How can I find the index of a character in a string in Rust?

Although a little more convoluted than I would like, another solution is to use the Chars iterator and its position() function:

"Program".chars().position(|c| c == 'g').unwrap()

find used in the accepted solution returns the byte offset and not necessarily the index of the character. It works fine with basic ASCII strings, such as the one in the question, and while it will return a value when used with multi-byte Unicode strings, treating the resulting value as a character index will cause problems.

This works:

let my_string = "Program";
let g_index = my_string.find("g");   // 3
let g: String = my_string.chars().skip(g_index).take(1).collect();
assert_eq!("g", g);   // g is "g"

This does not work:

let my_string = "プログラマーズ";
let g_index = my_string.find("グ");    // 6
let g: String = my_string.chars().skip(g_index).take(1).collect();
assert_eq!("グ", g);    // g is "ズ"

If your word has several g's you could use enumerate to find all indices:

"ඞggregate"
    .chars()
    .enumerate()
    .filter(|(_, c)| *c == 'g')
    .map(|(i, _)| i)
    .collect::<Vec<_>>();  // [1, 2, 5]

If the string contains ASCII characters only:

"aggregate"
    .bytes()
    .enumerate()
    .filter(|(_, b)| *b == b'g')
    .map(|(i, _)| i)
    .collect::<Vec<_>>();  // [1, 2, 5]

Playground

Finding a character that satisfies some condition and then using its byte index works like this:

fn example(s: &str) -> Option<char> {
    // find (the byte index of) some character satisfying some condition
    if let Some(byte_index) = s
        .char_indices()
        .find_map(|(p, c)| c.is_numeric().then_some(p))
    {
        // split the string there for further processing
        let (_first, last): (&str, &str) = s.split_at(byte_index);
        // the character that we found
        let found_char: char = last.chars().next().unwrap();
        Some(found_char)
    } else {
        // no such character
        None
    }
}