Remove usage of unstable feature conservative_impl_trait in layout

components/layout/flow_list.rs

@@ -7,6 +7,7 @@ use flow_ref::FlowRef;
 use serde::ser::{Serialize, SerializeSeq, Serializer};
 use serde_json::{Map, Value, to_value};
 use std::collections::{LinkedList, linked_list};
+use std::ops::Deref;
 use std::sync::Arc;
 
 /// This needs to be reworked now that we have dynamically-sized types in Rust.
@@ -53,6 +54,10 @@ pub struct MutFlowListIterator<'a> {
     it: linked_list::IterMut<'a, FlowRef>,
 }
 
+pub struct FlowListIterator<'a> {
+    it: linked_list::Iter<'a, FlowRef>,
+}
+
 impl FlowList {
     /// Add an element last in the list
     ///
@@ -101,8 +106,10 @@ impl FlowList {
     /// SECURITY-NOTE(pcwalton): This does not hand out `FlowRef`s by design. Do not add a method
     /// to do so! See the comment above in `FlowList`.
     #[inline]
-    pub fn iter<'a>(&'a self) -> impl DoubleEndedIterator<Item = &'a Flow> {
-        self.flows.iter().map(|flow| &**flow)
+    pub fn iter<'a>(&'a self) -> FlowListIterator {
+        FlowListIterator {
+            it: self.flows.iter(),
+        }
     }
 
     /// Provide a forward iterator with mutable references
@@ -150,12 +157,31 @@ impl FlowList {
     }
 }
 
+impl<'a> DoubleEndedIterator for FlowListIterator<'a> {
+    fn next_back(&mut self) -> Option<&'a Flow> {
+        self.it.next_back().map(Deref::deref)
+    }
+}
+
 impl<'a> DoubleEndedIterator for MutFlowListIterator<'a> {
     fn next_back(&mut self) -> Option<&'a mut Flow> {
         self.it.next_back().map(FlowRef::deref_mut)
     }
 }
 
+impl<'a> Iterator for FlowListIterator<'a> {
+    type Item = &'a Flow;
+    #[inline]
+    fn next(&mut self) -> Option<&'a Flow> {
+        self.it.next().map(Deref::deref)
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.it.size_hint()
+    }
+}
+
 impl<'a> Iterator for MutFlowListIterator<'a> {
     type Item = &'a mut Flow;
     #[inline]