How to Use the concat!, stringify!, and env! Macros

Use concat! to join string literals at compile time, stringify! to turn code into a string literal, and env! to read environment variables at compile time.

let joined = concat!("Hello, ", "world!");
let code = stringify!(let x = 5);
let version = env!("CARGO_PKG_VERSION");

println!("{} {} {}", joined, code, version);

concat! requires all arguments to be string literals. stringify! takes any Rust expression and returns its source code as a string. env! panics at compile time if the specified environment variable is not set.

These three macros help you work with text and configuration before your program even runs. concat! stitches together fixed text strings, stringify! turns code snippets into readable text, and env! grabs system settings like version numbers. Think of them as tools that bake information directly into your final program during the build process.