Throughout my time studying Latin in school, one grammatical construction in particular has always intrigued me to an extent — the future passive infinitive (eg. amatum iri). Whenever it came up (often in performing verb synopses), the teacher would always say something to the effect that it's the kind of form whose occurrences you can count on one hand. My questions is, is it indeed that rare of a form, and, if so, are there any primary examples of it in Latin literature?
This list will have some false positives, but the Packard Humanities Institute Latin Texts website generates this collection of all instances of "iri" in its corpus (179 total instances):
The first one I ever encountered was in Metellus' letter to Cicero (Fam. 5.1):
Existimaram pro mutuo inter nos animo et pro reconciliata gratia nec absentem umquam me abs te ludibrio laesum iri.
I had thought, on account of our mutual desire and reconciliation, that I while absent was not ever to be attacked with ridicule by you.
It's certainly rare. If we need more than four hands to count the instances in Cicero, it's because Cicero's corpus is so incredibly massive. That said, it's not completely unheard of, and this is a formal written style from one senator to another.
Let me give you a concrete example of how common the structure is. Caesar, whose language is considered good classical style, uses the word iri four times in De Bello Gallico, three of which are passive future infinitive. These are nocitum iri in 5.36.2, ductum iri in 7.11.4 and spoliatum iri in 7.66.5. The irrelevant one is ad castra iri oportere in 3.18.6.