Giving "const asserts" a whirl

[sethladd]

We should be able to experiment with "const asserts". The Dart team would love our feedback.

For analyzer: dart-lang/sdk#27142 (comment)

tldr: "enableAssertInitializer is included in the language section of the analysis options file"

For VM: dart-lang/sdk#24841 (comment)

tldr: "use flag --assert-initializer."

Thanks!

[sethladd]

cc @Hixie @abarth

[sethladd]

cc @floitschG

[abarth]

This issue is in the "top customer requests" milestone. Which customer has requested this?

[sethladd]

Mixed semantics for the Milestone. We wanted to give the Dart team feedback "ASAP", so this was put into the "sooner than later" milestone.

To help make this milestone's semantics more clear, I've moved it out. But, if we can, let's try this soonish and send feedback over.

[abarth]

I think the analyzer support might have a bug, which is making me unsure of the evaluation:

The analyzer allows this code:

  const MultiChildRenderObjectWidget({ Key key, this.children })
    : assert(children != null), assert(!children.any((Widget child) => child == null)), super(key: key);

But I would have expected it to be an error because https://gist.github.com/lrhn/45bdae95e017ebbf7a84bfbe26dffcae says "the expression in the assert must be a potentially compile-time constant expression". Is this really supported? If so, yay! If not, it will be easier to evaluate after the bug is fixed.

[abarth]

abarth@983be4b is my experiment so far. The analyzer seems happy with all those changes, but I'm worried that not all the expressions in the asserts are potentially compile-time constant expression.

[Hixie]

Does it run? I mean, if they're not compile-time expressions you'll presumably find out pretty quick if you try to violate the constraint at compile time and the compiler allows it.

Ideally the analyzer itself would evaluate the constants to see if they're true.

[sethladd]

cc @bwilkerson for analyzer question (#5865 (comment))

[abarth]

Does it run?

I haven't hacked up the engine yet to turn on the flag.

[lrhn]

Any progress?

[abarth]

@lrhn I never got an answer to my question above. Would you expect this snippet of code to be allowed:

const MultiChildRenderObjectWidget({ Key key, this.children })
    : assert(children != null), assert(!children.any((Widget child) => child == null)), super(key: key);

The type of this.children is List<Widget>. Specifically, I'm surprised that I can call the any function inside an assert in a const constructor.