request: add `remainder` method to `core::iter::Split`

[Lokathor]

It would be very helpful if the Split iterator let you get back the currently remaining slice. There's splitn when you know ahead of time the number of splits you need, but sometimes you don't, and you need to dynamically determine when to stop splitting.

[Patrick-Poitras]

@Lokathor could you give an example of how you would use it, or an example using it. I'm not sure I quite understand what is being requested here.

[Lokathor]

Sure, in this case I needed to do this because I wanted to parse some Netpbm files, which are ascii files that sometimes will transition to being binary data after a certain point.

Without getting too deep into the weeds, there's a tag, then width, then height, and then "pixel data", but pixel data might be more ascii data or it might actually be binary bytes. So one might start with a file_bytes.split(|&u| u == b'\n') to get the lines of the file, but after you've read far enough to get to the header info (which is always ascii), you may need to stop splitting on newline (if the header tag was P4, P5, or P6), and at that point you want to process the rest of the file data in a new way, without losing your position so far.

let mut header = MyHeaderData::default();
let mut pixel_data: &[u8] = &[];
let mut line_it = file_data.split(|&u| u == b'\n');
while let Some(line) = line_it.next() {
  // handle the header data by filling in the struct
  if all_header_data_has_been_parsed {
    pixel_data = line_it.remainder();
    break;
  }
}
// from here we know if we'll need to parse `pixel_data` as ascii or as binary