XO0161: list element followed by a newline instead of a comma

Newlines end statements (core 1.3), so a comma separated list (arguments, list, map, or struct literal elements, parameters) that spans lines needs a comma after every element, including the last one. Trailing commas are allowed in every list. The diagnostic carries a fix that inserts the comma. See core 1.3.

Example

fn f() -> List[Int] {
  let xs = [
    1,
    2
  ]
  xs
}

Fix

fn f() -> List[Int] {
  let xs = [
    1,
    2,
  ]
  xs
}

Run xo explain XO0161 for this text in a terminal, or see section 10.6 of the specification.