Its not that hard:

{
  let txn = Transaction::new()
}

In this case the compiler knows when it will be deconstructed. If note holds a reference to this beyond this scope then it is easily caught by the compiler. The ‘a parameter links the two references and can catch when one outlives the other. 

 I'm thinking that you can have code that conditionally at runtime might destroy an object and free its memory.
Probably rustc just assumes the worst 

 I don’t think you can explicitly do this in rust except in an unsafe block