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I agree with the first part of your comment, but not the second. Achieving 100% code coverage is very hard and sometimes unreasonable. E.g. How do you test a GUI? I think that often it is better to write tests for the application logic and test a GUI by using it.

question:

How do unit tests stay green when you rewrite in another language? Are your tests an external API you call?

Rewrites are not super common -- but IMHO if you must do one the first step is to move the tests over. Unless your goal with the rewrite is to rearchitect the interfaces at which point your basically greenfielding anyways.

this is simply not possible. if you move a method from one class to another every unit test breaks.