Merge #643

643: Implement `PeekingNext` transitively over mutable references. r=jswrenn a=olson-sean-k

This PR allows `PeekingNext` to be used as a trait object just like `Iterator`, allowing code to accept types like `&mut dyn PeekingNext<Item = T>` for some item type `T`. To accomplish this, generic methods now require that `Self: Sized` and `PeekingNext` now has a transitive implementation over mutable references to types that implement `PeekingNext`.

This mirrors the design of the `Iterator` trait ([here](https://doc.rust-lang.org/nightly/src/core/iter/traits/iterator.rs.html#776-782) and [here](https://doc.rust-lang.org/nightly/src/core/iter/traits/iterator.rs.html), for example). See also [this discussion on the Rust internals forum](https://internals.rust-lang.org/t/generic-methods-over-object-safe-traits/6774).

Co-authored-by: Sean Olson <olson.sean.k@gmail.com>

src/peeking_take_while.rs

@@ -16,7 +16,18 @@ pub trait PeekingNext : Iterator {
     /// if `accept` returns true, return it as the next element,
     /// else None.
     fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
-        where F: FnOnce(&Self::Item) -> bool;
+        where Self: Sized,
+              F: FnOnce(&Self::Item) -> bool;
+}
+
+impl<'a, I> PeekingNext for &'a mut I
+    where I: PeekingNext,
+{
+    fn peeking_next<F>(&mut self, accept: F) -> Option<Self::Item>
+        where F: FnOnce(&Self::Item) -> bool
+    {
+        (*self).peeking_next(accept)
+    }
 }
 
 impl<I> PeekingNext for Peekable<I>