Rule to mandate/forbid trailing commas

[3flex]

Expected Behavior

ktlint offers a rule that enforces a particular code style regarding trailing commas, which are now permitted by the Kotlin compiler as of 1.3.70, see https://youtrack.jetbrains.com/issue/KT-34743 for more details.

This may not be a good fit for ktlint as it's too opinionated and no conventions exist yet, but if it's suitable it's a better fit here than in detekt where it was originally proposed.

Observed Behavior

No such rule exists.

Steps to Reproduce

N/A, new rule required.

Your Environment

• Version of ktlint used:
• Name and version (or code for custom task) of integration used (Gradle plugin, Maven plugin, command line, custom Gradle task):
• Version of Gradle used (if applicable):
• Operating System and version:
• Link to your project (if it's a public repository):

[alexvanyo]

Ideally this rule could be as flexible as something like ESLint's comma dangling rule, which would allow specifying when trailing commas must be used (never, always, multiline-only, when entry, etc.)

Edit: Perhaps supporting that feature set would better live elsewhere?

[Tapchicoma]

Probably related to #701 to control enable/disable state for cases, though initial rule implementation could just enforce trailing commas and be in experimental set.

[s1monw1]

Any update?

[ivorsmorenburg]

Is breaking so many things this ,
https://stackoverflow.com/questions/65055798/kotlin-linting-rulesetprovider-trailing-comma

[3flex]

Is this on ktlint's roadmap? If not, we could implement in detekt instead. I don't think it makes sense to do that though if ktlint will implement the rule so we can avoid duplication of effort, but I think there's demand for it (I'm sure this is my most 👍 issue ever!)

[ivorsmorenburg]

Hopefully is or it will be 🤞, for now I'm trying to implement my own rule from the link above. I can confirm there is a high demand. Will be nice to autocorrect with ktlint...

[romtsn]

@3flex you are more than welcome to contribute this to ktlint directly and utilize it in detekt afterwards 👀 it is certainly on the roadmap, just the maintainers were sidetracked by the day jobs and we had other priorities in ktlint, but I see this one indeed has a high demand so we'll re-prioritize