Have the ability to keep and add commas to the end of a single element in a multi line list

[dickoff]

Precondition: Have multiElementCollectionTrailingCommas set to true in the swift format configuration.

Trailing commas in multi line arrays are great as they allow the git history for a line to be easily maintained when a new element is added later. For example:

let x =
[
    1,
    2,
    3
]

gets formatted to

let x =
[
    1,
    2,
    3,
]

This way, when someone comes along and adds "4," to this list later they will not have needed to add a "," after 3, leaving the git blame for that line alone. Unfortunately the rule for multiElementCollectionTrailingCommas does not keep or insert commas in single element lists. So in the above example if someone wrote

let x =
[
    1,
]

it will be formatted to:

let x =
[
    1
]

This means that when adding a second element to this list you must add a comma after the first element. It would be great if there was a way to configure swift-format to keep these trailing commas for single element multi line lists. Whether this is tied to multiElementCollectionTrailingCommas or some other configuration is certainly up to debate, I don't know the general guidance for whether or not configurations like this should change behavior.

There's this excerpt from PrettyPrint.swift describing this as a limitation.

      // We need to specifically disable trailing commas on elements of single item collections.
      // The syntax library can't distinguish a collection's initializer (where the elements are
      // types) from a literal (where the elements are the contents of a collection instance).
      // We never want to add a trailing comma in an initializer so we disable trailing commas on
      // single element collections.
      let shouldHaveTrailingComma =
        startLineNumber != openCloseBreakCompensatingLineNumber && !isSingleElement
        && configuration.multiElementCollectionTrailingCommas
      if shouldHaveTrailingComma && !hasTrailingComma {
        diagnose(.addTrailingComma, category: .trailingComma)
      } else if !shouldHaveTrailingComma && hasTrailingComma {
        diagnose(.removeTrailingComma, category: .trailingComma)
      }

[ahoppen]

Synced to Apple’s issue tracker as rdar://139474339