https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=2029ea20866ddae3e94f65745dc02e8e

numbered-placeholders.rs

fn main() {
    let input = "Some string {} another value {} third value {}";
    let output = numbered_placeholders(input);
    let expected_output = "Some string {0} another value {1} third value {2}";
    assert_eq!(output, expected_output);
}

fn numbered_placeholders(s: &str) -> String {
    let mut counter = 0;
    let mut result = String::new();
    let mut chars = s.chars().peekable();

    while let Some(c) = chars.next() {
        if c == '{' && chars.peek() == Some(&'}') {
            result.push_str(&format!("{{{}}}", counter));
            counter += 1;
            chars.next(); // consume the '}' character
        } else {
            result.push(c);
        }
    }
    result
}