You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lev_distance.rs
    path of file: ./repos/nushell/crates/nu-protocol/src
    
    the code of the file until where you have to start completion: // This file is copied from the rust compiler project:
// https://github.com/rust-lang/rust/blob/cf9ed0dd5836201843d28bbad50abfbe1913af2a/compiler/rustc_span/src/lev_distance.rs#L1
// https://github.com/rust-lang/rust/blob/cf9ed0dd5836201843d28bbad50abfbe1913af2a/LICENSE-MIT
//
// - the rust compiler-specific symbol::Symbol has been replaced by &str
// - unstable feature .then_some has been replaced by an if ... else expression

//! Levenshtein distances.
//!
//! The [Levenshtein distance] is a metric for measuring the difference between two strings.
//!
//! [Levenshtein distance]: https://en.wikipedia.org/wiki/Levenshtein_distance

use std::cmp;

/// Finds the Levenshtein distance between two strings.
///
/// Returns None if the distance exce
    
pub fn lev_distance_with_substrings(a: &str, b: &str, limit: usize) -> Option<usize> {
    let n = a.chars().count();
    let m = b.chars().count();

    // Check one isn't less than half the length of the other. If this is true then there is a
    // big difference in length.
    let big_len_diff = (n * 2) < m || (m * 2) < n;
    let len_diff = if n < m { m - n } else { n - m };
    let lev = lev_distance(a, b, limit + len_diff)?;

    // This is the crux, subtracting length difference means exact substring matches will now be 0
    let score = lev - len_diff;

    // If the score is 0 but the words have different lengths then it's a substring match not a full
    // word match
    let score = if score == 0 && len_diff > 0 && !big_len_diff {
        1 // Exact substring match, but not a total word match so return non-zero
    } else if !big_len_diff {
        // Not a big difference in length, discount cost of length difference
        score + (len_diff + 1) / 2
    } else {
        // A big difference in length, add back the difference in length to the score
        score + len_diff
    };

    if score <= limit {
        Some(score)
    } else {
        None
    }
}