multilineTrailingCommaBehavior: alwaysUsed breaks macro attributes e.g. @attached(...)

[ktoso]

A settings file:

  {
    "version": 1,
    "multilineTrailingCommaBehavior": "alwaysUsed"
  }

Insists on adding , at the @attached() macro parameters, which doesn't actually parse:

// --- BEFORE swift-format (compiles) ---

public protocol AnyJavaObject {}

@attached(
  member,
  names: named(fullJavaClassName),
  named(javaHolder),
  named(init(javaHolder:)),
  named(JavaSuperclass),
  named(`as`),
)
@attached(extension, conformances: AnyJavaObject)
public macro JavaClass(
  _ fullClassName: String,
  extends: (any AnyJavaObject.Type)? = nil
) = #externalMacro(module: "MyMacros", type: "JavaClassMacro")

Then apply swift-format with the above setting:

// --- AFTER swift-format (does NOT compile) ---
//
// @attached(
//   member,
//   names: named(fullJavaClassName),
//   named(javaHolder),
//   named(init(javaHolder:)),
//   named(JavaSuperclass),
//   named(`as`),           // <-- trailing comma added, breaks compilation
// )
// @attached(extension, conformances: AnyJavaObject)
// public macro JavaClass(
//   _ fullClassName: String,
//   extends: (any AnyJavaObject.Type)? = nil,  // <-- trailing comma added
// ) = #externalMacro(module: "MyMacros", type: "JavaClassMacro")

/tmp/swift-format-trailing-comma-bug.swift:30:1: error: unexpected ',' separator
28 |   named(JavaSuperclass),
29 |   named(`as`),
30 | )
   | `- error: unexpected ',' separator
31 | @attached(extension, conformances: AnyJavaObject)
32 | public macro JavaClass(

[allevato]

That seems like a bug in the parser that we should fix! The intent of SE-0439 was that the language would support trailing commas in lists that looked like standard argument lists, and @attached fits that bill since it is indeed being parsed as a LabeledExprListSyntax. 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.

In the meantime, I guess we need to explicitly special-case @attached and @freestanding in swift-format. ☹️