breaking change: remove support for old super-mixins from CFE and VM

[mraleph]

Background

Currently CFE allows certain targets (e.g. Flutter, Dart CLI) to use old super mixin feature - which permitted to classes to be used as mixins even when they extended something that was not an Object.

Meaning that CFE would compile (and VM would run) the following code:

class Base {}

class Mixin extends Base {
}

class C extends Base with Mixin {
}

Analyzer rejects this code with mixin_inherits_from_not_object error:

  error • test.dart:5:27 • The class 'Mixin' can't be used as a mixin because it extends a class other than 'Object'. • mixin_inherits_from_not_object

Though this error can be suppressed via // ignore:mixin_inherits_from_not_object.

The analyser used to support this feature (hidden under a flag) but this support has been removed.

Proposed change

We propose to remove support for this feature from CFE.

Migration

Users should migrate their class Mixin extends Base declarations to use mixin Mixin on Base declarations instead.

class Base {}

mixin Mixin on Base {
}

class C extends Base with Mixin {
}

Impact

Given that the analyzer issues an error by default we consider the risk to be minimal.

As far as we can neither Flutter no internally Google code base does not contain any suppressions for mixin_inherits_from_not_object which means it does not contain any uses of these feature.

Only users that explicitly suppressed analysis error will be affected and will be required to migrate.

[mraleph]

/cc @devoncarew @johnniwinther

[mraleph]

Announced on announce@dartlang.org. @devoncarew please kick of the process.

[devoncarew]

To paraphrase from above:

• we'd like to remove support for old super-mixins from CFE and VM
• the analyzer no longer supports this syntax (it emits an mixin_inherits_from_not_object error)
• we haven't found places (in flutter or in google3) where this error is ignored - assumed no remaining uses of the older syntax
• and, we've sent the notification + migration email

@vsm - can you review and sign off for Dart? @Hixie - can you do the same for Flutter? And @grouma for Angular? Thanks!

[Hixie]

I didn't realise we still supported this. Sure, this is fine by me.

[Hixie]

Does removing support leave an error message in place that explains how to migrate easily?

[devoncarew]

Does removing support leave an error message in place that explains how to migrate easily?

I assume it would leave the existing error message in place (The class 'Mixin' can't be used as a mixin because it extends a class other than 'Object'). @bwilkerson - perhaps this message could be modified to suggest the use of the mixin ... on syntax?

[devoncarew]

ping @vsm, @grouma

[grouma]

As far as I can tell this will not impact Angular.

[vsmenon]

lgtm

[devoncarew]

@mraleph - this breaking change request is approved; feel free to move to the next steps (https://github.com/dart-lang/sdk/blob/main/docs/process/breaking-changes.md#step-3-execution).

[mraleph]

We are ready to go @johnniwinther

[itsjustkevin]

@johnniwinther Is this something we are still moving forward with?

[johnniwinther]

It is. I'm currently looking into making some preparations in the CFE.

[johnniwinther]

This has landed.

[MaryaBelanger]

@johnniwinther can you confirm this section in the docs on mixins reflects the change?

Edit: also this section

Thanks!

[eernstg]

The section 'Mixins' is up to date. The section 'Adding features to a class: mixins', too.

The latter does contain the sentence

To implement a mixin, create a class that extends Object and declares no constructors.

It would have been better if it said 'To implement a mixin, write a mixin declaration. For backward compatibility, it is also possible to use a class as a mixin if it extends Object (that is, it has the clause extends Object or it does not have an extends clause), and it does not have any constructors', but the current wording isn't actually wrong.