Java 10+ Local-Variable Type Inference should be used `RSPEC-6212`

[yeikel]

This is controversial and the Sonar team had to remove it from the default "Sonar Way" profile

https://rules.sonarsource.com/java/tag/java10/RSPEC-6212

[tkvangorder]

This would be straightforward to implement with OpenRewrite, it can exist as a standalone recipe. This leverage UsesJavaVersion in its applicability test.

[MBoegers]

I'd like to work on this one and would like to clarify a few things.

To be sure, do we want to implement RSPEC-6212 100% Sonar like? I would argue against it because it also suggests using var for primitives, which is not recommended, as you can see in Project Ambers Local Variable Type Inference - G7.

[MBoegers]

Things I thought about, anything to add?

1. transform local Variables like: MyClass myClass = new MyClass(); to var myClass = new MyClass();
2. keep null assignments like String line = null
3. special treatment for generics and the diamond operator List<String> lines = new ArrayList<>() to var lines = new ArrayList<String>(), keep also var list = List.of() in mind
4. We depend on prior good naming, otherwise some information will be lost. But I think that's out of scope here, right?

[timtebeek]

Thank you for your offer to help out on this one! I think you have a decent list of restrictions in place already to limit applicability. I'd indeed say not to use this for primitives, and even then it's still not always a clear improvement, which makes it a tricky recipe to recommend.

Might be good to take into account the upcoming changes for OpenRewrite 8.0, which will have change the applicability tests for instance.

Maybe we can start with a suite of unit tests and do a round of feedback on that? Then we can more clearly decide which cases are worth covering, before we look into the finer details of the recipe implementation. Feel free to to open a draft pull request with what cases you would (not) want to cover as part of this work!

[yeikel]

I'd like to work on this one and would like to clarify a few things.

To be sure, do we want to implement RSPEC-6212 100% Sonar like? I would argue against it because it also suggests using var for primitives, which is not recommended, as you can see in Project Ambers Local Variable Type Inference - G7.

boolean seems to be an exception for this rule, right? (so not all primitives are excluded)