Based on https://internals.rust-lang.org/t/can-the-standard-library-s... it looks like that's probably the case, but that it's not beyond all doubt that it holds across all Unix-likes.

> On Linux, the answer is pretty obviously no. Linux file descriptors are stored in an array of structs that has its capacity bounded to INT_MAX, so any negative int would either be considered nonsense (if treated as negative) or be higher than INT_MAX (if it was bit-reinterpreted as an unsigned value).

> The Single UNIX Specification explicitly says that open can't return a negative file descriptor, and says in the page for dup2 that you should get EBADF if you try to claim a negative file descriptor.

> Unfortunately, their description of file descriptor allocation never explicitly says that the negative range is out of bounds, but open references this algorithm while simultaneously claiming that it never gives a negative result.

> Also, Wikipedia says it can't be negative, but they don't give a source on that particular claim (urgh!).

-1 was picked as a conservative choice that's enough for the common case of Option.

i don't know, but -1 has the prettiest two's complement representation of all negative ints :)