Rust Syntax

The main Function

Every executable Rust program needs exactly one function named main, which is the entry point the operating system calls to start your program. Function bodies are wrapped in curly braces, and Rust uses braces (not indentation) to define blocks of code.

fn main() {
    println!("Program started");
}

Statements and Semicolons

Rust is an expression-based language, but individual statements are still terminated with a semicolon, similar to C, Java, or JavaScript. A statement performs an action and does not itself produce a usable value. Forgetting a semicolon where one is required is one of the most common early mistakes new Rust developers make.

fn main() {
    let a = 5;
    let b = 10;
    let sum = a + b;
    println!("Sum: {}", sum);
}

Blocks as Expressions

A block, delimited by curly braces, is itself an expression in Rust and can evaluate to a value. If the last line inside a block has no trailing semicolon, that line's value becomes the value of the whole block. This is why Rust's if, match, and loop constructs can directly return values, unlike in most C-family languages.

fn main() {
    let result = {
        let x = 3;
        let y = 4;
        x * y // no semicolon: this is the block's value
    };
    println!("Block evaluated to: {}", result);
}

This same rule applies to if expressions. Because if is an expression, both branches must produce the same type, and the value can be assigned directly to a variable without needing a separate mutable variable declared beforehand.

fn main() {
    let temperature = 15;
    let description = if temperature > 20 {
        "warm"
    } else {
        "cool"
    };
    println!("It feels {}", description);
}

• Curly braces { } define blocks and function bodies, not indentation
• Statements end in a semicolon and produce no value
• Expressions produce a value and can be used anywhere a value is expected
• Omitting the semicolon on a block's final line makes it the block's return value
• Function bodies are themselves blocks, so the same rule lets you omit 'return' for the final value