The Rust SDK represents Fuel's Strings as SizedAsciiString<LEN>, where the generic parameter LEN is the length of a given string. This abstraction is necessary because all strings in Fuel and Sway are statically-sized, i.e., you must know the size of the string beforehand.

Here's how you can create a simple string using SizedAsciiString:

        let ascii_data = "abc".to_string();

            .expect("should have succeeded since we gave ascii data of correct length!");

To make working with SizedAsciiStrings easier, you can use try_into() to convert from Rust's String to SizedAsciiString, and you can use into() to convert from SizedAsciiString to Rust's String. Here are a few examples:

    fn can_be_constructed_from_str_ref() {
        let _: SizedAsciiString<3> = "abc".try_into().expect("should have succeeded");

    fn can_be_constructed_from_string() {
        let _: SizedAsciiString<3> = "abc".to_string().try_into().expect("should have succeeded");

    fn can_be_converted_into_string() {
        let sized_str = SizedAsciiString::<3>::new("abc".to_string()).unwrap();

        let str: String = sized_str.into();

        assert_eq!(str, "abc");

If your contract's method takes and returns, for instance, a Sway's str[23]. When using the SDK, this method will take and return a SizedAsciiString<23>, and you can pass a string to it like this:

    let _ = contract_instance.methods().takes_string(
            .expect("failed to convert string into SizedAsciiString"),