FIX: Fix iteration order debug assertion in Zip

We're trying to assert the iteration order is like C-contig or
C-preferred. The old assertion tripped when combining these two arrays:

- shape 1, 7 strides 7, 1  (layout: CFcf)
- shape 1, 7 strides 1, 2  (layout: f)

The result was that the first was judged to have "no preference" and
second "leaning f", total "leaning f".

The debug assertion tripped a false positive because the traversal is
one-dimensional, so elements are still visited in the required order.

The layout judgment for the second array should ideally be "CFcf" too,
but then it needs an understanding of being one-dimensional with
contiguous stride of 2, which is not implemented.

Author: bluss

src/zip/mod.rs

@@ -430,6 +430,25 @@ where
     }
 }
 
+impl<D, P1, P2> Zip<(P1, P2), D>
+    where D: Dimension,
+          P1: NdProducer<Dim=D>,
+          P1: NdProducer<Dim=D>,
+{
+    /// Debug assert traversal order is like c (including 1D case)
+    // Method placement: only used for binary Zip at the moment.
+    #[inline]
+    pub(crate) fn debug_assert_c_order(self) -> Self {
+        debug_assert!(self.layout.is(CORDER) || self.layout_tendency >= 0 ||
+                      self.dimension.slice().iter().filter(|&&d| d > 1).count() <= 1,
+                      "Assertion failed: traversal is not c-order or 1D for \
+                      layout {:?}, tendency {}, dimension {:?}",
+                      self.layout, self.layout_tendency, self.dimension);
+        self
+    }
+}
+
+
 /*
 trait Offset : Copy {
     unsafe fn offset(self, off: isize) -> Self;
@@ -673,13 +692,6 @@ macro_rules! map_impl {
                 self.build_and(part)
             }
 
-            #[allow(unused)]
-            #[inline]
-            pub(crate) fn debug_assert_c_order(self) -> Self {
-                debug_assert!(self.layout.is(CORDER) || self.layout_tendency >= 0);
-                self
-            }
-
             fn build_and<P>(self, part: P) -> Zip<($($p,)* P, ), D>
                 where P: NdProducer<Dim=D>,
             {