Statements vs Expressions

expressions return values while statements do not.

There are two kinds of statements, declaration statements and expression statements

everything else in Rust is an expression.

the statement x + 1; does not return a value.

Declaration statements

using let to introduce a binding is not an expression. so this code does not work:

let y = (let y = 5);

The compiler is telling us here that it was expecting to see the beginning of an expression, and a let can only begin a statement, not an expression.

Note that assigning to an already-bound variable (e.g. y = 5) is still an expression, although its value is not particularly useful. Unlike other languages where an assignment evaluates to the assigned value (e.g. 5 in the previous example), in Rust the value of an assignment is an empty tuple () because the assigned value can have only one owner, and any other returned value would be too surprising:

let mut y = 5;

let x = (y = 6);  // `x` has the value `()`, not `6`.

Early Returns

you can have your functions perform an early return by using the return keyword.

fn main(i: i32) -> i32 {
	return x + 1;

	i + 2 // this code never gets ran.
}

Diverging functions

you can also create variable bindings which point to functions:

fn plus_one(i: i32) -> i32 {
	i + 1
}

let f: fn(i32) -> i32 = plus_one;

// with type inference
let f = plus_one;