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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();
 
SizedAsciiString::<3>::new(ascii_data)
    .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: 
#[test]
fn can_be_constructed_from_str_ref() {
    let _: SizedAsciiString<3> = "abc".try_into().expect("Should have succeeded");
}
 
#[test]
fn can_be_constructed_from_string() {
    let _: SizedAsciiString<3> = "abc".to_string().try_into().expect("Should have succeeded");
}
 
#[test]
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 call_handler = contract_instance.methods().takes_string(
    "This is a full sentence"
        .try_into()
        .expect("failed to convert string into SizedAsciiString"),
);