Rustdoc has a "notable traits" tooltip when hovering the (i) icon next to some return values. It currently looks like this:
It was implemented in 2017, in response to a 2015 issue requesting more clarity about iterator returns. @GuillaumeGomez removed it in early 2020 for being too complex and not doing its job. @Manishearth re-added it in mid-2020 after a useful forum thread here.
The current implementation still doesn't do its job because the (i) is so unobtrusive. Most people don't notice it, as @GuillaumeGomez' Twitter poll suggested. Also, even if you notice the (i), the text in the box is inscrutable to a beginner (and still hard to eyeball for an experienced Rustacean).
The current version considers Iterator, Read, Write, and Future to be notable. Note that the original issue requested a way to show the iteration type of a returned iterator - it didn't request a generic way to show notable traits. And most of the discussion in @Manishearth's forum thread centered around iterators. I haven't seen someone make the case for Read, Write, and Future definitely needing this treatment.
I think we can do a lot better by specializing to just Iterator, and displaying some text inline rather than hiding it behind a tooltip: