Missing warning on partially-unused record destruction

[ChaserVasya]

Snippet

void main() {
  final (a,b) = fn();
  print(a);
}

(int,int) fn(){
  return (1,2);
}

Expected

Warning The value of the local variable 'b' isn't used. .

Actual

Nothing.

Context

Dart SDK version: 3.5.4 (stable) (Wed Oct 16 16:18:51 2024 +0000) on "macos_arm64"

[bwilkerson]

Just to note, the correct fix for this is to use a wildcard pattern in place of the unused variable pattern:

void main() {
  final (a,_) = fn();
  print(a);
}

(int,int) fn(){
  return (1,2);
}

[ChaserVasya]

@bwilkerson Yes. Your solution should be a quick fix suggested by analyzer.

[bwilkerson]

@pq @kallentu

[kallentu]

There's two separate issues here --

1. The snippet above doesn't trigger any UNUSED_VARIABLE warnings because of the design choice we made here: unused_local_variable flags parts of patterns that aren't used #54854. If we want to change that original decision, we'll need to do a few migrations, which isn't too bad. It's more of a question of if we want to start warning on individual variables in the pattern now.
2. The other issue is that the convert to wildcard variable fix doesn't run on these record destructuring patterns. So even if all the variables are unused, we don't currently encourage the user to use wildcard variables. With this, I've been working on a fix for in https://dart-review.googlesource.com/c/sdk/+/411087

[liranzairi]

Consider this case when using the Provider package:

class HomeScreen extends StatelessWidget {
  const HomeScreen();

  @override
  Widget build(BuildContext context) {
    final (users, messages) = context.select(
      (AppState state) => (state.users, state.messages),
    );
    return ListView(
      children: users.map(...).toList(),
    );
  }
}

A developer might miss the fact that he's not using the messages variable which may cause unnecessary builds and therefore reduce performance. One of the benefits of linter warnings is to help spot places where there can be a performance issue, such as not using const constructors. Therefore I think it's important to bring back a linter warning for this case. I understand the reasoning behind your design choice in #54854, so how about a new linter warning especially for the case of partially unused patterns?