Rollup merge of rust-lang#131717 - tgross35:stabilize-const_atomic_fr…

…om_ptr, r=RalfJung

Stabilize `const_atomic_from_ptr`

The API is already stable since rust-lang#115719, but const stability was blocked on `const_mut_refs`. Since that was recently stabilized, const stabilize the following:

```rust
// core::atomic

impl AtomicBool { pub const unsafe fn from_ptr<'a>(ptr: *mut bool) -> &'a AtomicBool; }

impl<T> AtomicPtr<T> { pub const unsafe fn from_ptr<'a>(ptr: *mut *mut T) -> &'a AtomicPtr<T>; }

impl AtomicU8    { pub const unsafe fn from_ptr<'a>(ptr: *mut u8)    -> &'a AtomicU8;    }
impl AtomicU16   { pub const unsafe fn from_ptr<'a>(ptr: *mut u16)   -> &'a AtomicU16;   }
impl AtomicU32   { pub const unsafe fn from_ptr<'a>(ptr: *mut u32)   -> &'a AtomicU32;   }
impl AtomicU64   { pub const unsafe fn from_ptr<'a>(ptr: *mut u64)   -> &'a AtomicU64;   }
impl AtomicUsize { pub const unsafe fn from_ptr<'a>(ptr: *mut usize) -> &'a AtomicUsize; }

impl AtomicI8    { pub const unsafe fn from_ptr<'a>(ptr: *mut i8)    -> &'a AtomicI8;    }
impl AtomicI16   { pub const unsafe fn from_ptr<'a>(ptr: *mut i16)   -> &'a AtomicI16;   }
impl AtomicI32   { pub const unsafe fn from_ptr<'a>(ptr: *mut i32)   -> &'a AtomicI32;   }
impl AtomicI64   { pub const unsafe fn from_ptr<'a>(ptr: *mut i64)   -> &'a AtomicI64;   }
impl AtomicIsize { pub const unsafe fn from_ptr<'a>(ptr: *mut isize) -> &'a AtomicIsize; }
```

library/core/src/sync/atomic.rs

@@ -469,7 +469,7 @@ impl AtomicBool {
     /// [valid]: crate::ptr#safety
     /// [Memory model for atomic accesses]: self#memory-model-for-atomic-accesses
     #[stable(feature = "atomic_from_ptr", since = "1.75.0")]
-    #[rustc_const_unstable(feature = "const_atomic_from_ptr", issue = "108652")]
+    #[rustc_const_stable(feature = "const_atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]
     pub const unsafe fn from_ptr<'a>(ptr: *mut bool) -> &'a AtomicBool {
         // SAFETY: guaranteed by the caller
         unsafe { &*ptr.cast() }
@@ -1264,7 +1264,7 @@ impl<T> AtomicPtr<T> {
     /// [valid]: crate::ptr#safety
     /// [Memory model for atomic accesses]: self#memory-model-for-atomic-accesses
     #[stable(feature = "atomic_from_ptr", since = "1.75.0")]
-    #[rustc_const_unstable(feature = "const_atomic_from_ptr", issue = "108652")]
+    #[rustc_const_stable(feature = "const_atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]
     pub const unsafe fn from_ptr<'a>(ptr: *mut *mut T) -> &'a AtomicPtr<T> {
         // SAFETY: guaranteed by the caller
         unsafe { &*ptr.cast() }
@@ -2263,7 +2263,7 @@ macro_rules! atomic_int {
             /// [valid]: crate::ptr#safety
             /// [Memory model for atomic accesses]: self#memory-model-for-atomic-accesses
             #[stable(feature = "atomic_from_ptr", since = "1.75.0")]
-            #[rustc_const_unstable(feature = "const_atomic_from_ptr", issue = "108652")]
+            #[rustc_const_stable(feature = "const_atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]
             pub const unsafe fn from_ptr<'a>(ptr: *mut $int_type) -> &'a $atomic_type {
                 // SAFETY: guaranteed by the caller
                 unsafe { &*ptr.cast() }