Eliminate unnecessary T: Clone bounds on tensor iterators

Replace derived `Clone` impls for tensor iterators with manual implementations
to remove the unnecessary `T: Clone` bound that the derived impl adds.

See also rust-lang/rust#26925.

rten-tensor/src/iterators.rs

@@ -184,22 +184,29 @@ impl IndexingIterBase {
     }
 }
 
-/// Iterator over elements of a tensor, in their logical order.
-#[derive(Clone)]
-pub struct Iter<'a, T> {
-    iter: IterKind<'a, T>,
-}
-
-/// Alternate implementations of `Elements`.
+/// Alternate implementations of [`Iter`].
 ///
 /// When the tensor has a contiguous layout, this iterator is just a thin
 /// wrapper around a slice iterator.
-#[derive(Clone)]
 enum IterKind<'a, T> {
     Direct(slice::Iter<'a, T>),
     Indexing(IndexingIter<'a, T>),
 }
 
+impl<'a, T> Clone for IterKind<'a, T> {
+    fn clone(&self) -> Self {
+        match self {
+            IterKind::Direct(slice_iter) => IterKind::Direct(slice_iter.clone()),
+            IterKind::Indexing(iter) => IterKind::Indexing((*iter).clone()),
+        }
+    }
+}
+
+/// Iterator over elements of a tensor, in their logical order.
+pub struct Iter<'a, T> {
+    iter: IterKind<'a, T>,
+}
+
 impl<'a, T> Iter<'a, T> {
     pub(super) fn new<L: Layout>(view: ViewRef<'a, '_, T, L>) -> Iter<'a, T> {
         if let Some(data) = view.contiguous_data() {
@@ -214,6 +221,14 @@ impl<'a, T> Iter<'a, T> {
     }
 }
 
+impl<'a, T> Clone for Iter<'a, T> {
+    fn clone(&self) -> Self {
+        Iter {
+            iter: self.iter.clone(),
+        }
+    }
+}
+
 impl<'a, T> Iterator for Iter<'a, T> {
     type Item = &'a T;
 
@@ -247,7 +262,6 @@ impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
 
 impl<'a, T> FusedIterator for Iter<'a, T> {}
 
-#[derive(Clone)]
 struct IndexingIter<'a, T> {
     base: IndexingIterBase,
 
@@ -264,6 +278,15 @@ impl<'a, T> IndexingIter<'a, T> {
     }
 }
 
+impl<'a, T> Clone for IndexingIter<'a, T> {
+    fn clone(&self) -> Self {
+        IndexingIter {
+            base: self.base.clone(),
+            data: self.data,
+        }
+    }
+}
+
 impl<'a, T> Iterator for IndexingIter<'a, T> {
     type Item = &'a T;