Merge rayon-rs#683

683: impl ParallelExtend<()> for () r=cuviper a=cuviper

Mirrors rust-lang/rust#50234

Co-authored-by: Josh Stone <cuviper@gmail.com>

Author: bors[bot]

src/iter/extend.rs

@@ -1,3 +1,4 @@
+use super::noop::NoopConsumer;
 use super::{IntoParallelIterator, ParallelExtend, ParallelIterator};
 
 use std::borrow::Cow;
@@ -368,3 +369,13 @@ where
         self.par_extend(par_iter.into_par_iter().cloned())
     }
 }
+
+/// Collapses all unit items from a parallel iterator into one.
+impl ParallelExtend<()> for () {
+    fn par_extend<I>(&mut self, par_iter: I)
+    where
+        I: IntoParallelIterator<Item = ()>,
+    {
+        par_iter.into_par_iter().drive_unindexed(NoopConsumer)
+    }
+}