`iter.last()` will drop all elements of `iter` in order, while
`iter.next_back()` will drop the non-last elements of `iter` when
`iter` goes out of scope since `.next_back()` does not consume its
argument.
When the transformation proposed by `double_ended_iterator_last` would
concern an iterator whose element type has a significant drop, a note is
added to warn about the possible drop order change, and the suggestion
is switched from `MachineApplicable` to `MaybeIncorrect`.
In the case where `iter` is a `DoubleEndedIterator`, replacing a call to
`iter.last()` (which consumes `iter`) by `iter.next_back()` (which
requires a mutable reference to `iter`) cannot be done when `iter`
Is not a mutable binding or a mutable reference.
When `iter` is a local binding, it can be made mutable by fixing its
definition site.
