Rollup merge of rust-lang#144515 - scottmcm:ptr_cast_array, r=Mark-Simulacrum

Implement `ptr_cast_array`

ACP: rust-lang/libs-team#602
Tracking Issue: rust-lang#144514

Author: GuillaumeGomez

core/src/ptr/const_ptr.rs

@@ -1569,6 +1569,15 @@ impl<T> *const [T] {
     }
 }
 
+impl<T> *const T {
+    /// Casts from a pointer-to-`T` to a pointer-to-`[T; N]`.
+    #[inline]
+    #[unstable(feature = "ptr_cast_array", issue = "144514")]
+    pub const fn cast_array<const N: usize>(self) -> *const [T; N] {
+        self.cast()
+    }
+}
+
 impl<T, const N: usize> *const [T; N] {
     /// Returns a raw pointer to the array's buffer.
     ///

core/src/ptr/mut_ptr.rs

@@ -1990,6 +1990,15 @@ impl<T> *mut [T] {
     }
 }
 
+impl<T> *mut T {
+    /// Casts from a pointer-to-`T` to a pointer-to-`[T; N]`.
+    #[inline]
+    #[unstable(feature = "ptr_cast_array", issue = "144514")]
+    pub const fn cast_array<const N: usize>(self) -> *mut [T; N] {
+        self.cast()
+    }
+}
+
 impl<T, const N: usize> *mut [T; N] {
     /// Returns a raw pointer to the array's buffer.
     ///

core/src/ptr/non_null.rs

@@ -193,6 +193,13 @@ impl<T: Sized> NonNull<T> {
         // requirements for a reference.
         unsafe { &mut *self.cast().as_ptr() }
     }
+
+    /// Casts from a pointer-to-`T` to a pointer-to-`[T; N]`.
+    #[inline]
+    #[unstable(feature = "ptr_cast_array", issue = "144514")]
+    pub const fn cast_array<const N: usize>(self) -> NonNull<[T; N]> {
+        self.cast()
+    }
 }
 
 impl<T: PointeeSized> NonNull<T> {

core/src/slice/mod.rs

@@ -328,7 +328,7 @@ impl<T> [T] {
         } else {
             // SAFETY: We explicitly check for the correct number of elements,
             //   and do not let the reference outlive the slice.
-            Some(unsafe { &*(self.as_ptr().cast::<[T; N]>()) })
+            Some(unsafe { &*(self.as_ptr().cast_array()) })
         }
     }
 
@@ -359,7 +359,7 @@ impl<T> [T] {
             // SAFETY: We explicitly check for the correct number of elements,
             //   do not let the reference outlive the slice,
             //   and require exclusive access to the entire slice to mutate the chunk.
-            Some(unsafe { &mut *(self.as_mut_ptr().cast::<[T; N]>()) })
+            Some(unsafe { &mut *(self.as_mut_ptr().cast_array()) })
         }
     }
 
@@ -387,7 +387,7 @@ impl<T> [T] {
 
         // SAFETY: We explicitly check for the correct number of elements,
         //   and do not let the references outlive the slice.
-        Some((unsafe { &*(first.as_ptr().cast::<[T; N]>()) }, tail))
+        Some((unsafe { &*(first.as_ptr().cast_array()) }, tail))
     }
 
     /// Returns a mutable array reference to the first `N` items in the slice and the remaining
@@ -420,7 +420,7 @@ impl<T> [T] {
         // SAFETY: We explicitly check for the correct number of elements,
         //   do not let the reference outlive the slice,
         //   and enforce exclusive mutability of the chunk by the split.
-        Some((unsafe { &mut *(first.as_mut_ptr().cast::<[T; N]>()) }, tail))
+        Some((unsafe { &mut *(first.as_mut_ptr().cast_array()) }, tail))
     }
 
     /// Returns an array reference to the last `N` items in the slice and the remaining slice.
@@ -448,7 +448,7 @@ impl<T> [T] {
 
         // SAFETY: We explicitly check for the correct number of elements,
         //   and do not let the references outlive the slice.
-        Some((init, unsafe { &*(last.as_ptr().cast::<[T; N]>()) }))
+        Some((init, unsafe { &*(last.as_ptr().cast_array()) }))
     }
 
     /// Returns a mutable array reference to the last `N` items in the slice and the remaining
@@ -482,7 +482,7 @@ impl<T> [T] {
         // SAFETY: We explicitly check for the correct number of elements,
         //   do not let the reference outlive the slice,
         //   and enforce exclusive mutability of the chunk by the split.
-        Some((init, unsafe { &mut *(last.as_mut_ptr().cast::<[T; N]>()) }))
+        Some((init, unsafe { &mut *(last.as_mut_ptr().cast_array()) }))
     }
 
     /// Returns an array reference to the last `N` items in the slice.
@@ -511,7 +511,7 @@ impl<T> [T] {
 
         // SAFETY: We explicitly check for the correct number of elements,
         //   and do not let the references outlive the slice.
-        Some(unsafe { &*(last.as_ptr().cast::<[T; N]>()) })
+        Some(unsafe { &*(last.as_ptr().cast_array()) })
     }
 
     /// Returns a mutable array reference to the last `N` items in the slice.
@@ -542,7 +542,7 @@ impl<T> [T] {
         // SAFETY: We explicitly check for the correct number of elements,
         //   do not let the reference outlive the slice,
         //   and require exclusive access to the entire slice to mutate the chunk.
-        Some(unsafe { &mut *(last.as_mut_ptr().cast::<[T; N]>()) })
+        Some(unsafe { &mut *(last.as_mut_ptr().cast_array()) })
     }
 
     /// Returns a reference to an element or subslice depending on the type of
@@ -846,7 +846,7 @@ impl<T> [T] {
     #[must_use]
     pub const fn as_array<const N: usize>(&self) -> Option<&[T; N]> {
         if self.len() == N {
-            let ptr = self.as_ptr() as *const [T; N];
+            let ptr = self.as_ptr().cast_array();
 
             // SAFETY: The underlying array of a slice can be reinterpreted as an actual array `[T; N]` if `N` is not greater than the slice's length.
             let me = unsafe { &*ptr };
@@ -864,7 +864,7 @@ impl<T> [T] {
     #[must_use]
     pub const fn as_mut_array<const N: usize>(&mut self) -> Option<&mut [T; N]> {
         if self.len() == N {
-            let ptr = self.as_mut_ptr() as *mut [T; N];
+            let ptr = self.as_mut_ptr().cast_array();
 
             // SAFETY: The underlying array of a slice can be reinterpreted as an actual array `[T; N]` if `N` is not greater than the slice's length.
             let me = unsafe { &mut *ptr };