JavaScript Dev Does Rust: Statements, expressions, and return values

7/6/2021

In Rust, a statement is a piece of code that does not return a value and always ends with a semicolon

let x = 12;

println!("x is {}", x);

An expression is code that evaluates to something. Most Rust code contains expressions.

let y = 2 + 2;  // 2 + 2 is an expression within a statement

do_something()  // do_something() is an expression the returns a value

This distinction is important, especially when reading functions. It might not be obvious what this function is returning at first glance. A function's return value can be the last expression evaluated by body of the function.

fn sum_and_double(x: i32, y: i32) -> i32 {
	let summed = x + y;
	summed * 2  // since this is the last expression, it is implicitly returned
}

summed * 2 is the last expression so it evaluates and returns that value from the function. This would break if we added a semicolon because that would turn the line into a statement.

This code would result in a compiler error for mismatched types because we are not returning an i32, we are returning nothing!

fn sum_and_double(x: i32, y: i32) -> i32 {
	let summed = x + y;
	summed * 2;  // Compiler error! This is now a statement and nothing gets returned from the function!
}

You can explicitly use the return keyword in functions, which is required for returning early, but the implicit return is a de facto standard and you will see it everywhere.

Pay attention to the semicolons!