DefaultImpl is a highly confusing name for what we now call auto impls,
as in `impl Send for ..`. The name auto impl is not formally decided
but for sanity anything is better than `DefaultImpl` which refers
neither to `default impl` nor to `impl Default`.
As part of the audit for #22820 the following feature gate tests have
been added:
* `negate_unsigned`
* `on_unimplemented`
* `optin_builtin_traits`
* `plugin`
* `rustc_attrs`
* `slice_patterns`