> I can’t recall a new Rust release ever breaking existing code. Is this what you’re talking about?

I have one case of code from late 2015 that stopped working in, IIRC, 2016, due to unsoundness being found in the borrow checker and it being fixed.

  error[E0713]: borrow may still be in use when destructor runs
     --> src/writer.rs:121:21
      |
  47  | impl<'a, F: Write + Read + Seek + 'a> Writer<'a, F> {
      |      -- lifetime `'a` defined here
  ...
  121 |         Reader::new(self.file)
      |         ------------^^^^^^^^^- returning this value requires that `*self.file` is borrowed for `'a`
  122 |     }
      |     - here, drop of `self` needs exclusive access to `*self.file`, because the type `Writer<'_, F>` implements the `Drop` trait

I ended up solving it by wrapping `self.file` in an "unnecessary" `Option` and using `Option::take` on `self.file` here. I guess I could have mucked around with `MaybeUninit` and risk UB instead, or removed the convenience `Drop` impl finalizer or made the impl non-generic.

I haven't had a case of code no longer compiling since then.