If that's in your threat model, fill the disk with random bytes.
This works on the economic argument that if you could easily recover data from a disk that's been rewritten, drive manufacturers would exploit that already to increase storage.
If your threat model includes attackers who bypass the firmware to read overwritten data, you are probably boned several other ways.
It is usually quicker to just zero the disk instead of random bytes, just because the randomness pool might be scarce in a system. This can save hours when clearing a large and fast disk, like a SSD. /dev/zero usually has much more throughput than /dev/random.
Of course this matters less on slow disks, where the bottleneck is the writing speeds anyway.
Many SSDs transform all-zero sector writes into a simple TRIM/DISCARD to reduce flash wear, so overwriting with pseudorandom data from /dev/urandom is likely more secure.
I don't understand why this question is asked in any context. What if an asteroid is on a collision course with Earth? What if Jesus comes back? What if the yeti is real?
Or whoever that has quantum computer in the near future. Encryption is just a cat and mouse game it is before long someone will figure out to decrypt it.
There is currently no reason to believe quantum computers will make breaking symmetric encryption like AES-256 any more practical than it is today. At best, grovers algorithm can modestly reduce the search time for brute forcing, but it's still brute force.
Asymmetric algorithms like RSA are the ones you should be worrying about, and I can't imagine any reason someone would do disk encryption with an asymmetric algorithm.
