Make some methods of Pin unstable const

Make the following methods unstable const under the `const_pin` feature:
- `new`
- `new_unchecked`
- `into_inner`
- `into_inner_unchecked`
- `get_ref`
- `into_ref`

Also adds tests for these methods in a const context.

Tracking issue: rust-lang#76654

library/core/src/lib.rs

@@ -80,6 +80,7 @@
 #![feature(const_int_pow)]
 #![feature(constctlz)]
 #![feature(const_panic)]
+#![feature(const_pin)]
 #![feature(const_fn_union)]
 #![feature(const_generics)]
 #![feature(const_option)]

library/core/src/pin.rs

@@ -471,9 +471,10 @@ impl<P: Deref<Target: Unpin>> Pin<P> {
     ///
     /// Unlike `Pin::new_unchecked`, this method is safe because the pointer
     /// `P` dereferences to an [`Unpin`] type, which cancels the pinning guarantees.
-    #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
-    pub fn new(pointer: P) -> Pin<P> {
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
+    #[stable(feature = "pin", since = "1.33.0")]
+    pub const fn new(pointer: P) -> Pin<P> {
         // SAFETY: the value pointed to is `Unpin`, and so has no requirements
         // around pinning.
         unsafe { Pin::new_unchecked(pointer) }
@@ -483,9 +484,10 @@ impl<P: Deref<Target: Unpin>> Pin<P> {
     ///
     /// This requires that the data inside this `Pin` is [`Unpin`] so that we
     /// can ignore the pinning invariants when unwrapping it.
-    #[stable(feature = "pin_into_inner", since = "1.39.0")]
     #[inline(always)]
-    pub fn into_inner(pin: Pin<P>) -> P {
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
+    #[stable(feature = "pin_into_inner", since = "1.39.0")]
+    pub const fn into_inner(pin: Pin<P>) -> P {
         pin.pointer
     }
 }
@@ -556,9 +558,10 @@ impl<P: Deref> Pin<P> {
     ///
     /// [`mem::swap`]: crate::mem::swap
     #[lang = "new_unchecked"]
-    #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
-    pub unsafe fn new_unchecked(pointer: P) -> Pin<P> {
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
+    #[stable(feature = "pin", since = "1.33.0")]
+    pub const unsafe fn new_unchecked(pointer: P) -> Pin<P> {
         Pin { pointer }
     }
 
@@ -589,9 +592,10 @@ impl<P: Deref> Pin<P> {
     ///
     /// If the underlying data is [`Unpin`], [`Pin::into_inner`] should be used
     /// instead.
-    #[stable(feature = "pin_into_inner", since = "1.39.0")]
     #[inline(always)]
-    pub unsafe fn into_inner_unchecked(pin: Pin<P>) -> P {
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
+    #[stable(feature = "pin_into_inner", since = "1.39.0")]
+    pub const unsafe fn into_inner_unchecked(pin: Pin<P>) -> P {
         pin.pointer
     }
 }
@@ -693,17 +697,18 @@ impl<'a, T: ?Sized> Pin<&'a T> {
     /// with the same lifetime as the original `Pin`.
     ///
     /// ["pinning projections"]: self#projections-and-structural-pinning
-    #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
-    pub fn get_ref(self) -> &'a T {
+    #[rustc_const_unstable(feature = "const_pin", issue = "76654")]
+    #[stable(feature = "pin", since = "1.33.0")]
+    pub const fn get_ref(self) -> &'a T {
         self.pointer
     }
 }
 
 impl<'a, T: ?Sized> Pin<&'a mut T> {
     /// Converts this `Pin<&mut T>` into a `Pin<&T>` with the same lifetime.
-    #[stable(feature = "pin", since = "1.33.0")]
     #[inline(always)]
+    #[stable(feature = "pin", since = "1.33.0")]
     pub fn into_ref(self) -> Pin<&'a T> {
         Pin { pointer: 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_pin)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_raw_ptr_deref)]
 #![feature(never_type)]
@@ -74,6 +75,7 @@ mod num;
 mod ops;
 mod option;
 mod pattern;
+mod pin;
 mod ptr;
 mod result;
 mod slice;

library/core/tests/pin.rs

@@ -0,0 +1,21 @@
+use core::pin::Pin;
+
+#[test]
+fn pin_const() {
+    // test that the methods of `Pin` are usable in a const context
+
+    const POINTER: &'static usize = &2;
+
+    const PINNED: Pin<&'static usize> = Pin::new(POINTER);
+    const PINNED_UNCHECKED: Pin<&'static usize> = unsafe { Pin::new_unchecked(POINTER) };
+    assert_eq!(PINNED_UNCHECKED, PINNED);
+
+    const INNER: &'static usize = Pin::into_inner(PINNED);
+    assert_eq!(INNER, POINTER);
+
+    const INNER_UNCHECKED: &'static usize = unsafe { Pin::into_inner_unchecked(PINNED) };
+    assert_eq!(INNER_UNCHECKED, POINTER);
+
+    const REF: &'static usize = PINNED.get_ref();
+    assert_eq!(REF, POINTER)
+}