Rollup merge of #123522 - dtolnay:constatomicintoinner, r=Nilstrieb

Stabilize const Atomic*::into_inner

Partial stabilization for #78729, for which the FCP has already completed.

The other `into_inner` functions in that tracking issue (`UnsafeCell`, `Cell`, `RefCell`) are blocked on #73255 for now.

```console
error[E0493]: destructor of `UnsafeCell<T>` cannot be evaluated at compile-time
    --> library/core/src/cell.rs:2076:29
     |
2076 |     pub const fn into_inner(self) -> T {
     |                             ^^^^ the destructor for this type cannot be evaluated in constant functions
2077 |         self.value
2078 |     }
     |     - value is dropped here
```

Author: matthiaskrgr

library/core/src/cell.rs

@@ -2071,6 +2071,7 @@ impl<T> UnsafeCell<T> {
     /// ```
     #[inline(always)]
     #[stable(feature = "rust1", since = "1.0.0")]
+    // When this is const stabilized, please remove `primitive_into_inner` below.
     #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
     pub const fn into_inner(self) -> T {
         self.value
@@ -2217,6 +2218,47 @@ impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T> {}
 
+// Special cases of UnsafeCell::into_inner where T is a primitive. These are
+// used by Atomic*::into_inner.
+//
+// The real UnsafeCell::into_inner cannot be used yet in a stable const function.
+// That is blocked on a "precise drop analysis" unstable const feature.
+// https://github.com/rust-lang/rust/issues/73255
+macro_rules! unsafe_cell_primitive_into_inner {
+    ($($primitive:ident $atomic:literal)*) => {
+        $(
+            #[cfg(target_has_atomic_load_store = $atomic)]
+            impl UnsafeCell<$primitive> {
+                pub(crate) const fn primitive_into_inner(self) -> $primitive {
+                    self.value
+                }
+            }
+        )*
+    };
+}
+
+unsafe_cell_primitive_into_inner! {
+    i8 "8"
+    u8 "8"
+    i16 "16"
+    u16 "16"
+    i32 "32"
+    u32 "32"
+    i64 "64"
+    u64 "64"
+    i128 "128"
+    u128 "128"
+    isize "ptr"
+    usize "ptr"
+}
+
+#[cfg(target_has_atomic_load_store = "ptr")]
+impl<T> UnsafeCell<*mut T> {
+    pub(crate) const fn primitive_into_inner(self) -> *mut T {
+        self.value
+    }
+}
+
 /// [`UnsafeCell`], but [`Sync`].
 ///
 /// This is just an `UnsafeCell`, except it implements `Sync`

library/core/src/sync/atomic.rs

@@ -578,9 +578,9 @@ impl AtomicBool {
     /// ```
     #[inline]
     #[stable(feature = "atomic_access", since = "1.15.0")]
-    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
+    #[rustc_const_stable(feature = "const_atomic_into_inner", since = "CURRENT_RUSTC_VERSION")]
     pub const fn into_inner(self) -> bool {
-        self.v.into_inner() != 0
+        self.v.primitive_into_inner() != 0
     }
 
     /// Loads a value from the bool.
@@ -1397,9 +1397,9 @@ impl<T> AtomicPtr<T> {
     /// ```
     #[inline]
     #[stable(feature = "atomic_access", since = "1.15.0")]
-    #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
+    #[rustc_const_stable(feature = "const_atomic_into_inner", since = "CURRENT_RUSTC_VERSION")]
     pub const fn into_inner(self) -> *mut T {
-        self.p.into_inner()
+        self.p.primitive_into_inner()
     }
 
     /// Loads a value from the pointer.
@@ -2378,9 +2378,9 @@ macro_rules! atomic_int {
             /// ```
             #[inline]
             #[$stable_access]
-            #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
+            #[rustc_const_stable(feature = "const_atomic_into_inner", since = "CURRENT_RUSTC_VERSION")]
             pub const fn into_inner(self) -> $int_type {
-                self.v.into_inner()
+                self.v.primitive_into_inner()
             }
 
             /// Loads a value from the atomic integer.