Test-Guided Migration Assistant

The test suite is a specification in executable form. During a migration, it's the oracle: if the tests pass, the migration preserved behavior (as far as the tests know).

The invariant

At every step of the migration: tests pass. If a step breaks a test, either:

• the step was wrong → revert and redo, or
• the test was wrong (tested implementation, not behavior) → fix the test first, in a separate commit, then redo the step.

Never proceed with a red test. Red accumulates.

Step 0 — Assess the suite

Before migrating, know what you're working with: