Mocking types that users not own [1] or are severely complicating test
logic [2] leads to brittle or wrong tests. In particular, the
StackOverflow answer is wrong, as the contract of java.util.Map is
violated. When a new key is added to the Map, the stubbed return would be wrong.
In Google we have used the DoNotMock annotation via ErrorProne [3]
to annotate these types, as well as an internal list of types that can't
be mocked (this includes several java.util types). We are using a custom
Mockmaker to enforce this on run-time.

Based on our successful experience with DoNotMock (we have seen a large
reduction in bad/broken tests for types involved), we are proposing to
open source this into Mockito itself.

The DoNotMock annotation can be added to any type, e.g. classes and
interfaces. If, in the type hierarchy of the class-to-be-mocked, there
is a type that is annotated with DoNotMock, Mockito will throw a
DoNotMockException.

This would help preventing issues such as #1827 and #1734 which is
in-line with the guidance on our wiki [1]. A follow-up change would
allow us to define external types (like the java.util types) that can't
be mocked. (We can't add the annotation to the types, as they live in the
JDK instead.)

[1]: https://github.com/mockito/mockito/wiki/How-to-write-good-tests#dont-mock-a-type-you-dont-own
[2]: https://stackoverflow.com/a/15820143
[3]: https://errorprone.info/api/latest/com/google/errorprone/annotations/DoNotMock.html

This introduces the DoNotMockEnforcer interface which users can override
to implement their special handling of types annotated with DoNotMock.

This name is clearer for implementers.

They won't compile for Android users