proposal: no constructor should give const suggestion?

[stephane-archer]

moving from:

class _FalseColorSwitch extends StatelessWidget {

  @override
  Widget build(BuildContext context) {

to

class _FalseColorSwitch extends StatelessWidget {
  const _FalseColorSwitch(); // adding a const constructor that does nothing

  @override
  Widget build(BuildContext context) {

give me so many linter optimization suggestions:

Use 'const' with the constructor to improve performance.
Try adding the 'const' keyword to the constructor invocation.

I think no constructor should give me the same recommendation as a const constructor that does nothing. is there an actual difference?

[srawlins]

Moving over to the language repo for the language request. I think this has been asked and answered before ("const by default"), but someone here can point to the past response.

[mateusfccp]

Refer to dart-lang/language#1410. I don't know if it's the exact same case, but it has a related discussion explaining how some things work.

[lrhn]

The reason you don't get const without asking for it, is that you might get const without wanting it.

A constructor being const is a promise to always allow const construction of the class. Changing that is a breaking change.
That kind of promise is not something the system should make for you, it's something you have to explicitly state.

If not, you could accidentally have a class that supports const construction in one version of a package, and not in the next one, because you added a mutable cache for some variable, and never even considered that the class could be const, because it was never intended as such. (So it's not tested.)
And that could break someone else, who relied on the implicit const-ness of the former version.

(I have no opinion on what lints should suggest here, but the default constructor you get if you write nothing is not a const constructor, whether it could be or not.)

[stephane-archer]

@lrhn I understand now why the constructor is not const by default if it can.
I think a linter suggestion to add a const constructor would be nice.
That way people don't miss the opportunity for extra optimization and it's explicit that they have a const constructor, so the breaking change will be clear to anyone (especially removing it).

[lrhn]

Ack. That's why I consider this an analyzer or linter issue, not a language issue.

[stephane-archer]

@lrhn I agree, that is why it was originally in dart-lang/linter but @srawlins transferred it here.
Could you please transfer it back?

[srawlins]

Thanks for the clarification 👍

[srawlins]

Looks like a duplicate of #59001 and #59273.

[stephane-archer]

I think it's a duplicate too, thanks for pointing out existing issues.