Constify DropGuard::into_inner and trait impls

library/core/src/mem/drop_guard.rs

@@ -1,4 +1,5 @@
 use crate::fmt::{self, Debug};
+use crate::marker::Destruct;
 use crate::mem::ManuallyDrop;
 use crate::ops::{Deref, DerefMut};
 
@@ -84,8 +85,12 @@ where
     /// assert_eq!(DropGuard::into_inner(guard), "Nori likes chicken");
     /// ```
     #[unstable(feature = "drop_guard", issue = "144426")]
+    #[rustc_const_unstable(feature = "const_drop_guard", issue = "none")]
     #[inline]
-    pub fn into_inner(guard: Self) -> T {
+    pub const fn into_inner(guard: Self) -> T
+    where
+        F: [const] Destruct,
+    {
         // First we ensure that dropping the guard will not trigger
         // its destructor
         let mut guard = ManuallyDrop::new(guard);
@@ -106,7 +111,8 @@ where
 }
 
 #[unstable(feature = "drop_guard", issue = "144426")]
-impl<T, F> Deref for DropGuard<T, F>
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl<T, F> const Deref for DropGuard<T, F>
 where
     F: FnOnce(T),
 {
@@ -118,7 +124,8 @@ where
 }
 
 #[unstable(feature = "drop_guard", issue = "144426")]
-impl<T, F> DerefMut for DropGuard<T, F>
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl<T, F> const DerefMut for DropGuard<T, F>
 where
     F: FnOnce(T),
 {
@@ -128,9 +135,10 @@ where
 }
 
 #[unstable(feature = "drop_guard", issue = "144426")]
-impl<T, F> Drop for DropGuard<T, F>
+#[rustc_const_unstable(feature = "const_drop_guard", issue = "none")]
+impl<T, F> const Drop for DropGuard<T, F>
 where
-    F: FnOnce(T),
+    F: [const] FnOnce(T),
 {
     fn drop(&mut self) {
         // SAFETY: `DropGuard` is in the process of being dropped.

library/core/src/mem/manually_drop.rs

@@ -217,8 +217,9 @@ impl<T> ManuallyDrop<T> {
     ///
     #[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"]
     #[stable(feature = "manually_drop_take", since = "1.42.0")]
+    #[rustc_const_unstable(feature = "const_manually_drop_take", issue = "none")]
     #[inline]
-    pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
+    pub const unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
         // SAFETY: we are reading from a reference, which is guaranteed
         // to be valid for reads.
         unsafe { ptr::read(&slot.value) }