What is the difference between having and not having a semicolon?

In Rust, almost everything is an expression. This means that almost everything returns a value. One exception is the variable binding statement let. In a let statement, and many others, the ending semicolon is a mandatory part of the syntax.

However, in expressions, the semicolon has a double role: it throws away a return value of the expression in addition to allowing further expressions. So if the expression is the last in a block, having a semicolon there means that the last value is thrown away, and not having a semicolon there means to return the last value.

An example should make it clear:

// 04/semicolon_block/src/main.rs
fn main()  
{ 
    let x = 5u32; 
 
    let y =  
    { 
        let x_squared = x * x; 
        let x_cube = x_squared * x; 
        x_cube + x_squared + x 
    }; 
 
    let z =  
    { 
        2 * x; 
    }; 
 
    println!("x is {:?}", x); 
    println!("y is {:?}", y); 
    println!("z is {:?}", z); 
} 

We have two different uses of the semicolon. Let's look at the let y line first:

let y =  
    { 
        let x_squared = x * x; 
        let x_cube = x_squared * x; 
        x_cube + x_squared + x // no semi-colon 
    }; 

This code does the following:

• The code within the braces is processed
• The final line, without the semicolon, is assigned to y

Essentially, this is considered as an inline function that returns the line without the semicolon into the variable.

The second line to consider is for z:

    let z =  
    { 
        2 * x; 
    }; 

Again, the code within the braces is evaluated. In this case, the line ends with a semicolon, so the result is thrown away and the empty value () gets bound to z.

When it is executed, we will get the following results:

In the code example, the line within fn main calling recurse gives the same result with or without the semicolon, because the Rust runtime doesn't use main's return value for anything.