Now that Apple has been forced to provide a somewhat standard charger with the iPhone, I was curious as to whether the iPhone charger would correctly charge the Librem 5.
Answer: yes
The Librem 5 negotiates USB-C PD and correctly negotiates max 3A, max 5V - and draws a reasonable amount of current (not obviously different from what happens with the official Purism charger). So all good.
So if you forget to bring your charger, you can try scrounging a charger from a late model iPhone user.
Just as a reminder, it’s always good practice to use a trusted [determination made by user] charger and cable: they are not just dumb devices, and in fact modern cables may have a lot of electronic circuits hidden in them. See: X-rays show the difference between a $5 and Apple's $129 USB-C cable