on distinct variables like _, __, ___ to indicate unused parameters. This practice is now redundant with the introduction of _, which improves clarity and reduces visual noise in the codebase.

Reference

• https://github.com/dart-lang/sdk/blob/main/CHANGELOG.md#wildcard-variables

Lint Rule Implementation

Rule Name: prefer_wildcard_variables
Description: Warns when distinct variable names such as __, ___, etc. are used for unused parameters, recommending the use of _ instead.
Severity Level: WARNING (as it’s a stylistic improvement and non-breaking).

Code Examples

Bad

ListView.builder(
  itemCount: 10,
  itemBuilder: (_, __) {
    return ListTile(
      title: Text('Item'),
    );
  },
);

Good

ListView.builder(
  itemCount: 10,
  itemBuilder: (_, _) {
    return ListTile(
      title: Text('Item'),
    );
  },
);

Behavior

• Triggers a warning: When multiple distinct unused variable names (__, ___) are declared.
• Does not trigger: For valid wildcard use cases, such as (_, _, _).

Benefits

1. Improved Code Readability: Reduces unnecessary differentiation in unused parameter names.
2. Consistent Dart Style: Encourages developers to adopt features introduced in newer Dart versions.
3. Simpler Refactoring: Avoids subtle issues where _, __, and ___ might accidentally be referenced or misinterpreted.

[dart-github-bot]

Summary: This proposal suggests a new Dart lint rule, prefer_wildcard_variables, to warn against using multiple underscores (_, __, ___) for unused parameters, recommending the single underscore wildcard (_) instead. This improves code readability and consistency.

[srawlins]

CC @pq @kallentu sounds like a win to me!

[kallentu]

Ah yes, we've agreed upon making a lint that's exactly this. We may call it unnecessary_underscores or something more specific than prefer_wildcard_variables though.

For more context: #56595 (comment)

[srawlins]

I like that name. too_many_underscores. why_so_many_underscores. 😁

[pq]

Hi! This has been implemented as unnecessary_underscores and should be in 3.7.

If it doesn't behave as expected, please raise issues.

Thanks!