How to use feature flag in Rust to capture multiple lines of code?

November 10, 2022

I realized that doing something like the following does not work in Rust as it throws temp not found error, because it seems the braces around #[cfg(not(feature = "my-feature"))] create a new scope, right?:

fn main() {
    #[cfg(not(feature = "my-feature"))] {
        let temp: usize = 1;
    }
    
    let mut output = 0;
    
    #[cfg(not(feature = "my-feature"))]
    output = temp + 1;
    
    println!("output: {:?}", output);
}

But what’s the proper way then to enclose multiple lines with a feature flag in Rust without introducing a new scope (i.e., to re-use the existing variables later)?

>Solution :

You can use/abuse the fact that blocks are expressions to write something like this.

#[cfg(stuff)]
let (x, y, z) = {
  let x = ...;
  let y = ...;
  let z = ...;

  (x, y, z)
}

let mut output = 0;

#[cfg(stuff)]
println!("{x}, {y}, {z}");

Its not pretty, but it’s better than adding the #[cfg(stuff)] to every line