We've done large schema migrations in a 1 million LoC Rust codebase. Most of the time, these are pretty straightforward. You mark things as deprecated and link

We've done large schema migrations in a 1 million LoC Rust codebase. Most of the time, these are pretty straightforward. You mark things as deprecated and link a tracking ticket for the migration in the doc-comment of the field/struct. What's a lot more difficult is making sure migrations don't break important forwards/backwards compatibility guarantees. Unfortunately, Rust won't help you much with these. One solution we've been using in our team to get confidence in the correctness of our schema evolution is to use proc-macros to generate a somewhat exhaustive set of test data, so that every field and enum variant is included at least once. In CI we test the branch version against master, and let each version serialize/deserialize the other version's test cases.