@huonw I agree; I am uneasy about putting these in structs, and I don't have a strong use-case for it.

The main reason for including it in the proposal was to treat `impl Trait` consistently as something you can write anywhere a type goes. But the more I think about it, the more I like the conservative alternative I outline in the end: restricting this RFC to function return types, and using the syntax "_ : Trait" instead.