`require_trailing_commas` is not triggered on switch expressions

[FMorschel]

Describe the issue
require_trailing_commas is not triggered on switch expressions.

To Reproduce

enum E {
  a,
  b,
  c,
  d,
  e,
  f;
}

int a(E e) {
  return switch (e) {
    E.a => 0,
    E.b => 1,
    E.c => 2,
    E.d => 3,
    E.e => 4,
    E.f => 5     // Should trigger
  };
}

Expected behavior
Triggering of require_trailing_commas.

[FMorschel]

Also, the current assist to transform switch statements into switch expressions doesn't place a trailing comma there even if the lint is enabled.

If the reviewer agrees, I can make a new issue if necessary.

[bwilkerson]

I'm not sure we want to trigger the lint in this case.

My understanding is that the lint exists because of the way a trailing comma in a parameter or argument list impacts the formatter, and some users wanted their code to always be formatted per the trailing-comma style. To the best of my knowledge, the formatter behaves the same whether of not there's a trailing comma in a switch expression, making this an odd choice.

If the formatter is updated so that the trailing comma no longer impacts the behavior, then we're likely to consider deprecating this lint.

[FMorschel]

I'm sorry but it behaves differently if the switch expression is short:

enum E {
  a,
  b,
  c;
}

int a(E e) {
  return switch (e) { E.a => 0, E.b => 1, E.c => 2 };
}

I'm sorry if I was not clear enough to start.

Edit

And if the trailing comma is added, it breaks the switch:

return switch (e) {
  E.a => 0,
  E.b => 1,
  E.c => 2,
};

[bwilkerson]

Thanks, I don't think I've written a switch expression short enough to fit on one line.

My expectation, given that, is that the future release of the formatter will likewise add and remove the trailing comma from switch expressions, making it unnecessary to extend this lint rule to support that case.

[dart-github-bot]

Summary: The require_trailing_commas linter rule doesn't flag missing trailing commas in switch expressions, despite being expected behavior.