main2.go

package main

import (
	"bufio"
	"errors"
	"flag"
	"fmt"
	"log"
	"os"
	"strings"
)

func main() {
	count, err := run()
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(count)
}

func run() (uint64, error) {
	flag.Parse()
	if flag.NArg() < 1 {
		return 0, errors.New("missing file to search")
	} else if flag.NArg() < 2 {
		return 0, errors.New("missing search query")
	}
	fpath, query := flag.Arg(0), flag.Arg(1)
	file, err := os.Open(fpath)
	if err != nil {
		return 0, err
	}
	scanner := bufio.NewScanner(file)
	count := uint64(0)
	for scanner.Scan() {
		if strings.Contains(scanner.Text(), query) {
			count++
		}
	}
	if err := scanner.Err(); err != nil {
		return 0, err
	}
	return count, nil
}

main2.rs

use std::env;
use std::error;
use std::fs::File;
use std::io::{self, BufRead, Write};
use std::process;

type Error = Box<error::Error + Send + Sync>;

fn main() {
    match run() {
        Ok(count) => println!("{}", count),
        Err(err) => {
            let _ = writeln!(&mut io::stderr(), "{}", err);
            process::exit(1);
        }
    }
}

fn run() -> Result<u64, Error> {
    let (fpath, query) = match (env::args_os().nth(1), env::args().nth(2)) {
        (None, _) => return Err(Error::from("missing file to search")),
        (_, None) => return Err(Error::from("missing search query")),
        (Some(fpath), Some(query)) => (fpath, query),
    };
    let mut count = 0;
    let rdr = io::BufReader::new(File::open(fpath)?);
    for line in rdr.lines() {
        let line = line?;
        if line.contains(&query) {
            count += 1;
        }
    }
    Ok(count)
}