The main Function

Every runnable Rust program needs a function named main — it's the entry point the compiled program starts executing from. You declare it with the fn keyword, followed by the name, a pair of parentheses for its parameters, and a pair of curly braces holding the body.

fn main() {
    println!("Hello, world!");
}

Rust uses curly braces {} to mark the body of a function, the same style used by C, Java, and JavaScript — unlike Python, which uses indentation alone. Each statement inside the body ends with a semicolon, and the compiler will refuse to build your program if you forget one on a statement that needs it.

A Rust program will fail to compile if it has no fn main() function defined anywhere.

In the program below, where does execution begin?

fn main() {
    println!("Hello, world!");
}