Consider `prefer_mixins`

[mit-mit]

We had initially selected prefer_mixins for inclusion in recommended lints, however ran into some issues that need resolving first, including:

• opt-out for old-school mixin-like classes from prefer_mixin sdk#45343, opt-out for old-school mixin-like classes from prefer_mixin
• Opt-out ChangeNotifier and WidgetsBindingObserver, enable lint in flutter codebase for dogfooding

[Levi-Lesches]

Many times, the class should be declared as mixin but just isn't for some reason. Having the lint in user code won't fix the problem, as the original API author is unaware and the user has no control of the declaration.

I believe new rules are being proposed in the language repo to prohibit mixing in non-mixin classes, so one side-effect is that the API author will be notified that they're not using mixin when testing their code.

[goderbauer]

As it stands, the lint is still not compatible with flutter's code, see dart-lang/sdk#45343. Some solutions have been proposed, but not implemented, see:

• opt-out for old-school mixin-like classes from prefer_mixin sdk#45343 (comment)
• Allow mixins as superclasses. Deprecate using classes as mixins. language#1529
• Do not allow mixing in a class, with migration path language#1643
• Allow mixins in "extends" clauses language#1942

[goderbauer]

Depending on how dart-lang/sdk#59039 gets resolved this may become obsolete.

[lrhn]

The lint has been removed.

[goderbauer]

The lint has been removed.

As far as I can tell, this lint still exists [1] and from a recent discussion [2] it sounded like it was still relevant.

[1] https://dart-lang.github.io/linter/lints/prefer_mixin.html
[2] dart-lang/sdk#59039

[lrhn]

ACK, must have mistyped something.
Lint still exists, and still applies to pre-3.0 code.

[devoncarew]

Closing - it's not clear this will help people in a pre-3.0 codebase or in a post-3.0 codebase.