nomicon/checked-uninit.md

% Checked Uninitialized Memory

Like C, all stack variables in Rust are uninitialized until a value is explicitly assigned to them. Unlike C, Rust statically prevents you from ever reading them until you do:

fn main() {
	let x: i32;
	println!("{}", x);
}

src/main.rs:3:20: 3:21 error: use of possibly uninitialized variable: `x`
src/main.rs:3     println!("{}", x);
                                 ^

This is based off of a basic branch analysis: every branch must assign a value to x before it is first used. Interestingly, Rust doesn't require the variable to be mutable to perform a delayed initialization if every branch assigns exactly once. However the analysis does not take advantage of constant analysis or anything like that. So this compiles:

fn main() {
	let x: i32;

	if true {
		x = 1;
	} else {
		x = 2;
	}

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

but this doesn't:

fn main() {
	let x: i32;
	if true {
		x = 1;
	}
	println!("{}", x);
}

src/main.rs:6:17: 6:18 error: use of possibly uninitialized variable: `x`
src/main.rs:6 	println!("{}", x);

while this does:

fn main() {
	let x: i32;
	if true {
		x = 1;
		println!("{}", x);
	}
	// Don't care that there are branches where it's not initialized
	// since we don't use the value in those branches
}

If a value is moved out of a variable, that variable becomes logically uninitialized if the type of the value isn't Copy. That is:

fn main() {
	let x = 0;
	let y = Box::new(0);
	let z1 = x; // x is still valid because i32 is Copy
	let z2 = y; // y is now logically uninitialized because Box isn't Copy
}

However reassigning y in this example would require y to be marked as mutable, as a Safe Rust program could observe that the value of y changed. Otherwise the variable is exactly like new.