It is not allowed to use or capture an uninitialized variable.
Erroneous code example:
```compile_fail,E0381
fn main() {
let x: i32;
let y = x; // error, use of possibly-uninitialized variable
}
```
To fix this, ensure that any declared variables are initialized before being
used. Example:
```
fn main() {
let x: i32 = 0;
let y = x; // ok!
}
```