b.rs

struct Parser<'a> {
    pub str: String,
    pub current_token: Option<&'a str>,
}

impl<'a> Parser<'a> {
    pub fn lex(&'a mut self) {
        let tmp: &'a str = &self.str[0..1];
        self.current_token = Some(tmp);
    }
}

fn main() {
    let mut parser = Parser{
        str: "hello".to_string(),
        current_token: None,
    };
    parser.lex();
}