Sdk: Add a hint `@nonVirtual` for class members

[matanlurey]

... or, to _help_ developers when defining the intended use of a class, and to _help_ users use a class properly (i.e. as intended by the class). I'd find this extremely _useful_ when working on fairly complex classes with lots and lots of member fields and methods.

Similar to the spirit of @sealed (https://github.com/dart-lang/sdk/issues/27372):

import 'package:meta/meta.dart';

class X {
  @nonVirtual
  void alwaysPrintFoo() {
    print('Foo');
  }
}

class Y extends B {
  // HINT: X.alwaysPrintFoo is marked @nonVirtual and should not be overridden.
  void alwaysPrintFoo() {
    print('Bar');
  }
}

General concept:

• Issue a _hint_ when a member annotated with @nonVirtual is overriden.

Because of how this annotation would need to work, I imagine it would _also_ need to:

• Issue a _hint_ when a class with one or more @nonVirtual members is _implemented_ (implements - not extends/with), as by definition a class with @nonVirtual member requires the _original_ implementation:

  import 'package:meta/meta.dart';
  
  class X {
  @nonVirtual
  void alwaysPrintFoo() {
    print('Foo');
  }
  }
  
  // HINT: Cannot implement a class with one or more `@nonVirtual` members.
  abstract class Y implements X {}
  

• Issue a _hint_ when an _abstract_ member is annotated with @nonVirtual. Also impossible:

  import 'package:meta/meta.dart';
  
  abstract class X {
  // HINT: Cannot define a `@nonVirtual` abstract member.
  @nonVirtual
  void alwaysPrintFoo();
  }
  

/cc @davidmorgan @srawlins for thoughts.

[rkj]

I do like it, would be definitely helpful in many APIs. Why not call it @final to align with Java?

[matanlurey]

I believe that is a reserved word/keyword in Dart. It could be @finalMember, but meh.

(I don't care about the name, to be clear, it can be whatever folks agree on)

[matanlurey]

An alternative is just to support @sealed on member methods/fields, of course!

/cc @srawlins

[filiph]

I keep getting into situations where I'd like to have this functionality.

For reference, an old school issue from @danrubel asking for language-level support of this is here: https://github.com/dart-lang/sdk/issues/3928. I'll update that issue to link here. I do think it's better (and more practical) to implement this via pkg:meta rather than having it in the language.

[rayfarias56]

Also just ran into a situation where this would be useful. Any updates on the priority? I'm willing to contribute the change, just not sure where to begin.

[hacker1024]

It's now October 2019, and this issue's been open for over a year. can someone on the Dart team let us know what they think about this?

[pq]

I'm all for revisiting this. Specifically, it could potentially be used to disallow overriding Widget equality in Flutter (which otherwise we'll have to analyze specifically for). See https://github.com/dart-lang/linter/issues/1783 and it's inspiration: https://github.com/flutter/flutter/pull/41220#discussion_r332737769.

[pq]

OK, running w/ @nonVirtual:

https://dart-review.googlesource.com/c/sdk/+/122701

[pq]

Implemented in server and landed in package:meta:

Looking forward to seeing how folks take advantage!