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The Ultimate Guide to Rust Programming

Let Without Assignment

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So far, we’ve immediately assigned values to variables with let. However, it’s valid in Rust to separate these steps out. For example:

Rust 1.40.0
fn main() {
    let x;
    x = 5;
    println!("x == {}", x);
    let mut y;
    y = 5;
    y += 1;
    println!("y == {}", y);
}

I point this out now because it may be surprising that:

  • x is an immutable variable

  • But we’re allowed to say x = 5 ...

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