Reading files is a fundamental operation in any programming language. In Rust, the standard library provides several ways to read files, each suited for different use cases.

The most basic way to read a file is using std::fs::read_to_string:

use std::fs;
 
let contents = fs::read_to_string("file.txt")?;
println!("{}", contents);

For more control, you can use File::open with Read trait methods:

use std::fs::File;
use std::io::Read;
 
let mut file = File::open("file.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;

For line-by-line reading, BufReader provides efficient buffered access:

use std::fs::File;
use std::io::{BufRead, BufReader};
 
let file = File::open("file.txt")?;
let reader = BufReader::new(file);
 
for line in reader.lines() {
    let line = line?;
    println!("{}", line);
}

The BufReader buffers reads internally, which is more efficient than reading one byte at a time, especially for large files.

Implement the following functions for reading file contents:

1. read_entire_file(path: &str) -> Result<String, io::Error> - Read the entire contents of a file into a String
2. count_lines(path: &str) -> Result<usize, io::Error> - Count the number of lines in a file
3. count_words(path: &str) -> Result<usize, io::Error> - Count the number of words in a file (words are whitespace-separated)
4. read_lines(path: &str) -> Result<Vec<String>, io::Error> - Read all lines into a Vec, with trailing newlines removed
5. first_n_lines(path: &str, n: usize) -> Result<Vec<String>, io::Error> - Read only the first n lines from a file

// Assuming a file "example.txt" contains:
// Hello World
// This is a test
// Rust is great
 
assert_eq!(
    read_entire_file("example.txt")?,
    "Hello World\nThis is a test\nRust is great\n"
);
assert_eq!(count_lines("example.txt")?, 3);
assert_eq!(count_words("example.txt")?, 9);
assert_eq!(
    read_lines("example.txt")?,
    vec!["Hello World", "This is a test", "Rust is great"]
);
assert_eq!(
    first_n_lines("example.txt", 2)?,
    vec!["Hello World", "This is a test"]
);