Check out example codes for "rust get crate version". It will help you in understanding the concepts better.

Code Example 1

const VERSION: &'static str = env!("CARGO_PKG_VERSION");
const OPTION_VERSION: Option<&'static str> = option_env!("CARGO_PKG_VERSION");

// ...

println!("MyProgram v{}", VERSION);
println!("MyProgram v{}", OPTION_VERSION.unwrap_or("unknown"));

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