Multiple self borrows

Question

I'm learning Rust and am trying to write a simple lexer. I have a function next_token in an impl block that reads the next token from an input character iterator. However, the code will not build because of multiple borrows of self. How do I get around this?

struct Lexer<'a> {
    input: Peekable<Chars<'a>>
}

...

impl<'a> Lexer<'a> {
    pub fn create(input: &str) -> Lexer {
        Lexer { input: input.chars().peekable() }
    }

    fn next_token(&mut self) -> Option<Token> {
        match self.input.by_ref().peek() {
            Some(c) if c.is_alphabetic() || *c == '_' => { // Identifier or keyword
                let literal = self.input.take_while(|&ch| {
                    match self.input.peek() {
                        Some(inner_c) => inner_c.is_alphanumeric() || *inner_c == '_',
                        None => false
                    }
                }).collect::<String>();
                if let Some(keyword) = check_keyword(&literal) { // Literal is a keyword
                    return Some(Token { literal: literal, token_type: keyword });
                } else { // Literal is identifier
                    return Some(Token { literal: literal, token_type: TokenType::Identifier });
                };
            },
            ... // Other cases
        }
    }
}

The error I am getting:

error[E0507]: cannot move out of borrowed content --> lexer.rs:110:20 | 110 | let literal = self.input.take_while(|&ch| { | ^^^^ cannot move out of borrowed content

error[E0500]: closure requires unique access to self but self.input is already borrowed --> lexer.rs:110:42 | 108 | match self.input.by_ref().peek() { | ---------- borrow occurs here 109 | Some(c) if c.is_alphabetic() || *c == '_' => { // Identifier or keyword 110 | let literal = self.input.take_while(|&ch| { | ^^^^^ closure construction occurs here 111 |
match self.input.peek() { | ---- borrow occurs due to use of self in closure ... 139 | } | - borrow ends here