First let me say that C-STRUCT-BOUNDS talks about derived bounds mainly, which is completely fine, just a bit confusing (as the text down below speaks about bounds in general). I'd like to see a more visible distinction between the two, but that's not my main point here.

I suggest adding a new execption: "when a struct is supposed to wrap a type implementing a specific trait" - that means specialized wrappers for Iterator, Future, Stream, Read, Write etc. Reason: if the user attempts to instantiate the type without generic implementing the trait, he hits the error somewhere else, which is confusing and causes long error messages.

Requiring the trait will cause the error sooner, pointing at the correct location.

As an alternative, one might omit the bounds from struct itself and put them on every constructing function instead. This works until the crate author forgets about one case and exactly that case will be experienced by the user. (Murphy law: Everything that can break will break.)

#6 reasoned that it's annoying, but since generics are just type-level functions, it's equally annoying as having to write types in function signatures, which was very reasonably considered as a good thing.