Actually, just noticed a minor issue - since there's no intermixing between the four lanes and the diffuse() function is the same for all of them, if any of the IVs match then I can swap all the blocks in those lanes and get the same hash out.

For example, if IV1 and IV2 match and the block pattern is ABCDABCDABCD, then BACDBACDBACD will produce the same hash value.

A minor finalizer change would fix it for any IV (pseudocode as I don't actually know Rust) -

vec[0] ^= diffuse(vec[1]); vec[1] ^= diffuse(vec[0]); vec[2] ^= diffuse(vec[3]); vec[3] ^= diffuse(vec[2]);

u64 result = diffuse(vec[1] ^ diffuse(vec[3]));

that's probably overkill but should work.