Why is `return` necessary in `match` arm when using `Result` in `main`?

Question

I am reading Rust by Example book. In this example removing return in Err(e) => return Err(e) causes an error: expected `i32`, found enum `Result`` . Why is that?

What is the difference between Err(e) => return Err(e) and Err(e) => Err(e)?

Here is the code from the example:

use std::num::ParseIntError;

fn main() -> Result<(), ParseIntError> {
    let number_str = "10";
    let number = match number_str.parse::<i32>() {
        Ok(number)  => number,
        Err(e) => return Err(e),
    };
    println!("{}", number);
    Ok(())
}

Answer 1

    let number = match number_str.parse::<i32>() {
        Ok(number)  => number,
        Err(e) => return Err(e),
    };

This says if number_str.parse::<i32>() returns an Ok to set number to number, an i32. If it returns an Err to immediately return Err(e), which is a Result, from main, number is not set.

That's fine because number can only be an i32 and main does return a Result.

    let number = match number_str.parse::<i32>() {
        Ok(number)  => number,
        Err(e) => Err(e),
    };

This says if number_str.parse::<i32>() returns an Ok to set number to number, an i32, same as before. If it returns an Err to set number to Err(e), which is a Result.

number cannot be both an i32 and a Result, so you get a compiler error.