Inconsistent removal of trailing comma in single-argument functions

[charliermarsh]

See: https://play.ruff.rs/96810c27-3971-44af-8eb5-5b5ebae1668e. We remove the trailing comma in the first case, but not in the second, when the trailing comment is present.

Note that this only happens with "skip-magic-trailing-comma": true. If you set "skip-magic-trailing-comma": false in the example above, we consistently insert the trailing comma.

Originally reported here: #1200 (comment).

[charliermarsh]

Another way to look at it: https://play.ruff.rs/5bf54dc0-235e-4cee-9902-90eff0a19150

Here, we don't insert a trailing comma, when we should.

[charliermarsh]

This seems tricky to fix because we need to know if the parent group breaks.

[MichaReiser]

This seems tricky to fix because we need to know if the parent group breaks.

Can you explain in detail or share some IR. You might be able to assign a group id to the parent group and use if_group_breaks(parent_group_id)

This is how it is implemented in biome

The array creates a group:

https://github.com/MichaReiser/biome/blob/59acbfd1e6458b13b5b44d4c5c33ead6a428435e/crates/biome_js_formatter/src/js/expressions/array_expression.rs#L45-L48

and we use it when formatting the trailing separator here

https://github.com/MichaReiser/biome/blob/59acbfd1e6458b13b5b44d4c5c33ead6a428435e/crates/biome_js_formatter/src/js/lists/array_element_list.rs#L42-L44

which ultimately resolves here

https://github.com/MichaReiser/biome/blob/63e4c40051ec15563cc586890c1c350b8c7b948e/crates/biome_formatter/src/separated.rs#L80-L82

But it depends on how the groups are structured in detail. But I thought it might help

[MichaReiser]

This could also be solved by associating the comments with the arguments. Because it would allow you to move the comment out of the inner group.

Using the parent group seems incorrect to me (which, breaks as well because the child group is breaking) because we want to have a trailing comma for

func(
	a, b, c,
): pass
``

[charliermarsh]

I don't think the comment is relevant. This, for example, needs a trailing comma:

def _example_function_xxxxxxx(
    variable: Optional[List[str]]
) -> List[example.ExampleConfig]:
    pass

[MichaReiser]

Hmm, but only if it is a single argument function. Could we avoid the inner group if the function only has a single argument? It seems unnecessary to emit two groups in that case.

[charliermarsh]

Maybe? I can try.