allow clippy::zero_sized_map_values

also move comment why using a hashmap in the first place

src/adaptors/multi_product.rs

@@ -114,7 +114,7 @@ impl<I> MultiProduct<I>
     /// Returns true if iteration has started and has not yet finished; false
     /// otherwise.
     fn in_progress(&self) -> bool {
-        self.0.last().map_or(false, |last| last.in_progress())
+        self.0.last().map_or(false, MultiProductIter::in_progress)
     }
 }
 

src/groupbylazy.rs

@@ -111,7 +111,7 @@ impl<K, I, F> GroupInner<K, I, F>
         if client < self.oldest_buffered_group {
             return None;
         }
-        let elt = self.buffer.get_mut(bufidx).and_then(|queue| queue.next());
+        let elt = self.buffer.get_mut(bufidx).and_then(Iterator::next);
         if elt.is_none() && client == self.oldest_buffered_group {
             // FIXME: VecDeque is unfortunately not zero allocation when empty,
             // so we do this job manually.

src/kmerge_impl.rs

@@ -215,7 +215,7 @@ impl<I, F> Iterator for KMergeBy<I, F>
     fn size_hint(&self) -> (usize, Option<usize>) {
         #[allow(deprecated)] //TODO: once msrv hits 1.51. replace `fold1` with `reduce`
         self.heap.iter()
-                 .map(|i| i.size_hint())
+                 .map(HeadTail::size_hint)
                  .fold1(size_hint::add)
                  .unwrap_or((0, Some(0)))
     }

src/tuple_impl.rs

@@ -63,7 +63,7 @@ impl<T> Iterator for TupleBuffer<T>
             0
         } else {
             buffer.iter()
-                  .position(|x| x.is_none())
+                  .position(Option::is_none)
                   .unwrap_or_else(|| buffer.len())
         };
         (len, Some(len))

src/unique_impl.rs

@@ -1,3 +1,8 @@
+// Use a Hashmap for the Entry API in order to prevent hashing twice.
+// This can maybe be replaced with a HashSet once `get_or_insert_with`
+// or a proper Entry API for Hashset is stable and meets this msrv
+#![allow(clippy::zero_sized_map_values)]
+
 use std::collections::HashMap;
 use std::collections::hash_map::Entry;
 use std::hash::Hash;
@@ -11,9 +16,7 @@ use std::iter::FusedIterator;
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
 pub struct UniqueBy<I: Iterator, V, F> {
     iter: I,
-    // Use a Hashmap for the Entry API in order to prevent hashing twice.
-    // This can maybe be replaced with a HashSet once `get_or_insert_with`
-    // or a proper Entry API for Hashset is stable and meets this msrv
+    // see comment for `allow(clippy::zero_sized_map_values)` for reasoning
     used: HashMap<V, ()>,
     f: F,
 }