Add more FusedIterator

Some Iterator is fused if the underlying Iterator is fused.
(UniqueBy, Unique, InterleaveShortest, Product, MergeBy, FilterOk,
FilterMapOk, Positions, Update)
Combinations is fused.

src/adaptors/mod.rs

@@ -15,7 +15,7 @@ pub use self::map::MapResults;
 pub use self::multi_product::*;
 
 use std::fmt;
-use std::iter::{Fuse, Peekable, FromIterator};
+use std::iter::{Fuse, Peekable, FromIterator, FusedIterator};
 use std::marker::PhantomData;
 use crate::size_hint;
 
@@ -157,6 +157,11 @@ impl<I, J> Iterator for InterleaveShortest<I, J>
     }
 }
 
+impl<I, J> FusedIterator for InterleaveShortest<I, J>
+    where I: FusedIterator,
+          J: FusedIterator<Item = I::Item>
+{}
+
 #[derive(Clone, Debug)]
 /// An iterator adaptor that allows putting back a single
 /// item to the front of the iterator.
@@ -361,6 +366,12 @@ impl<I, J> Iterator for Product<I, J>
     }
 }
 
+impl<I, J> FusedIterator for Product<I, J>
+    where I: FusedIterator,
+          J: Clone + FusedIterator,
+          I::Item: Clone
+{}
+
 /// A “meta iterator adaptor”. Its closure receives a reference to the iterator
 /// and may pick off as many elements as it likes, to produce the next iterator element.
 ///
@@ -588,6 +599,12 @@ impl<I, J, F> Iterator for MergeBy<I, J, F>
     }
 }
 
+impl<I, J, F> FusedIterator for MergeBy<I, J, F>
+    where I: FusedIterator,
+          J: FusedIterator<Item = I::Item>,
+          F: MergePredicate<I::Item>
+{}
+
 /// An iterator adaptor that borrows from a `Clone`-able iterator
 /// to only pick off elements while the predicate returns `true`.
 ///
@@ -876,6 +893,11 @@ impl<I, F, T, E> Iterator for FilterOk<I, F>
     }
 }
 
+impl<I, F, T, E> FusedIterator for FilterOk<I, F>
+    where I: FusedIterator<Item = Result<T, E>>,
+          F: FnMut(&T) -> bool,
+{}
+
 /// An iterator adapter to filter and apply a transformation on values within a nested `Result::Ok`.
 ///
 /// See [`.filter_map_ok()`](crate::Itertools::filter_map_ok) for more information.
@@ -947,6 +969,11 @@ impl<I, F, T, U, E> Iterator for FilterMapOk<I, F>
     }
 }
 
+impl<I, F, T, U, E> FusedIterator for FilterMapOk<I, F>
+    where I: FusedIterator<Item = Result<T, E>>,
+          F: FnMut(T) -> Option<U>,
+{}
+
 /// An iterator adapter to get the positions of each element that matches a predicate.
 ///
 /// See [`.positions()`](crate::Itertools::positions) for more information.
@@ -1006,6 +1033,11 @@ impl<I, F> DoubleEndedIterator for Positions<I, F>
     }
 }
 
+impl<I, F> FusedIterator for Positions<I, F>
+    where I: FusedIterator,
+          F: FnMut(I::Item) -> bool,
+{}
+
 /// An iterator adapter to apply a mutating function to each element before yielding it.
 ///
 /// See [`.update()`](crate::Itertools::update) for more information.
@@ -1081,3 +1113,9 @@ where
         }
     }
 }
+
+impl<I, F> FusedIterator for Update<I, F>
+where
+    I: FusedIterator,
+    F: FnMut(&mut I::Item),
+{}

src/combinations.rs

@@ -1,4 +1,5 @@
 use std::fmt;
+use std::iter::FusedIterator;
 
 use super::lazy_buffer::LazyBuffer;
 use alloc::vec::Vec;
@@ -122,3 +123,8 @@ impl<I> Iterator for Combinations<I>
         Some(self.indices.iter().map(|i| self.pool[*i].clone()).collect())
     }
 }
+
+impl<I> FusedIterator for Combinations<I>
+    where I: Iterator,
+          I::Item: Clone
+{}

src/unique_impl.rs

@@ -3,6 +3,7 @@ use std::collections::HashMap;
 use std::collections::hash_map::{Entry};
 use std::hash::Hash;
 use std::fmt;
+use std::iter::FusedIterator;
 
