How do I find the index of an element in an array, vector or slice?

TLDR Use an iterator with the position method, the Rust docs shows a good example.

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No, it's because indices are usize, not i32. In fact, i32 is completely inappropriate for this purpose; it may not be large enough, and there's no reason for it to be signed. Just use usize.

Some other notes: calling to_string() is not free, and you don't need it for the comparison; you can compare string slices just fine!

Also, if you really want to turn a usize into an i32, you can do that with a cast: x as i32, though this will not produce an error on over- or under-flow (i.e. the result may be negative).

All that said, as noted in Mathieu David's answer, there's a position method on iterators that does what you want.

I think you should look at the position method instead.

fn main() {
    let test = vec!["one", "two", "three"];
    let index = test.iter().position(|&r| r == "two").unwrap();
    println!("{}", index);
}

You can test it here.

Note that this works for any iterator, so it can be used for vectors, arrays, and slices, all of which produce iterators.