I'm not a member of this community seeing as I've barely used Rust, but...
It's not quite that simple (from Wiki, bolding by me):
In the 14th edition (1993) of The Chicago Manual of Style, the University of Chicago Press explicitly recommended use of singular use of they and their, noting a "revival" of this usage and citing "its venerable use by such writers as Addison, Austen, Chesterfield, Fielding, Ruskin, Scott, and Shakespeare."[87] From the 15th edition, this was changed. In Chapter 5 of the 16th edition, now written by Bryan A. Garner, the recommendations are: "The singular they. A singular antecedent requires a singular referent pronoun. Because he is no longer accepted as a generic pronoun referring to a person of either sex, it has become common in speech and in informal writing to substitute the third-person plural pronouns they, them, their, and themselves, and the nonstandard singular themself. While this usage is accepted in casual context, it is still considered ungrammatical in formal writing."[88] and "Gender bias. . . . On the one hand, it is unacceptable to a great many reasonable readers to use the generic masculine pronoun (he in reference to no one in particular). On the other hand, it is unacceptable to a great many readers (often different readers) either to resort to non-traditional gimmicks to avoid the generic masculine (by using he/she of s/he, for example) or to use they as a kind of singular pronoun. Either way, credibility is lost with some readers."[79]
To me that reads more like ¯\_(ツ)_/¯.
The OED seems to come down on the side of "they" (though I can't find an official document)
it’s the policy of current English Oxford Dictionaries to use plural pronouns and determiners such as they and their in definitions in cases where, formerly, singular forms such as he and his would have been selected.
though
Although also very common, the use of they after a singular noun is still anathema to many people, especially in formal contexts.
IMO it's better to be ungrammatical than potentially offensive, even in formal contexts like the Rust docs. But I'm a descriptivist so... ¯\_(ツ)_/¯.
PS. you would write "The person you mentioned, are they coming to dinner with us tonight?". There's no ambiguity in this case because there's one subject, and it's singular.