b.rs

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

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

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

// error[E0495]: cannot infer an appropriate lifetime for lifetime parameter in function call due to conflicting requirements
//  --> b.rs:8:36
//   |
// 8 |         self.current_token = Some(&self.str[0..1]);
//   |                                    ^^^^^^^^^^^^^^
//   |
// note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 7:5...
//  --> b.rs:7:5
//   |
// 7 | /     pub fn lex(&mut self) {
// 8 | |         self.current_token = Some(&self.str[0..1]);
// 9 | |     }
//   | |_____^
// note: ...so that reference does not outlive borrowed content
//  --> b.rs:8:36
//   |
// 8 |         self.current_token = Some(&self.str[0..1]);
//   |                                    ^^^^^^^^
// note: but, the lifetime must be valid for the lifetime 'a as defined on the impl at 6:6...
//  --> b.rs:6:6
//   |
// 6 | impl<'a> Parser<'a> {
//   |      ^^
//   = note: ...so that the expression is assignable:
//           expected std::option::Option<&'a str>
//              found std::option::Option<&str>
// 
// error: aborting due to previous error
// 
// For more information about this error, try `rustc --explain E0495`.