`parseMacroRoleArguments` should allow trailing commas

[allevato]

Description

Discovered via swiftlang/swift-format#1178.

parseMacroRoleArguments explicitly disallows trailing commas, but that seems like an oversight; as far as the user is concerned, that looks (and is represented in the AST) as a regular LabeledExprListSyntax, and the intent of SE-0439 was that the language would support trailing commas in those kinds of lists.

From what I can tell in the Git history, as trailing comma support was rolled out, the argument was added and defaulted to false and then it was updated for custom attributes but not for macro role attributes.

Steps to Reproduce

No response

[allevato]

@mateusrodriguesxyz Do you remember if there was a specific reason that macro role arguments weren't given trailing commas, or was this just missed during the implementation?

[hamishknight]

Based on the discussion on swiftlang/swift#74522 (comment), it sounds like we decided this should fall under "built-in attributes", quoting https://forums.swift.org/t/accepted-with-modifications-se-0439-allow-trailing-comma-in-comma-separated-lists/73216:

Similarly, many built-in attributes make use of custom parsing for parenthesized contents that aren't treated as comma-separated argument lists; in those circumstances, too, we would not add trailing comma support--for instance, @inline(never,).

[allevato]

I guess the quote "make use of custom parsing for parenthesized contents that aren't treated as comma-separated argument lists" is meant to be interpreted as they aren't semantically treated as comma-separated lists, even though the parser parses them as such.

That's unfortunate, because there's no way from a purely syntactic POV to distinguish between a built-in attribute like this and a custom attribute. We've made a problem for ourselves 🙁

The safest choice would be to simply not ever insert trailing commas in attributes (although we could support keeping them if the user has provided one).

[mateusrodriguesxyz]

Based on the discussion on swiftlang/swift#74522 (comment), it sounds like we decided this should fall under "built-in attributes", quoting https://forums.swift.org/t/accepted-with-modifications-se-0439-allow-trailing-comma-in-comma-separated-lists/73216:

Similarly, many built-in attributes make use of custom parsing for parenthesized contents that aren't treated as comma-separated argument lists; in those circumstances, too, we would not add trailing comma support--for instance, @inline(never,).

Yeah, I think everything that uses a custom parsing was left out and macro role was one of them, I believe.