Statements perform actions, expressions return values

In Rust, function definitions and variable declarations are statements. You can’t bind a statement to a variable, because it doesn’t return anything. Calling functions or macros, or making scope blocks are expressions.

Expressions do not include ending semicolons. If you add a semicolon to the end of an expression, you turn it into a statement, and it will then not return a value. This will probably bite you at some point.

// this doesn't compile!

fn main() {
    let x = plus_one(5);

    println!("The value of x is: {x}");
}

fn plus_one(x: i32) -> i32 {
    x + 1;  // this has a semicolon so is a statement
}

// cargo is sad, but helpfully reminds you that you probably didn't want to include that semicolon

Function bodies are made up of a series of statements optionally ending in an expression. The last expression is implicitly returned, although you can return early using the return keyword.

if is an expression, so you can use it in a let statement like this, kind of like a ternary operator.

fn main() {
    let condition = true;
    let number = if condition { 5 } else { 6 };
    // note that both of these ^^         ^^ must be the same type
    println!("The value of number is: {number}");
    //                                   5
}

Sources

• The Rust Book chapter 3.3 and chapter 3.5