Make do_collapse_axis return offset instead of performing it

Otherwise, `do_collapse_axis` should really be an `unsafe` function.

Author: jturner314

src/dimension/mod.rs

@@ -302,24 +302,20 @@ impl<'a> DimensionExt for [Ix]
 ///
 /// **Panics** if `index` is larger than the size of the axis
 // FIXME: Move to Dimension trait
-pub fn do_collapse_axis<A, D: Dimension>(
+pub fn do_collapse_axis<D: Dimension>(
     dims: &mut D,
-    ptr: &mut *mut A,
     strides: &D,
     axis: usize,
     index: usize,
-) {
+) -> isize {
     let dim = dims.slice()[axis];
     let stride = strides.slice()[axis];
     ndassert!(index < dim,
               "collapse_axis: Index {} must be less than axis length {} for \
                array with shape {:?}",
              index, dim, *dims);
     dims.slice_mut()[axis] = 1;
-    let off = stride_offset(index, stride);
-    unsafe {
-        *ptr = ptr.offset(off);
-    }
+    stride_offset(index, stride)
 }
 
 /// Compute the equivalent unsigned index given the axis length and signed index.

src/impl_methods.rs

@@ -635,13 +635,9 @@ impl<A, S, D> ArrayBase<S, D> where S: Data<Elem=A>, D: Dimension
     ///
     /// **Panics** if `axis` or `index` is out of bounds.
     pub fn collapse_axis(&mut self, axis: Axis, index: usize) {
-        dimension::do_collapse_axis(
-            &mut self.dim,
-            &mut self.ptr,
-            &self.strides,
-            axis.index(),
-            index,
-        )
+        let offset = dimension::do_collapse_axis(&mut self.dim, &self.strides, axis.index(), index);
+        self.ptr = unsafe { self.ptr.offset(offset) };
+        debug_assert!(self.pointer_is_inbounds());
     }
 
     /// Along `axis`, select the subview `index` and return a