cross: DimSlice: add as_ref, as_mut methods to re-borrow the data with a different lifetime

we can't use the actual AsRef, AsMut trait because we aren't returning a
reference but a new type with an inner reference.

crates/cross/src/dim/dim_slice.rs

@@ -1,3 +1,4 @@
+use core::convert::{AsMut, AsRef};
 use core::iter::Zip;
 use core::ops::{Index, IndexMut};
 
@@ -5,6 +6,8 @@ use crate::vec::Vec3u;
 
 /// use this to wrap a flat region of memory into something which can be indexed by coordinates in
 /// 3d space.
+#[cfg_attr(feature = "fmt", derive(Debug))]
+#[derive(PartialEq)]
 pub struct DimSlice<T> {
     dim: Vec3u,
     items: T,
@@ -20,6 +23,19 @@ impl<T> DimSlice<T> {
     pub fn dim(&self) -> Vec3u {
         self.dim
     }
+
+    /// re-borrow the slice with a different lifetime.
+    pub fn as_ref<R: ?Sized>(&self) -> DimSlice<&R>
+        where T: AsRef<R>
+    {
+        DimSlice::new(self.dim, self.items.as_ref())
+    }
+    /// re-borrow the slice with a different lifetime.
+    pub fn as_mut<R: ?Sized>(&mut self) -> DimSlice<&mut R>
+        where T: AsMut<R>
+    {
+        DimSlice::new(self.dim, self.items.as_mut())
+    }
 }
 
 #[cfg(feature = "iter")]
@@ -122,6 +138,13 @@ mod test {
         assert_eq!(index(Vec3u::new(1, 2, 2), dim), 17);
     }
 
+    #[test]
+    fn as_ref() {
+        let data = [1, 2];
+        let s = DimSlice::new(Vec3u::new(1, 2, 1), &data[..]);
+        assert_eq!(s.as_ref(), DimSlice::new(Vec3u::new(1, 2, 1), &data[..]));
+    }
+
     #[test]
     fn dim_slice_index() {
         let data = [

crates/cross/src/dim/offset_dim_slice.rs

@@ -1,3 +1,4 @@
+use core::convert::{AsMut, AsRef};
 use core::iter::Zip;
 use core::ops::{Index, IndexMut};
 
@@ -5,6 +6,8 @@ use crate::dim::{DimIter, DimSlice};
 use crate::vec::Vec3u;
 
 
+#[cfg_attr(feature = "fmt", derive(Debug))]
+#[derive(PartialEq)]
 pub struct OffsetDimSlice<T> {
     offset: Vec3u,
     inner: DimSlice<T>,
@@ -17,6 +20,19 @@ impl<T> OffsetDimSlice<T> {
     pub fn indices(&self) -> OffsetDimIter {
         OffsetDimIter::new(self.offset, self.inner.indices())
     }
+
+    /// re-borrow the slice with a different lifetime.
+    pub fn as_ref<R: ?Sized>(&self) -> OffsetDimSlice<&R>
+        where T: AsRef<R>
+    {
+        OffsetDimSlice { offset: self.offset, inner: self.inner.as_ref()}
+    }
+    /// re-borrow the slice with a different lifetime.
+    pub fn as_mut<R: ?Sized>(&mut self) -> OffsetDimSlice<&mut R>
+        where T: AsMut<R>
+    {
+        OffsetDimSlice { offset: self.offset, inner: self.inner.as_mut()}
+    }
 }
 
 #[cfg(feature = "iter")]