Kotlin - Enable trailing comma

[tinsukE]

Trailing commas on Kotlin sources has many advantages:

• It makes version-control diffs cleaner – as all the focus is on the changed value.
• It makes it easy to add and reorder elements – there is no need to add or delete the comma if you manipulate elements.
• It simplifies code generation, for example, for object initializers. The last element can also have a comma.

This PR doesn't go as far as require it, but tweaks KtLint to at least allow it.

The two .kt files prove that the KtLint rules have been properly disabled.

[nikclayton]

If I've understood this correctly (I've tried it locally to confirm) this will allow a mix of trailing and non-trailing commas in future code, with no clean way to migrate naturally as the relevant files get modified over time.

Can this be done so that existing non-trailing-comma usage is grandfathered in, but new usage is forbidden?

E.g., in .editorconfig have:

ktlint_code_style = intellij_idea
ij_kotlin_allow_trailing_comma = true
ij_kotlin_allow_trailing_comma_on_call_site = true

and then create a baseline that includes all the lint "violations" that this will introduce (per the discussion of the baseline file, in https://pinterest.github.io/ktlint/0.50.0/install/cli/#violation-reporting).

Over time violations would shrink, and the baseline file will get smaller (every time it's regenerated) until there are none (or only a handful) to clean up.

For this to work ktlintFormat would need to ignore violations in the baseline file too. I haven't tested that.

[Tak]

IMO it should be situation dependent - neither mandated nor forbidden.
For example, trailing commas are pretty awkward in single-line usage.

[nikclayton]

IMO it should be situation dependent - neither mandated nor forbidden. For example, trailing commas are pretty awkward in single-line usage.

These changes don't affect single-line usage, the rule / format change is only triggered on multi-line call sites and declarations.

// No trailing comma required, formatter does not add one
data class Foo(val x: String, val y: String)

// Trailing comma would be required, lint would fail, formatter adds one
data class Foo(
    val x: String,
    val y: String  // <-- currently allowed, would require trailing comma
)

[tinsukE]

@Tak since this is approved, shall we merge it?

Or would you prefer I look into merging/improving this other one? #3968

[Tak]

Yes, let's merge this one, thank you.