 /// An iterator adapter to filter out duplicate elements.
 ///
@@ -92,6 +93,12 @@ impl<I, V, F> DoubleEndedIterator for UniqueBy<I, V, F>
     }
 }
 
+impl<I, V, F> FusedIterator for UniqueBy<I, V, F>
+    where I: FusedIterator,
+          V: Eq + Hash,
+          F: FnMut(&I::Item) -> V
+{}
+
 impl<I> Iterator for Unique<I>
     where I: Iterator,
           I::Item: Eq + Hash + Clone
@@ -136,6 +143,11 @@ impl<I> DoubleEndedIterator for Unique<I>
     }
 }
 
+impl<I> FusedIterator for Unique<I>
+    where I: FusedIterator,
+          I::Item: Eq + Hash + Clone
+{}
+
 /// An iterator adapter to filter out duplicate elements.
 ///
 /// See [`.unique()`](crate::Itertools::unique) for more information.

tests/quick.rs

@@ -1598,3 +1598,76 @@ quickcheck! {
         TestResult::from_bool(itertools::equal(x, y))
     }
 }
+
+
+fn is_fused<I: Iterator>(mut it: I) -> bool
+{
+    while let Some(_) = it.next() {}
+    for _ in 0..10{
+        if it.next().is_some(){
+            return false;
+        }
+    }
+    true
+}
+
+quickcheck! {
+    fn fused_combination(a: Iter<i16>) -> bool
+    {
+        is_fused(a.clone().combinations(1)) &&
+        is_fused(a.combinations(3))
+    }
+
+    fn fused_unique(a: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().unique())
+    }
+
+    fn fused_unique_by(a: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().unique_by(|x| x % 100))
+    }
+
+    fn fused_interleave_shortest(a: Iter<i16>, b: Iter<i16>) -> bool
+    {
+        !is_fused(a.clone().interleave_shortest(b.clone())) &&
+        is_fused(a.fuse().interleave_shortest(b.fuse()))
+    }
+
+    fn fused_product(a: Iter<i16>, b: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().cartesian_product(b.fuse()))
+    }
+
+    fn fused_merge(a: Iter<i16>, b: Iter<i16>) -> bool
+    {
+        is_fused(a.fuse().merge(b.fuse()))
+    }
+
+    fn fused_filter_ok(a: Iter<i16>) -> bool
+    {
+        is_fused(a.map(|x| if x % 2 == 0 {Ok(x)} else {Err(x)} )
+                 .filter_ok(|x| x % 3 == 0)
+                 .fuse())
+    }
+
+    fn fused_filter_map_ok(a: Iter<i16>) -> bool
+    {
+        is_fused(a.map(|x| if x % 2 == 0 {Ok(x)} else {Err(x)} )
+                 .filter_map_ok(|x| if x % 3 == 0 {Some(x / 3)} else {None})
+                 .fuse())
+    }
+
+    fn fused_positions(a: Iter<i16>) -> bool
+    {
+        !is_fused(a.clone().positions(|x|x%2==0)) &&
+        is_fused(a.fuse().positions(|x|x%2==0))
+    }
+
+    fn fused_update(a: Iter<i16>) -> bool
+    {
+        !is_fused(a.clone().update(|x|*x+=1)) &&
+        is_fused(a.fuse().update(|x|*x+=1))
+    }
+}
+