Rollup merge of #145325 - clarfonthey:cast-init, r=scottmcm

Add `cast_init` and `cast_uninit` methods for pointers

ACP: rust-lang/libs-team#627
Tracking issue: #145036

This includes an incredibly low-effort search to find uses that could be switched to using these methods. I only searched for `cast::<\w>` and `cast::<MaybeUninit` because there would otherwise be way too much to look through, and I also didn't modify anything inside submodules/subtrees.

Author: Kobzol

library/alloc/src/lib.rs

@@ -102,6 +102,7 @@
 #![feature(async_iterator)]
 #![feature(bstr)]
 #![feature(bstr_internals)]
+#![feature(cast_maybe_uninit)]
 #![feature(char_internals)]
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]

library/alloc/src/vec/mod.rs

@@ -3176,7 +3176,7 @@ impl<T, A: Allocator> Vec<T, A> {
         // - but the allocation extends out to `self.buf.capacity()` elements, possibly
         // uninitialized
         let spare_ptr = unsafe { ptr.add(self.len) };
-        let spare_ptr = spare_ptr.cast::<MaybeUninit<T>>();
+        let spare_ptr = spare_ptr.cast_uninit();
         let spare_len = self.buf.capacity() - self.len;
 
         // SAFETY:

library/core/src/iter/adapters/map_windows.rs

@@ -195,7 +195,7 @@ impl<T, const N: usize> Buffer<T, N> {
 
         // SAFETY: the index is valid and this is element `a` in the
         // diagram above and has not been dropped yet.
-        unsafe { ptr::drop_in_place(to_drop.cast::<T>()) };
+        unsafe { ptr::drop_in_place(to_drop.cast_init()) };
     }
 }
 

library/core/src/ptr/const_ptr.rs

@@ -1430,6 +1430,28 @@ impl<T: PointeeSized> *const T {
     }
 }
 
+impl<T> *const T {
+    /// Casts from a type to its maybe-uninitialized version.
+    #[must_use]
+    #[inline(always)]
+    #[unstable(feature = "cast_maybe_uninit", issue = "145036")]
+    pub const fn cast_uninit(self) -> *const MaybeUninit<T> {
+        self as _
+    }
+}
+impl<T> *const MaybeUninit<T> {
+    /// Casts from a maybe-uninitialized type to its initialized version.
+    ///
+    /// This is always safe, since UB can only occur if the pointer is read
+    /// before being initialized.
+    #[must_use]
+    #[inline(always)]
+    #[unstable(feature = "cast_maybe_uninit", issue = "145036")]
+    pub const fn cast_init(self) -> *const T {
+        self as _
+    }
+}
+
 impl<T> *const [T] {
     /// Returns the length of a raw slice.
     ///

library/core/src/ptr/mut_ptr.rs

@@ -1687,6 +1687,31 @@ impl<T: PointeeSized> *mut T {
     }
 }
 
+impl<T> *mut T {
+    /// Casts from a type to its maybe-uninitialized version.
+    ///
+    /// This is always safe, since UB can only occur if the pointer is read
+    /// before being initialized.
+    #[must_use]
+    #[inline(always)]
+    #[unstable(feature = "cast_maybe_uninit", issue = "145036")]
+    pub const fn cast_uninit(self) -> *mut MaybeUninit<T> {
+        self as _
+    }
+}
+impl<T> *mut MaybeUninit<T> {
+    /// Casts from a maybe-uninitialized type to its initialized version.
+    ///
+    /// This is always safe, since UB can only occur if the pointer is read
+    /// before being initialized.
+    #[must_use]
+    #[inline(always)]
+    #[unstable(feature = "cast_maybe_uninit", issue = "145036")]
+    pub const fn cast_init(self) -> *mut T {
+        self as _
+    }
+}
+
 impl<T> *mut [T] {
     /// Returns the length of a raw slice.
     ///

library/core/src/ptr/non_null.rs

@@ -1357,6 +1357,28 @@ impl<T: PointeeSized> NonNull<T> {
     }
 }
 
+impl<T> NonNull<T> {
+    /// Casts from a type to its maybe-uninitialized version.
+    #[must_use]
+    #[inline(always)]
+    #[unstable(feature = "cast_maybe_uninit", issue = "145036")]
+    pub const fn cast_uninit(self) -> NonNull<MaybeUninit<T>> {
+        self.cast()
+    }
+}
+impl<T> NonNull<MaybeUninit<T>> {
+    /// Casts from a maybe-uninitialized type to its initialized version.
+    ///
+    /// This is always safe, since UB can only occur if the pointer is read
+    /// before being initialized.
+    #[must_use]
+    #[inline(always)]
+    #[unstable(feature = "cast_maybe_uninit", issue = "145036")]
+    pub const fn cast_init(self) -> NonNull<T> {
+        self.cast()
+    }
+}
+
 impl<T> NonNull<[T]> {
     /// Creates a non-null raw slice from a thin pointer and a length.
     ///

library/std/src/lib.rs

@@ -329,6 +329,7 @@
 // tidy-alphabetical-start
 #![feature(bstr)]
 #![feature(bstr_internals)]
+#![feature(cast_maybe_uninit)]
 #![feature(char_internals)]
 #![feature(clone_to_uninit)]
 #![feature(const_cmp)]

library/std/src/sys/fs/windows.rs

@@ -1606,7 +1606,7 @@ pub fn junction_point(original: &Path, link: &Path) -> io::Result<()> {
     };
     unsafe {
         let ptr = header.PathBuffer.as_mut_ptr();
-        ptr.copy_from(abs_path.as_ptr().cast::<MaybeUninit<u16>>(), abs_path.len());
+        ptr.copy_from(abs_path.as_ptr().cast_uninit(), abs_path.len());
 
         let mut ret = 0;
         cvt(c::DeviceIoControl(