[New rule] Implement a class instead of extending it when it is semantically an interface

[purplenoodlesoop]

Please describe what the rule should do:

The rule should suggest swapping the extends keyword to the implements keyword if the class that is being extended follows two rules:

1. Is abstract.
2. Does not contain any implementations.

What category of rule is this? (place an "X" next to just one item)

[ ] Warns about a potential error (problem) [x] Suggests an alternate way of doing something (suggestion) [ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about (it will be better if you can provide both good and bad examples):

BAD:

abstract class A {}

class B extends A {}

BAD:

abstract class A {
  int get a;
}

class B extends A {
  // Implementation
}

BAD:

abstract class A {
  int get a;
}

abstract class B extends A {
  String get b;
}

class C extends B {
  // Implementation
}

GOOD:

abstract class A {
  int get a;
}

class B implements A {
  // Implementation
}

GOOD:

abstract class A {
  int get a;
}

abstract class B {
  String get b;
}

abstract class C implements A, B {
  @override
  int get a => 1;
}

class D extends C {
  // Implementation
}

Are you willing to submit a pull request to implement this rule?

Not sure at the moment.

[incendial]

@vlkonoshenko is this still in progress?

[vlkonoshenko]

@vlkonoshenko is this still in progress?

It’s on pause for now. I can find extends of abstract classes. But I can`t check if there are implementation of any methods in these classes.

[incendial]

@vlkonoshenko what the problem with checking the implementation? You can collect class declarations and compare them to the base class to find the implemented methods, right?

[incendial]

Feels like this one is covered by new Dart 3.0 features.