Finding the largest product in a series with Rust

The eight Project Euler problem - Largest Product in a Series - is stated as follows. The four adjacent digits in the 1000-digit number below that have the greatest product are 9 × 9 × 8 × 9 = 5832.

73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

fn char_to_u64(c: char) -> u64 {
    // Convert the char to a digit in base 10.
    c.to_digit(10) 
        // Return the value or default to 0.
        .unwrap_or(0) as u64
}

fn product_of_string(s: &str) -> u64 {
    // Returns an iterator over the chars.
    s.chars()
        // Convert chars to u64 and get their product.
        .fold(1, |product, n| char_to_u64(n) * product)
}

fn largest_product_in_a_series(series: &str, len: usize) -> u64 {
    // Iterate from 0 to the series length minus the length
    // of the number of adjacent digits we are looking for.
    (0..series.len() - (len - 1))
        // Get the digits we are looking for as a string.
        .map(|x| &series[x..x + len])
        // Convert the chars to u64 and get their product.
        .map(|x| product_of_string(x))
        // Get the largest product.
        .max()
        // Return the value or default to 0.
        .unwrap_or(0)
}

fn main() {
    let l = 13;
    let s = "73167176531330624919225119674426574742355349194934\
             96983520312774506326239578318016984801869478851843\
             85861560789112949495459501737958331952853208805511\
             12540698747158523863050715693290963295227443043557\
             66896648950445244523161731856403098711121722383113\
             62229893423380308135336276614282806444486645238749\
             30358907296290491560440772390713810515859307960866\
             70172427121883998797908792274921901699720888093776\
             65727333001053367881220235421809751254540594752243\
             52584907711670556013604839586446706324415722155397\
             53697817977846174064955149290862569321978468622482\
             83972241375657056057490261407972968652414535100474\
             82166370484403199890008895243450658541227588666881\
             16427171479924442928230863465674813919123162824586\
             17866458359124566529476545682848912883142607690042\
             24219022671055626321111109370544217506941658960408\
             07198403850962455444362981230987879927244284909188\
             84580156166097919133875499200524063689912560717606\
             05886116467109405077541002256983155200055935729725\
             71636269561882670428252483600823257530420752963450";

    println!(
        "{} adjacent digits = {}",
        l,
        largest_product_in_a_series(s, l)
    );
}