Previously, the test code would emit E0615, thus revealing the existence
of private methods that the programmer probably does not care about.
Now it ignores their existence instead, producing error E0609 (no field).
The motivating example is:
```rust
let x = std::rc::Rc::new(());
x.inner;
```
which would previously mention the private method `Rc::inner()`, even
though `Rc<T>` intentionally has no public methods so that it can be a
transparent smart pointer for any `T`.
