proposal: `unused_ignore`

[rrousselGit]

unused_ignore

Description

An // ignore: code line was defined, but it isn't actually ignoring anything

Details

When dealing with // ignore, it is possible that the issue disappears but the // ignore stays.

Kind

warning

Bad Examples

If the ignored code is a known lint code, warn if the next line doesn't have this warning:

// ignore: unused_import
import 'dart:math';

void main() {
  min(0 ,1); // math is used
}

Good Examples

// ignore: unused_import
import 'dart:math'; // The import is correctly unused

We also shouldn't warn if the code is not recognised by the SDK:

// ignore: unknown_code

This avoids false positives if:

• the code is tested against multiple SDK versions, including some where a lint is not implemented
• the lint comes from an analyzer_plugin, so analyzer may not know about this ignore

Discussion

Add any other motivation or useful context here.

Discussion checklist

• [ ] List any existing rules this proposal modifies, complements, overlaps or conflicts with.
• [ ] List any relevant issues (reported here, the SDK Tracker, or elsewhere).
• [ ] If there's any prior art (e.g., in other linters), please add references here.
• [x] If this proposal corresponds to Effective Dart or Flutter Style Guide advice, please call it out. (If there isn't any corresponding advice, should there be?)
• [x] If this proposal is motivated by real-world examples, please provide as many details as you can. Demonstrating potential impact is especially valuable.

[bwilkerson]

We have already implemented this as a warning, but have been unable to land it because it's too big a breaking change.

We have discussed implementing this as a lint, and have agreed (I think) that this is the right approach, but I haven't yet been able to make the time to convert the warning-based implementation to produce a lint. (We won't be able to implement it as a traditional lint because the check has to occur after all the other lints have been run, not as part of running the lints. But that's just an implementation detail.)

[srawlins]

I think duplicate of https://github.com/dart-lang/sdk/issues/35234 and https://github.com/dart-lang/sdk/issues/25917