Is it possible to declare a variable in the conditional of an if expression in Rust?

Question

In Go, we can declare a variable inside the conditional of an if expression. This variable will be valid inside the if scope, and not outside of it. For example:

func main() {
    if n := 4; n != 0 {
        fmt.Printf("%d is not zero", n)
    } else {
        fmt.Printf("%d is zero", n)
    }

    fmt.Printf("%d", n) // error, n doesn't exist here!
}

Is there a similar syntax in Rust?

Answer 1

Rust does have if let expressions:

if let n = 4 {}

println!("{}", n); // error: cannot find value `n` in this scope

These are primarily used for pattern matching:

let optional_num = Some(1);

if let Some(num) = optional_num {
    println!("optional_num contained", num);
} else {
    println!("optional_num was None");
}

There is an RFC for if let chains that would allow for something like this:

if let n = 4 && n != 0 {
    println!("{} is not zero", n);
}

println!("{}", n); // error, n doesn't exist here!

However, the variables declared in if let chains are scoped only to the if statement, not the else, so your example would not be possible:

if let n = 4 && n != 0 {
    println!("{} is not zero", n);
} else {
    println!("{}", n); // error, n doesn't exist here! 
}