produce "raise on execute" symbols that we can put in places we know that "None" will fail, like UniqueConstraint(None), etc.

Migrated issue, originally created by Ulrich Petri (@ulope)

When adding unique=True to an existing column the generated migration looks like this:

def upgrade():
    op.create_unique_constraint(None, 'table', ['field'])


def downgrade():
    op.drop_constraint(None, 'table')

I think alembic should either autogenerate a constraint name (I realize this might be difficult across backends) or print a very explicit warning that the migration needs to be 'fixed' manually.

Start by tracing how autogeneration turns an existing column with unique=True into upgrade() and downgrade() calls using None for constraint names. The issue names no files or tests, so first locate the relevant migration-generation and constraint-handling entry points, then clarify whether completion means generating names or emitting an explicit warning, with coverage for both generated functions.