peppergrep 0.1.1

A simple grep utility written following the 12th chapter of the Rust book. Some little modifications were made.
Documentation 
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use std::{env, error::Error, fs};

pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
    let contents = fs::read_to_string(config.file_path)?;

    let results = if config.ignore_case {
        search_case_insensitive(&config.query, &contents)
    } else {
        search(&config.query, &contents)
    };

    for line in results {
        println!("{line}");
    }

    Ok(())
}

pub struct Config {
    pub query: String,
    pub file_path: String,
    pub ignore_case: bool,
}

impl Config {
    pub fn build(
        mut args: impl Iterator<Item = String>
    ) -> Result<Config, &'static str> {
        args.next();

        let query: String = match args.next() {
            Some(arg) => arg,
            None => return Err("Didn't get a query string"),
        };

        let file_path = match args.next() {
            Some(arg) => arg,
            None => return Err("Didn't get any source file string")
        };
        
        let mut ignore_case = env::var("IGNORE_CASE").is_ok(); 
        if let Some(arg) = args.next() {
            if arg == "--ignore-case" {
                ignore_case = true;
            } else if arg == "--attend-case" {
                ignore_case = false;
            } else {
                return Err("Invalid `case` parameter; please use `--ignore-case` or `--attend-case`");
            }
        }

        Ok(Config {
            query, 
            file_path, 
            ignore_case
        })
    }
}

pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    contents
        .lines()
        .filter(|line| line.contains(query))
        .collect()
}

pub fn search_case_insensitive<'a>(query: &str, contents: &'a str) -> Vec<&'a str>{
    let query = query.to_lowercase();
    contents
        .lines()
        .filter(|line| line.to_lowercase().contains(&query))
        .collect()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn case_sensitive() {
        let query = "duct";
        let contents = "\
Rust:
safe, fast, productive.
Pick three.
Duct tape.";

        assert_eq!(vec!["safe, fast, productive."], search(query, contents));
    }

    #[test]
    fn case_insensitive() {
        let query = "rUsT";
        let contents = "\
Rust:
safe, fast, productive.
Pick three.
Trust me.";

        assert_eq!(
            vec!["Rust:", "Trust me."],
            search_case_insensitive(query, contents)
        );
    }
}