mustBeConst top-level constant

@experimental _MustBeConst const mustBeConst

Used to annotate a parameter which should be constant.

The Dart type system does not allow distinguishing values of constant expressions from other values of the same type, so a function cannot ask to have only constant values as arguments. This annotation marks a parameter as requiring a constant expression as argument. The analyzer can warn, or err if so configured, if a non-constant expression is used as argument.

The annotation can be applied to any parameter, but if it is applied to a parameter of an instance member, subclasses overriding the member will not inherit the annotation. If the subclass member also wants a constant argument, it must annotate its own parameter as well.

Notice that if an annotatated instance member overrides a superclass member where the same parameter is not annotated with this annotation, then a user can cast to the superclass and invoke with a non-constant argument without any warnings.

An example use could be the arguments to functions annotated with ResourceIdentifier, as only constant arguments can be made available to the post-compile steps.

import 'package:meta/meta.dart' show mustBeConst;

void main() {
  f();
  A().i = 3;
}

const v = 3;

int f() => g(v);

int g(@mustBeConst int value) => value + 1;

class A {
  int? _i;

  int? get i => _i;

  set i(@mustBeConst int? value) {
    _i = value;
  }
}

Implementation

@experimental
const _MustBeConst mustBeConst = _MustBeConst();