Allow destructuring into class fields

[rubenferreira97]

This issue proposes extending the language to support destructuring into class fields. This enhancement would enable developers to directly destructure and assign values from records or objects into corresponding class fields.

void main() {
  int a = 3;
  int b = 2;
  MapEntry<int, int> entry = MapEntry(1, 2);
  (a, b) = (1,2); // dart allows record destructuring into variables
  MapEntry(key: a, value: b) = entry; // dart allows object destructuring into variables
  print('$a,$b');
}

class RecordDestructingExample {
  // Error: dart does not allow record destructuring into fields
  RecordDestructingExample(): (a, b) = (1,2);  
  
  final int a;
  final int b;
}

class ObjectDestructingExample {
  // Error: dart does not allow object destructuring into fields
  ObjectDestructingExample(MapEntry<int, int> entry): MapEntry(key: a, value: b) = entry;  
  
  final int a;
  final int b;
}

// dumb concrete example why this could be useful
class Example {
  // Error: dart does not allow record destructuring into fields
  Example(List<(int, int)> records): (a, b) = _getSum(records);
  
  static (int, int) _getSum(List<(int, int)> records) {
    int sumA = 0;
    int sumB = 0;
    for(final (a,b) in records) {
      sumA += a;
      sumB += b;
    }
    return (sumA, sumB);
  }
  
  final int a;
  final int b;
}

[julemand101]

Sounds like a duplicate of: #2774
(or at least closely related)

[munificent]

Not quite the same, but I'm certain that it we allow destructuring in constructor initializers, then we'll also allow it at field declarations and vice versa, so I'm going to go ahead and close this issue in favor of that one.