We have written a custom code generator mix task. Working with external APIs, we wrote the same set of API provider-specific modules every time a new integration would be added.
The generator is simple/naive in its implementation, but it saves hours of repetitive copying, pasting and adjusting. It spits out the modules alongside some unit tests and prints out a to-do list of tasks that need to be performed manually.
The task itself was maybe half a day‘s work and has paid for that time investment many times over by now. (Refactoring the code base to a point where it‘s extensible in a way that is so straightforward that a codegen is even an option was naturally a bit more involved, but that is a different story 
)
