Make some methods of `Pin<&mut T>` unstable const

Make the following methods unstable const under the `const_pin` feature:
- `into_ref`
- `get_mut`
- `get_unchecked_mut`

library/core/src/pin.rs

@@ -708,8 +708,9 @@ impl<'a, T: ?Sized> Pin<&'a T> {
 impl<'a, T: ?Sized> Pin<&'a mut T> {
     /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
     #[inline(always)]
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
     #[stable(feature = "pin", since = "1.33.0")]
-    pub fn into_ref(self) -> Pin<&'a T> {
+    pub const fn into_ref(self) -> Pin<&'a T> {
         Pin { pointer: self.pointer }
     }
 
@@ -722,9 +723,10 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
     /// that lives for as long as the borrow of the `Pin`, not the lifetime of
     /// the `Pin` itself. This method allows turning the `Pin` into a reference
     /// with the same lifetime as the original `Pin`.
-    #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
-    pub fn get_mut(self) -> &'a mut T
+    #[stable(feature = "pin", since = "1.33.0")]
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
+    pub const fn get_mut(self) -> &'a mut T
     where
         T: Unpin,
     {
@@ -741,9 +743,10 @@ impl<'a, T: ?Sized> Pin<&'a mut T> {
     ///
     /// If the underlying data is `Unpin`, `Pin::get_mut` should be used
     /// instead.
-    #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
-    pub unsafe fn get_unchecked_mut(self) -> &'a mut T {
+    #[stable(feature = "pin", since = "1.33.0")]
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
+    pub const unsafe fn get_unchecked_mut(self) -> &'a mut T {
         self.pointer
     }
 

library/core/tests/lib.rs

@@ -39,6 +39,7 @@
 #![feature(iter_order_by)]
 #![feature(cmp_min_max_by)]
 #![feature(iter_map_while)]
+#![feature(const_mut_refs)]
 #![feature(const_pin)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_raw_ptr_deref)]

library/core/tests/pin.rs

@@ -17,5 +17,15 @@ fn pin_const() {
     assert_eq!(INNER_UNCHECKED, POINTER);
 
     const REF: &'static usize = PINNED.get_ref();
-    assert_eq!(REF, POINTER)
+    assert_eq!(REF, POINTER);
+
+    // Note: `pin_mut_const` tests that the methods of `Pin<&mut T>` are usable in a const context.
+    // A const fn is used because `&mut` is not (yet) usable in constants.
+    const fn pin_mut_const() {
+        let _ = Pin::new(&mut 2).into_ref();
+        let _ = Pin::new(&mut 2).get_mut();
+        let _ = unsafe { Pin::new(&mut 2).get_unchecked_mut() };
+    }
+
+    pin_mut_const();
 }