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); }
| 3 | println!("{}", x); | ^ use of possibly uninitialized `x`

This is based off of a basic branch analysis: every branch must assign a value to x before it is first used. For short, we also say that "x is init" or "x is uninit".

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); }
| 6 | println!("{}", x); | ^ use of possibly uninitialized `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 }

Of course, while the analysis doesn't consider actual values, it does have a relatively sophisticated understanding of dependencies and control flow. For instance, this works:

#![allow(unused)] fn main() { let x: i32; loop { // Rust doesn't understand that this branch will be taken unconditionally, // because it relies on actual values. if true { // But it does understand that it will only be taken once because // we unconditionally break out of it. Therefore `x` doesn't // need to be marked as mutable. x = 0; break; } } // It also knows that it's impossible to get here without reaching the break. // And therefore that `x` must be initialized here! println!("{}", x); }

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:

fn main() { let mut y = Box::new(0); let z = y; // y is now logically uninitialized because Box isn't Copy y = Box::new(1); // reinitialize y }

Otherwise it's like y is a brand new variable.