From 1a5c668cbf7341d7c0afe452f0ac809307bce9a6 Mon Sep 17 00:00:00 2001
From: bluss <bluss@users.noreply.github.com>
Date: Thu, 25 Jan 2018 19:14:45 +0100
Subject: [PATCH] FEAT: Add .reborrow() methods to array views

`ArrayView<'a, T, D>` is invariant in parameters `T` and `'a`, because
`ArrayBase<S, D>` is invariant in the parameter `S` because the array
base is using the projection `<S as Data>::Elem` in the struct
definition.

This means that the equivalent of the `&'a T -> &'b T` conversion (valid
whenever `'a: 'b` is not naturally available for array views; we can
provide it manually, with a method.
---
 src/impl_views.rs | 21 ++++++++++++++++++++-
 tests/array.rs    | 14 ++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/src/impl_views.rs b/src/impl_views.rs
index 7f9329af0..7bd96ffeb 100644
--- a/src/impl_views.rs
+++ b/src/impl_views.rs
@@ -82,6 +82,16 @@ impl<'a, A, D> ArrayView<'a, A, D>
         ArrayView::new_(ptr, dim, strides)
     }
 
+    /// Convert the view into an `ArrayView<'b, A, D>` where `'b` is a lifetime
+    /// outlived by `'a'`.
+    pub fn reborrow<'b>(self) -> ArrayView<'b, A, D>
+        where 'a: 'b
+    {
+        unsafe {
+            ArrayView::new_(self.as_ptr(), self.dim, self.strides)
+        }
+    }
+
     /// Split the array view along `axis` and return one view strictly before the
     /// split and one view after the split.
     ///
@@ -133,7 +143,6 @@ impl<'a, A, D> ArrayView<'a, A, D>
             None
         }
     }
-
 }
 
 
@@ -328,6 +337,16 @@ impl<'a, A, D> ArrayViewMut<'a, A, D>
         ArrayViewMut::new_(ptr, dim, strides)
     }
 
+    /// Convert the view into an `ArrayViewMut<'b, A, D>` where `'b` is a lifetime
+    /// outlived by `'a'`.
+    pub fn reborrow<'b>(mut self) -> ArrayViewMut<'b, A, D>
+        where 'a: 'b
+    {
+        unsafe {
+            ArrayViewMut::new_(self.as_mut_ptr(), self.dim, self.strides)
+        }
+    }
+
     /// Split the array view along `axis` and return one mutable view strictly
     /// before the split and one mutable view after the split.
     ///
diff --git a/tests/array.rs b/tests/array.rs
index ef98f4a42..a98a743a2 100644
--- a/tests/array.rs
+++ b/tests/array.rs
@@ -40,6 +40,20 @@ fn test_matmul_rcarray()
     }
 }
 
+#[allow(unused)]
+fn arrayview_shrink_lifetime<'a, 'b: 'a>(view: ArrayView1<'b, f64>)
+    -> ArrayView1<'a, f64>
+{
+    view.reborrow()
+}
+
+#[allow(unused)]
+fn arrayviewmut_shrink_lifetime<'a, 'b: 'a>(view: ArrayViewMut1<'b, f64>)
+    -> ArrayViewMut1<'a, f64>
+{
+    view.reborrow()
+}
+
 #[test]
 fn test_mat_mul() {
     // smoke test, a big matrix multiplication of uneven size