fix some more clippy lints

Author: hellow554

src/adaptors/multi_product.rs

@@ -123,7 +123,7 @@ where
     /// 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/flatten_ok.rs

@@ -76,8 +76,7 @@ where
         let inner_hint = |inner: &Option<T::IntoIter>| {
             inner
                 .as_ref()
-                .map(Iterator::size_hint)
-                .unwrap_or((0, Some(0)))
+                .map_or((0, Some(0)), Iterator::size_hint)
         };
         let inner_front = inner_hint(&self.inner_front);
         let inner_back = inner_hint(&self.inner_back);

src/groupbylazy.rs

@@ -112,7 +112,7 @@ where
         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

@@ -226,7 +226,7 @@ where
         #[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/unique_impl.rs

@@ -11,9 +11,7 @@ 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,
 }