It depends very much on the language, e.g. common Java tools work with byte code at runtime (so no 'recompilation' is necessary), and tools for scripting langua

It depends very much on the language, e.g. common Java tools work with byte code at runtime (so no 'recompilation' is necessary), and tools for scripting languages have no option but to generate source. Actually the approach has a weakness: The code may have hidden state that could change test results between runs. As mutations need to run the code many times, this can skew the results.