[Omin]

That definition of what is valid or not depends on the type of checker. When you're talking about Rust's borrow checker, then you can take any property that it checks for, such as "there are no aliasing mutable references" or "any variable that is read from has been initialized" and build an example for it that would be correct, but is rejected by the borrow checker.

Here is an example where a variable is definitely initialized, but it will be rejected.

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