Arrays can span multiple lines. A terminating comma (also called trailing comma) is ok after the last value of the array. There can be an arbitrary number of newlines and comments before a value and before the closing bracket.

My question is this: Can comments appear before commas in in-line arrays?

integers3 = [
  1,
  2 # is this a valid toml document?
  , 3
]

According to the ABNF file, the above is not a valid toml document.

However, tomlplusplus parses the above document just fine: https://godbolt.org/z/f6MG38