Rollup merge of rust-lang#88887 - fee1-dead:const-deref, r=oli-obk

Const Deref

Implements `const Deref`/`const DerefMut` for `&mut T`, `&T`, `Cow<'_, B>` and `ManuallyDrop<T>`

Author: JohnTitor

library/alloc/src/borrow.rs

@@ -330,7 +330,11 @@ impl<B: ?Sized + ToOwned> Cow<'_, B> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<B: ?Sized + ToOwned> Deref for Cow<'_, B> {
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
+impl<B: ?Sized + ToOwned> const Deref for Cow<'_, B>
+where
+    B::Owned: ~const Borrow<B>,
+{
     type Target = B;
 
     fn deref(&self) -> &B {

library/core/src/mem/manually_drop.rs

@@ -145,7 +145,8 @@ impl<T: ?Sized> ManuallyDrop<T> {
 }
 
 #[stable(feature = "manually_drop", since = "1.20.0")]
-impl<T: ?Sized> Deref for ManuallyDrop<T> {
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
+impl<T: ?Sized> const Deref for ManuallyDrop<T> {
     type Target = T;
     #[inline(always)]
     fn deref(&self) -> &T {
@@ -154,7 +155,8 @@ impl<T: ?Sized> Deref for ManuallyDrop<T> {
 }
 
 #[stable(feature = "manually_drop", since = "1.20.0")]
-impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
+impl<T: ?Sized> const DerefMut for ManuallyDrop<T> {
     #[inline(always)]
     fn deref_mut(&mut self) -> &mut T {
         &mut self.value

library/core/src/ops/deref.rs

@@ -76,7 +76,8 @@ pub trait Deref {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Deref for &T {
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
+impl<T: ?Sized> const Deref for &T {
     type Target = T;
 
     #[rustc_diagnostic_item = "noop_method_deref"]
@@ -89,7 +90,8 @@ impl<T: ?Sized> Deref for &T {
 impl<T: ?Sized> !DerefMut for &T {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Deref for &mut T {
+#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
+impl<T: ?Sized> const Deref for &mut T {
     type Target = T;
 
     fn deref(&self) -> &T {