You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: validate.rs
    path of file: ./repos/sd/src/replacer
    
    the code of the file until where you have to start completion: use std::{error::Error, fmt, str::CharIndices};

#[derive(Debug)]
pub struct InvalidReplaceCapture {
    original_replace: String,
    invalid_ident: Span,
    num_leading_digits: usize,
}

impl Error for InvalidReplaceCapture {}

// NOTE: This code is much more allocation heavy than it needs to be, but it's
//       only displayed as a hard error to the user, so it's not a big deal
impl fmt::Display for InvalidReplaceCapture {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        #[derive(Clone, Copy)]
        enum SpecialChar {
            Newline,
            CarriageReturn,
            Tab,
        }

        impl SpecialChar {
            fn new(c: char) -> Option<Self> {
                match c {
                    '\n' => Some(Self::Newline),
                    '\r' => Some(Self::CarriageReturn),
                    '\t' => Some(Self::Tab),
                    _ => None,
                }
            }

            /// Renders as the character from the "Control Pictures" block
            ///
            /// https://en.wikipedia.org/wiki/Control_Pictures
            fn render(sel
    
    fn next(&mut self) -> Option<Self::Item> {
        // Continually seek to `$` until we find one that has a capture group
        loop {
            let (start, _) = self.0.find(|(_, c)| *c == '$')?;

            let replacement = self.0.as_str();
            let rep = replacement.as_bytes();
            let open_span = Span::start_at(start + 1);
            let maybe_cap = match rep.first()? {
                // Handle escaping of '$'.
                b'$' => {
                    self.0.next().unwrap();
                    None
                }
                b'{' => find_cap_ref_braced(rep, open_span),
                _ => find_cap_ref(rep, open_span),
            };

            if let Some(cap) = maybe_cap {
                // Advance the inner iterator to consume the capture
                let mut remaining_bytes = cap.name.len();
                while remaining_bytes > 0 {
                    let (_, c) = self.0.next().unwrap();
                    remaining_bytes =
                        remaining_bytes.checked_sub(c.len_utf8()).unwrap();
                }
                return Some(cap);
            }
        }
    }
}