If we're listing language footguns, I would argue that narrowing 'as' is a footgun.

Rust's 'as' is a general cast. It will try to do something reasonable to get from one type to the other, but while a few of the casts are totally unremarkable, some others open up too much opportunity to blow your feet off in my opinion (and I am not alone). Most obviously casting from a larger to a smaller integer is silently truncating. Sometimes, often even, this is what you intended, sometimes it's unimportant, but once in a while it's a trap.

Obviously it's nothing compared to the landmines in some languages, but I think there's room to teach safer options here, warn for and eventually (in a future edition) prohibit the narrowing cast.