Unstably constify ptr::drop_in_place and related methods

library/core/src/mem/manually_drop.rs

@@ -1,3 +1,4 @@
+use crate::marker::Destruct;
 use crate::ops::{Deref, DerefMut, DerefPure};
 use crate::ptr;
 
@@ -249,7 +250,11 @@ impl<T: ?Sized> ManuallyDrop<T> {
     /// [pinned]: crate::pin
     #[stable(feature = "manually_drop", since = "1.20.0")]
     #[inline]
-    pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
+    #[rustc_const_unstable(feature = "const_drop_in_place", issue = "109342")]
+    pub const unsafe fn drop(slot: &mut ManuallyDrop<T>)
+    where
+        T: [const] Destruct,
+    {
         // SAFETY: we are dropping the value pointed to by a mutable reference
         // which is guaranteed to be valid for writes.
         // It is up to the caller to make sure that `slot` isn't dropped again.

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};
 
@@ -1438,8 +1438,12 @@ 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()) }
     }

library/coretests/tests/lib.rs

@@ -17,6 +17,7 @@
 #![feature(clone_to_uninit)]
 #![feature(const_convert)]
 #![feature(const_destruct)]
+#![feature(const_drop_in_place)]
 #![feature(const_eval_select)]
 #![feature(const_ops)]
 #![feature(const_option_ops)]

library/coretests/tests/ptr.rs

@@ -1,6 +1,6 @@
 use core::cell::RefCell;
 use core::marker::Freeze;
-use core::mem::MaybeUninit;
+use core::mem::{ManuallyDrop, MaybeUninit};
 use core::num::NonZero;
 use core::ptr;
 use core::ptr::*;
@@ -1036,3 +1036,42 @@ fn test_ptr_default() {
     let default = PtrMutDefaultTest::default();
     assert!(default.ptr.is_null());
 }
+
+#[test]
+fn test_const_drop_in_place() {
+    const COUNTER: usize = {
+        let mut counter = 0;
+        let counter_ptr = &raw mut counter;
+
+        // only exists to make `Drop` indirect impl
+        #[allow(dead_code)]
+        struct Test(Dropped);
+
+        struct Dropped(*mut usize);
+        impl const Drop for Dropped {
+            fn drop(&mut self) {
+                unsafe {
+                    *self.0 += 1;
+                }
+            }
+        }
+
+        let mut one = ManuallyDrop::new(Test(Dropped(counter_ptr)));
+        let mut two = ManuallyDrop::new(Test(Dropped(counter_ptr)));
+        let mut three = ManuallyDrop::new(Test(Dropped(counter_ptr)));
+        assert!(counter == 0);
+        unsafe {
+            ManuallyDrop::drop(&mut one);
+        }
+        assert!(counter == 1);
+        unsafe {
+            ManuallyDrop::drop(&mut two);
+        }
+        assert!(counter == 2);
+        unsafe {
+            ManuallyDrop::drop(&mut three);
+        }
+        counter
+    };
+    assert_eq!(COUNTER, 3);
+}

src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.stderr

@@ -1,8 +1,10 @@
 error: Undefined Behavior: trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag only grants SharedReadOnly permission for this location
   --> RUSTLIB/core/src/ptr/mod.rs:LL:CC
    |
-LL | pub unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T) {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this error occurs as part of retag at ALLOC[0x0..0x1]
+LL | / pub const unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T)
+LL | | where
+LL | |     T: [const] Destruct,
+   | |________________________^ this error occurs as part of retag at ALLOC[0x0..0x1]
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

src/tools/miri/tests/fail/unaligned_pointers/drop_in_place.stderr

@@ -1,8 +1,10 @@
 error: Undefined Behavior: constructing invalid value: encountered an unaligned reference (required ALIGN byte alignment but found ALIGN)
   --> RUSTLIB/core/src/ptr/mod.rs:LL:CC
    |
-LL | pub unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T) {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Undefined Behavior occurred here
+LL | / pub const unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T)
+LL | | where
+LL | |     T: [const] Destruct,
+   | |________________________^ Undefined Behavior occurred here
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information