Rollup merge of #133512 - bjoernager:slice-as-array, r=Amanieu

Add `as_array` and `as_mut_array` conversion methods to slices.

Tracking issue: #133508

This PR unstably implements the `as_array` and `as_mut_array` converters to `[T]`, `*const [T]`, and `*mut [T]`.

library/core/src/array/mod.rs

@@ -214,8 +214,8 @@ impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
     }
 }
 
-/// Tries to create an array `[T; N]` by copying from a slice `&[T]`. Succeeds if
-/// `slice.len() == N`.
+/// Tries to create an array `[T; N]` by copying from a slice `&[T]`.
+/// Succeeds if `slice.len() == N`.
 ///
 /// ```
 /// let bytes: [u8; 3] = [1, 0, 2];
@@ -282,13 +282,7 @@ impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N] {
 
     #[inline]
     fn try_from(slice: &'a [T]) -> Result<&'a [T; N], TryFromSliceError> {
-        if slice.len() == N {
-            let ptr = slice.as_ptr() as *const [T; N];
-            // SAFETY: ok because we just checked that the length fits
-            unsafe { Ok(&*ptr) }
-        } else {
-            Err(TryFromSliceError(()))
-        }
+        slice.as_array().ok_or(TryFromSliceError(()))
     }
 }
 
@@ -310,13 +304,7 @@ impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N] {
 
     #[inline]
     fn try_from(slice: &'a mut [T]) -> Result<&'a mut [T; N], TryFromSliceError> {
-        if slice.len() == N {
-            let ptr = slice.as_mut_ptr() as *mut [T; N];
-            // SAFETY: ok because we just checked that the length fits
-            unsafe { Ok(&mut *ptr) }
-        } else {
-            Err(TryFromSliceError(()))
-        }
+        slice.as_mut_array().ok_or(TryFromSliceError(()))
     }
 }
 

library/core/src/lib.rs

@@ -144,6 +144,7 @@
 #![feature(ptr_alignment_type)]
 #![feature(ptr_metadata)]
 #![feature(set_ptr_value)]
+#![feature(slice_as_array)]
 #![feature(slice_as_chunks)]
 #![feature(slice_ptr_get)]
 #![feature(str_internals)]

library/core/src/ptr/const_ptr.rs

@@ -1526,6 +1526,22 @@ impl<T> *const [T] {
         self as *const T
     }
 
+    /// Gets a raw pointer to the underlying array.
+    ///
+    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
+    #[unstable(feature = "slice_as_array", issue = "133508")]
+    #[rustc_const_unstable(feature = "slice_as_array", issue = "133508")]
+    #[inline]
+    #[must_use]
+    pub const fn as_array<const N: usize>(self) -> Option<*const [T; N]> {
+        if self.len() == N {
+            let me = self.as_ptr() as *const [T; N];
+            Some(me)
+        } else {
+            None
+        }
+    }
+
     /// Returns a raw pointer to an element or subslice, without doing bounds
     /// checking.
     ///

library/core/src/ptr/mut_ptr.rs

@@ -1760,6 +1760,22 @@ impl<T> *mut [T] {
         self.len() == 0
     }
 
+    /// Gets a raw, mutable pointer to the underlying array.
+    ///
+    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
+    #[unstable(feature = "slice_as_array", issue = "133508")]
+    #[rustc_const_unstable(feature = "slice_as_array", issue = "133508")]
+    #[inline]
+    #[must_use]
+    pub const fn as_mut_array<const N: usize>(self) -> Option<*mut [T; N]> {
+        if self.len() == N {
+            let me = self.as_mut_ptr() as *mut [T; N];
+            Some(me)
+        } else {
+            None
+        }
+    }
+
     /// Divides one mutable raw slice into two at an index.
     ///
     /// The first will contain all indices from `[0, mid)` (excluding

library/core/src/slice/mod.rs

@@ -855,6 +855,44 @@ impl<T> [T] {
         start..end
     }
 
+    /// Gets a reference to the underlying array.
+    ///
+    /// If `N` is not exactly equal to slice's the length of `self`, then this method returns `None`.
+    #[unstable(feature = "slice_as_array", issue = "133508")]
+    #[rustc_const_unstable(feature = "slice_as_array", issue = "133508")]
+    #[inline]
+    #[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];
+
+            // 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 };
+            Some(me)
+        } else {
+            None
+        }
+    }
+
+    /// Gets a mutable reference to the slice's underlying array.
+    ///
+    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
+    #[unstable(feature = "slice_as_array", issue = "133508")]
+    #[rustc_const_unstable(feature = "slice_as_array", issue = "133508")]
+    #[inline]
+    #[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];
+
+            // 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 };
+            Some(me)
+        } else {
+            None
+        }
+    }
+
     /// Swaps two elements in the slice.
     ///
     /// If `a` equals to `b`, it's guaranteed that elements won't change value.