Make SliceInfo be Sized, and remove pointer casts

The original reason for `SliceInfo` being `?Sized` and these
conversions was to work around the limitations of the slicing methods
taking a type involving `Dimension::SliceArg`. Now that the slicing
methods take `I: SliceArg<D>` as the parameter type, these conversions
are no longer necessary, and `SliceInfo` doesn't need to be `?Sized`.

Author: jturner314

src/slice.rs

@@ -406,7 +406,7 @@ unsafe impl SliceArg<IxDyn> for [AxisSliceInfo] {
 
 /// Represents all of the necessary information to perform a slice.
 ///
-/// The type `T` is typically `[AxisSliceInfo; n]`, `[AxisSliceInfo]`, or
+/// The type `T` is typically `[AxisSliceInfo; n]`, `&[AxisSliceInfo]`, or
 /// `Vec<AxisSliceInfo>`. The type `Din` is the dimension of the array to be
 /// sliced, and `Dout` is the output dimension after calling [`.slice()`]. Note
 /// that if `Din` is a fixed dimension type (`Ix0`, `Ix1`, `Ix2`, etc.), the
@@ -415,14 +415,13 @@ unsafe impl SliceArg<IxDyn> for [AxisSliceInfo] {
 ///
 /// [`.slice()`]: struct.ArrayBase.html#method.slice
 #[derive(Debug)]
-#[repr(transparent)]
-pub struct SliceInfo<T: ?Sized, Din: Dimension, Dout: Dimension> {
+pub struct SliceInfo<T, Din: Dimension, Dout: Dimension> {
     in_dim: PhantomData<Din>,
     out_dim: PhantomData<Dout>,
     indices: T,
 }
 
-impl<T: ?Sized, Din, Dout> Deref for SliceInfo<T, Din, Dout>
+impl<T, Din, Dout> Deref for SliceInfo<T, Din, Dout>
 where
     Din: Dimension,
     Dout: Dimension,
@@ -482,14 +481,7 @@ where
             indices,
         }
     }
-}
 
-impl<T, Din, Dout> SliceInfo<T, Din, Dout>
-where
-    T: AsRef<[AxisSliceInfo]>,
-    Din: Dimension,
-    Dout: Dimension,
-{
     /// Returns a new `SliceInfo` instance.
     ///
     /// Errors if `Din` or `Dout` is not consistent with `indices`.
@@ -508,14 +500,7 @@ where
             indices,
         })
     }
-}
 
-impl<T: ?Sized, Din, Dout> SliceInfo<T, Din, Dout>
-where
-    T: AsRef<[AxisSliceInfo]>,
-    Din: Dimension,
-    Dout: Dimension,
-{
     /// Returns the number of dimensions of the input array for
     /// [`.slice()`](struct.ArrayBase.html#method.slice).
     ///
@@ -546,7 +531,7 @@ where
     }
 }
 
-impl<'a, Din, Dout> TryFrom<&'a [AxisSliceInfo]> for &'a SliceInfo<[AxisSliceInfo], Din, Dout>
+impl<'a, Din, Dout> TryFrom<&'a [AxisSliceInfo]> for SliceInfo<&'a [AxisSliceInfo], Din, Dout>
 where
     Din: Dimension,
     Dout: Dimension,
@@ -555,16 +540,11 @@ where
 
     fn try_from(
         indices: &'a [AxisSliceInfo],
-    ) -> Result<&'a SliceInfo<[AxisSliceInfo], Din, Dout>, ShapeError> {
-        check_dims_for_sliceinfo::<Din, Dout>(indices)?;
+    ) -> Result<SliceInfo<&'a [AxisSliceInfo], Din, Dout>, ShapeError> {
         unsafe {
-            // This is okay because we've already checked the correctness of
-            // `Din` and `Dout`, and the only non-zero-sized member of
-            // `SliceInfo` is `indices`, so `&SliceInfo<[AxisSliceInfo], Din,
-            // Dout>` should have the same bitwise representation as
-            // `&[AxisSliceInfo]`.
-            Ok(&*(indices as *const [AxisSliceInfo]
-                as *const SliceInfo<[AxisSliceInfo], Din, Dout>))
+            // This is okay because `&[AxisSliceInfo]` always returns the same
+            // value for `.as_ref()`.
+            Self::new(indices)
         }
     }
 }
@@ -630,20 +610,18 @@ where
     }
 }
 
-impl<T, Din, Dout> AsRef<SliceInfo<[AxisSliceInfo], Din, Dout>> for SliceInfo<T, Din, Dout>
+impl<'a, T, Din, Dout> From<&'a SliceInfo<T, Din, Dout>>
+    for SliceInfo<&'a [AxisSliceInfo], Din, Dout>
 where
     T: AsRef<[AxisSliceInfo]>,
     Din: Dimension,
     Dout: Dimension,
 {
-    fn as_ref(&self) -> &SliceInfo<[AxisSliceInfo], Din, Dout> {
-        unsafe {
-            // This is okay because the only non-zero-sized member of
-            // `SliceInfo` is `indices`, so `&SliceInfo<[AxisSliceInfo], Din, Dout>`
-            // should have the same bitwise representation as
-            // `&[AxisSliceInfo]`.
-            &*(self.indices.as_ref() as *const [AxisSliceInfo]
-                as *const SliceInfo<[AxisSliceInfo], Din, Dout>)
+    fn from(info: &'a SliceInfo<T, Din, Dout>) -> SliceInfo<&'a [AxisSliceInfo], Din, Dout> {
+        SliceInfo {
+            in_dim: info.in_dim,
+            out_dim: info.out_dim,
+            indices: info.indices.as_ref(),
         }
     }
 }