message: caller_span only makes sense when there are 2 frames on the stack
diff: @@ -939,6 +939,7 @@ impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {
  /// Get the current span in the topmost function which is workspace-local and not
  /// `#[track_caller]`.
  /// This function is backed by a cache, and can be assumed to be very fast.
  + /// It will work even when the stack is empty.
  pub fn current_span(&self) -> Span {
  self.top_user_relevant_frame()
  .map(|frame_idx| self.stack()[frame_idx].current_span())
  @@ -953,10 +954,9 @@ impl<'mir, 'tcx> MiriMachine<'mir, 'tcx> {
  pub fn caller_span(&self) -> Span {
  // We need to go down at least to the caller (len - 2), or however
  // far we have to go to find a frame in a local crate which is also not #[track_caller].
  - self.top_user_relevant_frame()
  - .map(|frame_idx| cmp::min(frame_idx, self.stack().len() - 2))
  - .map(|frame_idx| self.stack()[frame_idx].current_span())
  - .unwrap_or(rustc_span::DUMMY_SP)
  + let frame_idx = self.top_user_relevant_frame().unwrap();
  + let frame_idx = cmp::min(frame_idx, self.stack().len().checked_sub(2).unwrap());
  + self.stack()[frame_idx].current_span()
  }
  fn stack(&self) -> &[Frame<'mir, 'tcx, Provenance, machine::FrameData<'tcx>>] {
  