Function unprintable_chars 

pub fn unprintable_chars(text: &str) -> String

The distinct characters of text this card’s font has no glyph for, in order of first use; empty when every character prints.

Resolved against the SAME face generate_dope_card_pdf draws with (the system copy of Liberation Sans when one is installed, the embedded copy otherwise), so this cannot disagree with the document. A font that will not parse at all reports nothing — that failure surfaces from the generator itself, as an error, rather than being recast here as “every character is unprintable”.