Update C-NEWTYPE-HIDE now impl Trait is shipped

src/future-proofing.md

@@ -108,14 +108,19 @@ less to the client. The client does not know _how_ the result iterator is
 constructed or represented, which means the representation can change in the
 future without breaking client code.
 
-In the future the same thing can be accomplished more concisely with the [`impl
-Trait`] feature but this is currently unstable.
+Rust 1.26 also introduces the [`impl Trait`][] feature, which is more concise
+than the newtype pattern but with some additional trade offs, namely with `impl
+Trait` you are limited in what you can express.  For example, returning an
+iterator that impls `Debug` or `Clone` or some combination of the other iterator
+extension traits can be problematic.  In summary `impl Trait` as a return type
+is probably great for internal APIs and may even be appropriate for public APIs,
+but probably not in all cases.  See the ["`impl Trait` for returning complex
+types with ease"][impl-trait-2] section of the Edition Guide for more details.
 
 [`impl Trait`]: https://github.com/rust-lang/rfcs/blob/master/text/1522-conservative-impl-trait.md
+[impl-trait-2]: https://rust-lang-nursery.github.io/edition-guide/rust-2018/trait-system/impl-trait-for-returning-complex-types-with-ease.html
 
 ```rust
-#![feature(conservative_impl_trait)]
-
 pub fn my_transform<I: Iterator>(input: I) -> impl Iterator<Item = (usize, I::Item)> {
     input.skip(3).enumerate()
 }