We couldn’t just start replacing old code with new code willy-nilly; without some type of structure to keep the old and new code separate, they’d end up getting hopelessly tangled together and we’d never have our modern codebase. To solve this problem, we introduced a few rules and functions in a concept called legacy-interop:

• old code cannot directly import new code: only new code that has been “exported” for use by the old code is available
• new code cannot directly import old code: only old code that has been “adapted” for use by modern code is available.

The progressive approach to the rebuild is interesting. Especially the rules that enforced how the rewritten parts of the code base could interact with the old code they were ultimately replacing.