Unstably constify ptr::drop_in_place and related methods

Author: clarfonthey

library/core/src/mem/maybe_uninit.rs

@@ -1,4 +1,5 @@
 use crate::any::type_name;
+use crate::marker::Destruct;
 use crate::mem::ManuallyDrop;
 use crate::{fmt, intrinsics, ptr, slice};
 
@@ -714,7 +715,11 @@ impl<T> MaybeUninit<T> {
     ///
     /// [`assume_init`]: MaybeUninit::assume_init
     #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
-    pub unsafe fn assume_init_drop(&mut self) {
+    #[rustc_const_unstable(feature = "const_drop_in_place", issue = "109342")]
+    pub const unsafe fn assume_init_drop(&mut self)
+    where
+        T: [const] Destruct,
+    {
         // SAFETY: the caller must guarantee that `self` is initialized and
         // satisfies all invariants of `T`.
         // Dropping the value in place is safe if that is the case.
@@ -1390,7 +1395,11 @@ impl<T> [MaybeUninit<T>] {
     /// behaviour.
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
     #[inline(always)]
-    pub unsafe fn assume_init_drop(&mut self) {
+    #[rustc_const_unstable(feature = "const_drop_in_place", issue = "109342")]
+    pub const unsafe fn assume_init_drop(&mut self)
+    where
+        T: [const] Destruct,
+    {
         if !self.is_empty() {
             // SAFETY: the caller must guarantee that every element of `self`
             // is initialized and satisfies all invariants of `T`.

library/core/src/ptr/mod.rs

@@ -403,7 +403,7 @@
 
 use crate::cmp::Ordering;
 use crate::intrinsics::const_eval_select;
-use crate::marker::{FnPtr, PointeeSized};
+use crate::marker::{Destruct, FnPtr, PointeeSized};
 use crate::mem::{self, MaybeUninit, SizedTypeProperties};
 use crate::num::NonZero;
 use crate::{fmt, hash, intrinsics, ub_checks};
@@ -801,7 +801,11 @@ pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
 #[lang = "drop_in_place"]
 #[allow(unconditional_recursion)]
 #[rustc_diagnostic_item = "ptr_drop_in_place"]
-pub unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T) {
+#[rustc_const_unstable(feature = "const_drop_in_place", issue = "109342")]
+pub const unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T)
+where
+    T: [const] Destruct,
+{
     // Code here does not matter - this is replaced by the
     // real drop glue by the compiler.
 

library/core/src/ptr/mut_ptr.rs

@@ -1,7 +1,7 @@
 use super::*;
 use crate::cmp::Ordering::{Equal, Greater, Less};
 use crate::intrinsics::const_eval_select;
-use crate::marker::PointeeSized;
+use crate::marker::{Destruct, PointeeSized};
 use crate::mem::{self, SizedTypeProperties};
 use crate::slice::{self, SliceIndex};
 
@@ -1437,8 +1437,12 @@ impl<T: PointeeSized> *mut T {
     ///
     /// [`ptr::drop_in_place`]: crate::ptr::drop_in_place()
     #[stable(feature = "pointer_methods", since = "1.26.0")]
+    #[rustc_const_unstable(feature = "const_drop_in_place", issue = "109342")]
     #[inline(always)]
-    pub unsafe fn drop_in_place(self) {
+    pub const unsafe fn drop_in_place(self)
+    where
+        T: [const] Destruct,
+    {
         // SAFETY: the caller must uphold the safety contract for `drop_in_place`.
         unsafe { drop_in_place(self) }
     }

library/core/src/ptr/non_null.rs

@@ -1,5 +1,5 @@
 use crate::cmp::Ordering;
-use crate::marker::{PointeeSized, Unsize};
+use crate::marker::{Destruct, PointeeSized, Unsize};
 use crate::mem::{MaybeUninit, SizedTypeProperties};
 use crate::num::NonZero;
 use crate::ops::{CoerceUnsized, DispatchFromDyn};
@@ -1118,7 +1118,11 @@ impl<T: PointeeSized> NonNull<T> {
     /// [`ptr::drop_in_place`]: crate::ptr::drop_in_place()
     #[inline(always)]
     #[stable(feature = "non_null_convenience", since = "1.80.0")]
-    pub unsafe fn drop_in_place(self) {
+    #[rustc_const_unstable(feature = "const_drop_in_place", issue = "109342")]
+    pub const unsafe fn drop_in_place(self)
+    where
+        T: [const] Destruct,
+    {
         // SAFETY: the caller must uphold the safety contract for `drop_in_place`.
         unsafe { ptr::drop_in_place(self.as_ptr()) }
     }