From ffacb8861a860e5797e2835668336e60147ef454 Mon Sep 17 00:00:00 2001
From: JoJoJet <21144246+JoJoJet@users.noreply.github.com>
Date: Tue, 16 May 2023 15:36:05 -0400
Subject: [PATCH] add `UnsafeCell::from_mut`

---
 compiler/rustc_span/src/symbol.rs |  1 +
 library/core/src/cell.rs          | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 9951d8f4fc153..02505457efa4d 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1583,6 +1583,7 @@ symbols! {
         unrestricted_attribute_tokens,
         unsafe_block_in_unsafe_fn,
         unsafe_cell,
+        unsafe_cell_from_mut,
         unsafe_no_drop_flag,
         unsafe_pin_internals,
         unsize,
diff --git a/library/core/src/cell.rs b/library/core/src/cell.rs
index 2c3c14853a44b..744767aae44ca 100644
--- a/library/core/src/cell.rs
+++ b/library/core/src/cell.rs
@@ -2030,6 +2030,27 @@ impl<T> UnsafeCell<T> {
 }
 
 impl<T: ?Sized> UnsafeCell<T> {
+    /// Converts from `&mut T` to `&mut UnsafeCell<T>`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(unsafe_cell_from_mut)]
+    /// use std::cell::UnsafeCell;
+    ///
+    /// let mut val = 42;
+    /// let uc = UnsafeCell::from_mut(&mut val);
+    ///
+    /// *uc.get_mut() -= 1;
+    /// assert_eq!(*uc.get_mut(), 41);
+    /// ```
+    #[inline(always)]
+    #[unstable(feature = "unsafe_cell_from_mut", issue = "111645")]
+    pub const fn from_mut(value: &mut T) -> &mut UnsafeCell<T> {
+        // SAFETY: `UnsafeCell<T>` has the same memory layout as `T` due to #[repr(transparent)].
+        unsafe { &mut *(value as *mut T as *mut UnsafeCell<T>) }
+    }
+
     /// Gets a mutable pointer to the wrapped value.
     ///
     /// This can be cast to a pointer of any kind.