Rollup merge of rust-lang#74532 - fusion-engineering-forks:atomic-from-mut, r=KodrAus

Add Atomic*::from_mut.

The atomic equivalent of [`Cell::from_mut`](https://doc.rust-lang.org/stable/std/cell/struct.Cell.html#method.from_mut).

Author: Dylan-DPC

library/core/src/sync/atomic.rs

@@ -110,6 +110,7 @@ use self::Ordering::*;
 use crate::cell::UnsafeCell;
 use crate::fmt;
 use crate::intrinsics;
+use crate::mem::align_of;
 
 use crate::hint::spin_loop;
 
@@ -327,6 +328,27 @@ impl AtomicBool {
         unsafe { &mut *(self.v.get() as *mut bool) }
     }
 
+    /// Get atomic access to a `&mut bool`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(atomic_from_mut)]
+    /// use std::sync::atomic::{AtomicBool, Ordering};
+    ///
+    /// let mut some_bool = true;
+    /// let a = AtomicBool::from_mut(&mut some_bool);
+    /// a.store(false, Ordering::Relaxed);
+    /// assert_eq!(some_bool, false);
+    /// ```
+    #[inline]
+    #[unstable(feature = "atomic_from_mut", issue = "76314")]
+    pub fn from_mut(v: &mut bool) -> &Self {
+        // SAFETY: the mutable reference guarantees unique ownership, and
+        // alignment of both `bool` and `Self` is 1.
+        unsafe { &*(v as *mut bool as *mut Self) }
+    }
+
     /// Consumes the atomic and returns the contained value.
     ///
     /// This is safe because passing `self` by value guarantees that no other threads are
@@ -820,6 +842,30 @@ impl<T> AtomicPtr<T> {
         unsafe { &mut *self.p.get() }
     }
 
+    /// Get atomic access to a pointer.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(atomic_from_mut)]
+    /// use std::sync::atomic::{AtomicPtr, Ordering};
+    ///
+    /// let mut some_ptr = &mut 123 as *mut i32;
+    /// let a = AtomicPtr::from_mut(&mut some_ptr);
+    /// a.store(&mut 456, Ordering::Relaxed);
+    /// assert_eq!(unsafe { *some_ptr }, 456);
+    /// ```
+    #[inline]
+    #[unstable(feature = "atomic_from_mut", issue = "76314")]
+    pub fn from_mut(v: &mut *mut T) -> &Self {
+        let [] = [(); align_of::<Self>() - align_of::<*mut T>()];
+        // SAFETY:
+        //  - the mutable reference guarantees unique ownership.
+        //  - the alignment of `*mut T` and `Self` is the same on all platforms
+        //    supported by rust, as verified above.
+        unsafe { &*(v as *mut *mut T as *mut Self) }
+    }
+
     /// Consumes the atomic and returns the contained value.
     ///
     /// This is safe because passing `self` by value guarantees that no other threads are
@@ -1104,6 +1150,12 @@ impl<T> From<*mut T> for AtomicPtr<T> {
     }
 }
 
+macro_rules! if_not_8_bit {
+    (u8, $($tt:tt)*) => { "" };
+    (i8, $($tt:tt)*) => { "" };
+    ($_:ident, $($tt:tt)*) => { $($tt)* };
+}
+
 #[cfg(target_has_atomic_load_store = "8")]
 macro_rules! atomic_int {
     ($cfg_cas:meta,
@@ -1115,7 +1167,8 @@ macro_rules! atomic_int {
      $stable_nand:meta,
      $const_stable:meta,
      $stable_init_const:meta,
-     $s_int_type:expr, $int_ref:expr,
+     $(from_mut: cfg($from_mut_cfg:meta),)?
+     $s_int_type:literal, $int_ref:expr,
      $extra_feature:expr,
      $min_fn:ident, $max_fn:ident,
      $align:expr,
@@ -1226,6 +1279,45 @@ assert_eq!(some_var.load(Ordering::SeqCst), 5);
                 }
             }
 
+            doc_comment! {
+                concat!("Get atomic access to a `&mut ", stringify!($int_type), "`.
+
+",
+if_not_8_bit! {
+    $int_type,
+    concat!(
+        "**Note:** This function is only available on targets where `",
+        stringify!($int_type), "` has an alignment of ", $align, " bytes."
+    )
+},
+"
+
+# Examples
+
+```
+#![feature(atomic_from_mut)]
+", $extra_feature, "use std::sync::atomic::{", stringify!($atomic_type), ", Ordering};
+
+let mut some_int = 123;
+let a = ", stringify!($atomic_type), "::from_mut(&mut some_int);
+a.store(100, Ordering::Relaxed);
+assert_eq!(some_int, 100);
+```
+                "),
+                #[inline]
+                $(#[cfg($from_mut_cfg)])?
+                #[unstable(feature = "atomic_from_mut", issue = "76314")]
+                pub fn from_mut(v: &mut $int_type) -> &Self {
+                    let [] = [(); align_of::<Self>() - align_of::<$int_type>()];
+                    // SAFETY:
+                    //  - the mutable reference guarantees unique ownership.
+                    //  - the alignment of `$int_type` and `Self` is the
+                    //    same on all platforms enabled by `$from_mut_cfg`
+                    //    as verified above.
+                    unsafe { &*(v as *mut $int_type as *mut Self) }
+                }
+            }
+
             doc_comment! {
                 concat!("Consumes the atomic and returns the contained value.
 
@@ -1984,6 +2076,7 @@ atomic_int! {
     stable(feature = "integer_atomics_stable", since = "1.34.0"),
     rustc_const_stable(feature = "const_integer_atomics", since = "1.34.0"),
     unstable(feature = "integer_atomics", issue = "32976"),
+    from_mut: cfg(not(target_arch = "x86")),
     "i64", "../../../std/primitive.i64.html",
     "",
     atomic_min, atomic_max,
@@ -2002,6 +2095,7 @@ atomic_int! {
     stable(feature = "integer_atomics_stable", since = "1.34.0"),
     rustc_const_stable(feature = "const_integer_atomics", since = "1.34.0"),
     unstable(feature = "integer_atomics", issue = "32976"),
+    from_mut: cfg(not(target_arch = "x86")),
     "u64", "../../../std/primitive.u64.html",
     "",
     atomic_umin, atomic_umax,