I'm not sure if this is the correct approach; 2**61 bytes is 2.3 exabytes, which probably wouldn't get hit before an actual OOM error occurs in the process. There is a RFC that allows for catching OOMs in a few different ways (try_reserve -> Result<(), ...> functions on Vec or String, oom=panic rustc option that will panic and unwind instead of just aborting) but none of them are stable yet. I'm not sure how to approach this problem, honestly.

That is true, but this is also a reasonable sanity check, since a list with 8 byte pointers can only contain up to usize::MAX / 8 elements. Beyond that, allocating becomes the virtual machine's problem and will need to dealt with accordingly.