Function edit_line 

fn edit_line(
    e: &LineEdit,
    prompt: &'static str,
    formula: bool,
    d: Option<&Diagnostic>,
) -> Line<'static>

The prompt (edit> , or formula> for a formula) and the text, the cell under the caret reversed and a diagnostic’s span red and underlined. In a formula with the caret at the end inside a call, the parameters still to type follow, dimmed.