Make ArrayViewMut::into_view public and implement From conversion (#1595)

- Change ArrayViewMut::into_view visibility to pub
- Add From<ArrayViewMut> for ArrayView implementation
- Add regression tests in tests/views.rs

Author: at264939-ctrl

src/arraytraits.rs

@@ -647,6 +647,17 @@ where
     }
 }
 
+/// Implementation of `ArrayView::from(ArrayViewMut)`
+impl<'a, A, D> From<ArrayViewMut<'a, A, D>> for ArrayView<'a, A, D>
+where D: Dimension
+{
+    /// Create a read-only array view from a mutable array view.
+    fn from(view: ArrayViewMut<'a, A, D>) -> Self
+    {
+        view.into_view()
+    }
+}
+
 impl<A, D> From<Array<A, D>> for ArcArray<A, D>
 where D: Dimension
 {

src/impl_views/conversions.rs

@@ -273,8 +273,11 @@ where D: Dimension
 impl<'a, A, D> ArrayViewMut<'a, A, D>
 where D: Dimension
 {
-    // Convert into a read-only view
-    pub(crate) fn into_view(self) -> ArrayView<'a, A, D>
+    /// Convert into a read-only view.
+    ///
+    /// Unlike [`Self::view`], this method consumes the mutable view and preserves
+    /// the original lifetime of the data.
+    pub fn into_view(self) -> ArrayView<'a, A, D>
     {
         unsafe { ArrayView::new(self.parts.ptr, self.parts.dim, self.parts.strides) }
     }

tests/views.rs

@@ -15,3 +15,44 @@ fn cell_view()
     }
     assert_eq!(a, answer);
 }
+
+#[test]
+fn test_view_conversion()
+{
+    let mut a = Array2::<f32>::zeros((4, 4));
+    let view_mut = a.view_mut();
+    let view = view_mut.into_view();
+    assert_eq!(view.shape(), &[4, 4]);
+
+    let view_mut = a.view_mut();
+    let view: ArrayView2<'_, f32> = view_mut.into();
+    assert_eq!(view.shape(), &[4, 4]);
+}
+
+#[test]
+fn test_view_conversion_lifetime()
+{
+    // Regression test for #1595
+    struct Foo<'a> {
+        data: ArrayViewMut2<'a, f32>,
+    }
+
+    impl<'a> Foo<'a> {
+        fn into_shared(self) -> ArrayView2<'a, f32> {
+            self.data.into_view()
+        }
+
+        fn into_shared_from(self) -> ArrayView2<'a, f32> {
+            self.data.into()
+        }
+    }
+
+    let mut a = Array2::<f32>::zeros((4, 4));
+    let foo = Foo { data: a.view_mut() };
+    let shared = foo.into_shared();
+    assert_eq!(shared.shape(), &[4, 4]);
+
+    let foo = Foo { data: a.view_mut() };
+    let shared = foo.into_shared_from();
+    assert_eq!(shared.shape(), &[4, 4]);
+}