Function reedline::menu_functions::completer_input

pub fn completer_input(
    buffer: &str,
    insertion_point: usize,
    prev_input: Option<&str>,
    only_buffer_difference: bool
) -> (String, usize)

Get the part of the line that should be given as input to the completer, as well as the index of the end of that piece of text

prev_input is the text in the buffer when the menu was activated. Needed if only_buffer_difference is true