When APIs evolve, structs change shape. Fields get added, renamed, or dropped. Code that handles multiple versions of a schema — REST API consumers, database migration layers, message bus subscribers — has to deal with this, usually through ad-hoc conversion functions that silently discard data.

interface makes those conversions explicit and type-safe:

• Lossy vs. lossless is a compile-time distinction. Downgrading a v2::User to v1::User drops the email field. That destruction is encoded in the type as Translation<_, _, Lossy>, and the call site must use .translate_lossy() — the compiler won't let you silently throw data away.

• Every translation carries a diff. Before committing to a conversion you can inspect exactly which fields are added or removed.

• Conversions are deferred values, not immediate calls. .upgrade() and .downgrade() return a Translation you can log, forward, or discard before executing.

MIT