How to set lifetime for this Iterator trait impl?

Question

I have these structs

pub struct Lexer {
    input: Vec<char>,
    // ...
}

pub struct Token<'a> {
    pub t: TokenType,
    pub literal: Literal<'a>,
}

pub enum Literal<'a> {
    Chars(&'a [char]), // here borrows from the origin input string in Lexer
    // ...
}

The field literal of Token with type Literal has some slice borrowing directly from origin input string in the input field of struct Lexer. There is a method of Lexer called next_token like this:

pub fn next_token(&mut self) -> Token

I'm intending to let Lexer implement the traitor Iterator like :

impl<'a> Iterator for Lexer {
    type Item = Token<'a>;

    fn next(&mut self) -> Option<Self::Item> {

        let t = self.next_token();
        Some(t)
    }

But get errors like

the lifetime parameter 'a is not constrained by the impl trait, self type, or predicates"

How can I set the correct lifetime and implement Iterator trait for Lexer?