Rust Essentials - Second Edition

Expressions

Rust is an expression-oriented language, which means that most pieces of code are in fact expressions, that is, they compute a value and return that value. However, expressions on themselves do not form meaningful code; they must be used in statements.

let bindings like the following are declaration statements; they are not expressions:

// see Chapter 2/code/expressions.rs 
let a = 2;    // a binds to 2 
let b = 5;    // b binds to 5 
let n = a + b;   // n binds to 7 

But, here, a + b is an expression and, if we omit the semicolon at the end, the resulting value (here the value 7) is returned. This is often used when a function needs to return its value (see examples in the next chapter). Ending an expression with a semicolon like a + b; ...