@A1-Triard:

By the way, looks like this is not 100%-correct strerror_r result check. The strerror_r Linux manpage says, that starting from glibc 2.13 it returns (positive) errno as a result instead of returning -1 and setting errno. The corresponding manpage in FreeBSD also specifies 0, EINVAL, and ERANGE as only possible return values.

https://linux.die.net/man/3/strerror_r

Thinking further, the fact that a Display impl (which most people assume is non–side-effecting) can change global state is kind of surprising. We'll need to document that at least. Not sure what else we should do.