Rollup merge of rust-lang#137928 - RalfJung:const_cell, r=m-ou-se

stabilize const_cell

``@rust-lang/libs-api`` ``@rust-lang/wg-const-eval`` I see no reason to wait any longer, so I propose we stabilize the use of `Cell` in `const fn`  -- specifically the APIs listed here:
```rust
// core::cell

impl<T> Cell<T> {
    pub const fn replace(&self, val: T) -> T;
}

impl<T: Copy> Cell<T> {
    pub const fn get(&self) -> T;
}

impl<T: ?Sized> Cell<T> {
    pub const fn get_mut(&mut self) -> &mut T;
    pub const fn from_mut(t: &mut T) -> &Cell<T>;
}

impl<T> Cell<[T]> {
    pub const fn as_slice_of_cells(&self) -> &[Cell<T>];
}
```
Unfortunately, `set` cannot be made `const fn` yet as it drops the old contents.

Fixes rust-lang#131283

Author: matthiaskrgr

library/core/src/cell.rs

@@ -495,7 +495,7 @@ impl<T> Cell<T> {
     /// ```
     #[inline]
     #[stable(feature = "move_cell", since = "1.17.0")]
-    #[rustc_const_unstable(feature = "const_cell", issue = "131283")]
+    #[rustc_const_stable(feature = "const_cell", since = "CURRENT_RUSTC_VERSION")]
     #[rustc_confusables("swap")]
     pub const fn replace(&self, val: T) -> T {
         // SAFETY: This can cause data races if called from a separate thread,
@@ -537,7 +537,7 @@ impl<T: Copy> Cell<T> {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_cell", issue = "131283")]
+    #[rustc_const_stable(feature = "const_cell", since = "CURRENT_RUSTC_VERSION")]
     pub const fn get(&self) -> T {
         // SAFETY: This can cause data races if called from a separate thread,
         // but `Cell` is `!Sync` so this won't happen.
@@ -617,7 +617,7 @@ impl<T: ?Sized> Cell<T> {
     /// ```
     #[inline]
     #[stable(feature = "cell_get_mut", since = "1.11.0")]
-    #[rustc_const_unstable(feature = "const_cell", issue = "131283")]
+    #[rustc_const_stable(feature = "const_cell", since = "CURRENT_RUSTC_VERSION")]
     pub const fn get_mut(&mut self) -> &mut T {
         self.value.get_mut()
     }
@@ -637,7 +637,7 @@ impl<T: ?Sized> Cell<T> {
     /// ```
     #[inline]
     #[stable(feature = "as_cell", since = "1.37.0")]
-    #[rustc_const_unstable(feature = "const_cell", issue = "131283")]
+    #[rustc_const_stable(feature = "const_cell", since = "CURRENT_RUSTC_VERSION")]
     pub const fn from_mut(t: &mut T) -> &Cell<T> {
         // SAFETY: `&mut` ensures unique access.
         unsafe { &*(t as *mut T as *const Cell<T>) }
@@ -695,7 +695,7 @@ impl<T> Cell<[T]> {
     /// assert_eq!(slice_cell.len(), 3);
     /// ```
     #[stable(feature = "as_cell", since = "1.37.0")]
-    #[rustc_const_unstable(feature = "const_cell", issue = "131283")]
+    #[rustc_const_stable(feature = "const_cell", since = "CURRENT_RUSTC_VERSION")]
     pub const fn as_slice_of_cells(&self) -> &[Cell<T>] {
         // SAFETY: `Cell<T>` has the same memory layout as `T`.
         unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }