in Sources/SwiftFormatPrettyPrint/Comment.swift [23:32]
func trimmingTrailingWhitespace() -> String {
if isEmpty { return String() }
let scalars = unicodeScalars
var idx = scalars.index(before: scalars.endIndex)
while scalars[idx].properties.isWhitespace {
if idx == scalars.startIndex { return String() }
idx = scalars.index(before: idx)
}
return String(String.UnicodeScalarView(scalars[...idx]))
}