Somehow prevent unique constraint violations in migration at compile time

[LHolten]

Unique constraint violations are already checked at compile time and you need to handle potential conflicts when inserting new rows.
However, currently you don't need to handle unique constraint conflicts in migrations and instead the migration will just panic and roll back when a unique constraint is violated.

It is unclear what would be a good API for migrating columns to a new column with a unique constraint.
There can be many unique constraint violations in these cases, so there would need to be a callback of sorts to handle each one.

It is also unclear what the options of the callback should even be. Migrations are generally not allowed to drop rows because that could violate foreign key constraints.