Wrong warning message in rust

help
#1

For below code snippet, rust compiler with cargo check is giving wrong warning
I am using rust version rustc 1.48.0 (7eac88abb 2020-11-16) and
cargo version cargo 1.48.0 (65cbdd2dc 2020-10-14)
Here is the smallest possible code to replicate the issue.

fn main() {
    let mut input = String::new();
    let result = Some("test");
    match result {
        Some(value) => input = value.to_string(),
        None => return,
    }

    println!("The value is {}", input);
}

Below is the warning message:

warning: value assigned to `input` is never read
 --> src/main.rs:2:9
  |
2 |     let mut input = String::new();
  |         ^^^^^^^^^
  |
  = note: `#[warn(unused_assignments)]` on by default
  = help: maybe it is overwritten before being read?

warning: 1 warning emitted

Is this valid thing or I need to report bug for this. Also where to report bugs for such thing.

#2

Why is it wrong?

#3

The error isn't wrong. The very first value that you assign input to, String::new() is never used. You reassign input to value.to_string() without ever reading the String::new().
The following code produces no warning:

fn main() {
    let input;
    let result = Some("test");
    match result {
        Some(value) => input = value.to_string(),
        None => return,
    }

    println!("The value is {}", input);
}
6 Likes
#4

Ohh, I got it now. I thought that if we declare variable then its mandatory to initialize it.
Thank you.

#5

It's mandatory to initialize a variable if you're trying to read it, or put it into a structure. This variable:

let x: String;

Is in a kind of limbo "write-once-only" state.

#6

The more idiomatic version would be:

fn main() {
    let result = Some("test");
    let input = match result {
        Some(value) => value.to_string(),
        None => return,
    };

    println!("The value is {}", input);
}
12 Likes