Statement-Expression

some theory

Rust is a expression-based language. Fundamental definitions are:

• Statements: are instructions that perform some action and do not return a value.

#![allow(unused)]
fn main() {
let a = 1; // is an statement.
}

#![allow(unused)]
fn main() {
let a = (let b = 1); // since it do not return anything, a statement cannot be part of an assignment.
}

• Expressions: evaluate to a resultant value.

#![allow(unused)]
fn main() {
let a = {
    let b = 1;
    b + 1 // this is an expression.
    // an expression at the end of a scope is the return value of the scope.
};
}

#![allow(unused)]
fn main() {
let a: u32 = {
    let b = 1u32;
    b + 1; // the semicolon at the end transforms an Expression into a Statement.
};
}

For details please check the Rust Book.