Rollup merge of #60667 - oli-obk:raw_from_raw_parts, r=sfackler

Add functions for building raw slices to libcore

implement https://github.com/rust-lang/rust/issues/36925

src/libcore/ptr/mod.rs

@@ -230,6 +230,53 @@ pub const fn null<T>() -> *const T { 0 as *const T }
 #[rustc_promotable]
 pub const fn null_mut<T>() -> *mut T { 0 as *mut T }
 
+#[repr(C)]
+pub(crate) union Repr<T> {
+    pub(crate) rust: *const [T],
+    rust_mut: *mut [T],
+    pub(crate) raw: FatPtr<T>,
+}
+
+#[repr(C)]
+pub(crate) struct FatPtr<T> {
+    data: *const T,
+    pub(crate) len: usize,
+}
+
+/// Forms a slice from a pointer and a length.
+///
+/// The `len` argument is the number of **elements**, not the number of bytes.
+///
+/// # Examples
+///
+/// ```rust
+/// #![feature(slice_from_raw_parts)]
+/// use std::ptr;
+///
+/// // create a slice pointer when starting out with a pointer to the first element
+/// let mut x = [5, 6, 7];
+/// let ptr = &mut x[0] as *mut _;
+/// let slice = ptr::slice_from_raw_parts_mut(ptr, 3);
+/// assert_eq!(unsafe { &*slice }[2], 7);
+/// ```
+#[inline]
+#[unstable(feature = "slice_from_raw_parts", reason = "recently added", issue = "36925")]
+pub fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
+    unsafe { Repr { raw: FatPtr { data, len } }.rust }
+}
+
+/// Performs the same functionality as [`from_raw_parts`], except that a
+/// mutable slice is returned.
+///
+/// See the documentation of [`from_raw_parts`] for more details.
+///
+/// [`from_raw_parts`]: ../../std/slice/fn.from_raw_parts.html
+#[inline]
+#[unstable(feature = "slice_from_raw_parts", reason = "recently added", issue = "36925")]
+pub fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
+    unsafe { Repr { raw: FatPtr { data, len } }.rust_mut }
+}
+
 /// Swaps the values at two mutable locations of the same type, without
 /// deinitializing either.
 ///

src/libcore/slice/mod.rs

@@ -45,19 +45,6 @@ pub mod memchr;
 mod rotate;
 mod sort;
 
-#[repr(C)]
-union Repr<'a, T: 'a> {
-    rust: &'a [T],
-    rust_mut: &'a mut [T],
-    raw: FatPtr<T>,
-}
-
-#[repr(C)]
-struct FatPtr<T> {
-    data: *const T,
-    len: usize,
-}
-
 //
 // Extension traits
 //
@@ -78,7 +65,7 @@ impl<T> [T] {
     #[rustc_const_unstable(feature = "const_slice_len")]
     pub const fn len(&self) -> usize {
         unsafe {
-            Repr { rust: self }.raw.len
+            crate::ptr::Repr { rust: self }.raw.len
         }
     }
 
@@ -5195,7 +5182,7 @@ pub unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
     debug_assert!(data as usize % mem::align_of::<T>() == 0, "attempt to create unaligned slice");
     debug_assert!(mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
                   "attempt to create slice covering half the address space");
-    Repr { raw: FatPtr { data, len } }.rust
+    &*ptr::slice_from_raw_parts(data, len)
 }
 
 /// Performs the same functionality as [`from_raw_parts`], except that a
@@ -5216,7 +5203,7 @@ pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T]
     debug_assert!(data as usize % mem::align_of::<T>() == 0, "attempt to create unaligned slice");
     debug_assert!(mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
                   "attempt to create slice covering half the address space");
-    Repr { raw: FatPtr { data, len } }.rust_mut
+    &mut *ptr::slice_from_raw_parts_mut(data, len)
 }
 
 /// Converts a reference to T into a slice of length 1 (without copying).