Who borrowed a variable?

Question

I\'m fighting with the borrow checker. I have two similar pieces of code, one working as I expect, and the other not.

The one that works as I expect:

Answer 1

I put the body of f4() in a main() and implemented Drop for Bar2 to find out when it is dropped (i.e. goes out of scope):

impl<'b> Drop for Bar2<'b> {
    fn drop(&mut self) { println!("dropping Bar2!"); }
}

And the result was:

error: `bar2` does not live long enough
  --> :24:5
   |
24 |     bar2.f();
   |     ^^^^ does not live long enough
25 | }
   | - borrowed value dropped before borrower
   |
   = note: values in a scope are dropped in the opposite order they are created

Something's fishy; let's examine it in detail, with helper scopes:

fn main() {
    {
        let foo = Foo {}; // foo scope begins
        {
            let mut bar2 = Bar2 { x: &foo }; // bar2 scope begins; bar2 borrows foo
            bar2.f();
        } // bar2 should be dropped here, but it has the same lifetime as foo, which is still live
    } // foo is dropped (its scope ends)
}

It looks to me that there is a leak here and bar2 is never dropped (and thus Drop cannot be implemented for it). That's why you cannot re-borrow it.