Code shared from the Rust Playground

playground.rs

use std::iter::Peekable;
use std::str::Chars;

pub enum Token {
    E,
    Rest,
}

pub struct Lexer<'a> {
    chars: Peekable<Chars<'a>>,
}

impl Lexer<'_> {
    pub fn new(input: &str) -> Lexer {
        Lexer {
            chars: input.chars().peekable(),
        }
    }

    fn foo(&mut self) {}

    // compiles
    pub fn next(&mut self) -> Option<Token> {
        match self.chars.peek() {
            Some(&'e') => {
                self.foo();
                Some(Token::E)
            }
            Some(_) => Some(Token::Rest),
            _ => None,
        }
    }

    // compiler error
    pub fn next2(&mut self) -> Option<Token> {
        self.chars.peek().cloned().map(|ch| match ch {
            'e' => {
                self.foo();
                Token::E
            }
            _ => Token::Rest,
        })
    }
}

fn main() {
    let input = "Hello";
    let mut l = Lexer::new(input);
    l.next();
}