Merge #498

498: FromParallelIterator and ParallelExtend Cow for String r=cuviper a=cuviper

Parallel version of rust-lang/rust#41449.

src/iter/extend.rs

@@ -1,5 +1,6 @@
 use super::{ParallelExtend, IntoParallelIterator, ParallelIterator};
 
+use std::borrow::Cow;
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::hash::{BuildHasher, Hash};
 use std::collections::LinkedList;
@@ -262,6 +263,36 @@ impl ParallelExtend<String> for String {
     }
 }
 
+/// Extend a string with string slices from a parallel iterator.
+impl<'a> ParallelExtend<Cow<'a, str>> for String {
+    fn par_extend<I>(&mut self, par_iter: I)
+        where I: IntoParallelIterator<Item = Cow<'a, str>>
+    {
+        // This is like `extend`, but `Extend<Cow<'a, str>> for String`
+        // wasn't added until Rust 1.19, so we can't use it directly yet.
+        let list = par_iter
+            .into_par_iter()
+            .fold(Vec::new, |mut vec, elem| {
+                vec.push(elem);
+                vec
+            })
+        .map(|vec| {
+            let mut list = LinkedList::new();
+            list.push_back(vec);
+            list
+        })
+        .reduce(LinkedList::new, |mut list1, mut list2| {
+            list1.append(&mut list2);
+            list1
+        });
+
+        self.reserve(str_len(&list));
+        for vec in list {
+            self.extend(vec.iter().map(|cow| &**cow));
+        }
+    }
+}
+
 
 /// Extend a deque with items from a parallel iterator.
 impl<T> ParallelExtend<T> for VecDeque<T>

src/iter/from_par_iter.rs

@@ -154,6 +154,15 @@ impl FromParallelIterator<String> for String {
     }
 }
 
+/// Collect string slices from a parallel iterator into a string.
+impl<'a> FromParallelIterator<Cow<'a, str>> for String {
+    fn from_par_iter<I>(par_iter: I) -> Self
+        where I: IntoParallelIterator<Item = Cow<'a, str>>
+    {
+        collect_extended(par_iter)
+    }
+}
+
 /// Collect an arbitrary `Cow` collection.
 ///
 /// Note, the standard library only has `FromIterator` for `Cow<'a, str>` and