How can I get user input without receiving an “Unsed Variable” warning?

Question

I'm taking a look at Rust and decided to build a small program that takes a user's input and prints it, but also want to do some math stuff with it for practice. Currently, this is how I am taking user input:

let mut number = String::new();

let input = io::stdin().read_line(&mut number)
    .ok()
    .expect("Failed to read line");

println!("You entered {}", number);

However, although I do get the correct input this way, Cargo gives me the following warning:

src/main.rs:10:9: 10:14 warning: unused variable: input, #[warn(unused_variables)] on by default

src/main.rs:10 let input = reader.read_line(&mut number)

If I were to just use the input variable, no matter what number I enter I would get a "2" in return when I print the number.

How can I avoid the warning? Is there another way for me to take input without creating 2 variable bindings?

Answer 1

You can simply not write the value to a variable. As long as the type of the value is not marked must_use, you can ignore the value.

let mut number = String::new();

io::stdin().read_line(&mut number)
           .ok()
           .expect("Failed to read line");

println!("You entered {}", number);

---

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You can use the text_io crate for super short and readable input like

let i: i32 = read!()
let tup: (i32, String) = read!("{}, {}");

[/commercial]

Answer 2

It creates a warning because you are allocating space for a variable that is never used.

When faced with such warning you can either replace offending variable with _

let _ = io::stdin().read_line(&mut number) ...

or as ker noted just remove the variable altogether

io::stdin().read_line(&mut number)...

The _ will also work in other situation like parameters or in match clauses.

---

One additional option is to add #[allow(unused_variables)] in the module or crate and disable unused variable warnings. Although, I don't recommend it.