You can add interface ConvertAssigner like this

type ConvertAssigner interface {
	ConvertAssign(dest, src interface{}) error
}

And check it in Scan

func (rs *Rows) Scan(dest ...interface{}) error {
	rs.closemu.RLock()
	if rs.closed {
		rs.closemu.RUnlock()
		return errors.New("sql: Rows are closed")
	}
	rs.closemu.RUnlock()

	if rs.lastcols == nil {
		return errors.New("sql: Scan called without calling Next")
	}
	if len(dest) != len(rs.lastcols) {
		return fmt.Errorf("sql: expected %d destination arguments in Scan, not %d", len(rs.lastcols), len(dest))
	}

	convertAssign := defaultConvertAssign // defaultConvertAssign is a new name for the convertAssign function 
	if rows, ok := rs.rowsi.(ConvertAssigner); ok {
		convertAssign = rows.ConvertAssign
	}

	for i, sv := range rs.lastcols {
		if err := convertAssign(dest[i], sv); err != nil {
			return fmt.Errorf("sql: Scan error on column index %d: %v", i, err)
		}
	}
	return nil
}

Concrete driver can be more effective than defaultConvertAssign

database/sql: value converter interface for rows.Scan #22544

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