const drop

Author: bend-n

library/core/src/array/mod.rs

@@ -996,16 +996,10 @@ impl<T> const Drop for Guard<'_, T> {
     #[inline]
     fn drop(&mut self) {
         debug_assert!(self.initialized <= self.array_mut.len());
-        crate::intrinsics::const_eval_select!(
-            @capture [T] { x: &mut Guard<'_, T> = self } -> ():
-            if const {}
-            else {
-                // SAFETY: this slice will contain only initialized objects.
-                unsafe {
-                    x.array_mut.get_unchecked_mut(..x.initialized).assume_init_drop();
-                }
-            }
-        );
+        // SAFETY: this slice will contain only initialized objects.
+        unsafe {
+            self.array_mut.get_unchecked_mut(..self.initialized).assume_init_drop();
+        }
     }
 }
 

library/core/src/mem/maybe_uninit.rs

@@ -1389,8 +1389,12 @@ impl<T> [MaybeUninit<T>] {
     /// non-null. Dropping such a `Vec<T>` however will cause undefined
     /// behaviour.
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
     #[inline(always)]
-    pub unsafe fn assume_init_drop(&mut self) {
+    pub const unsafe fn assume_init_drop(&mut self)
+    where
+        T: [const] crate::marker::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 +801,8 @@ 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 = "none")]
+pub const unsafe fn drop_in_place<T: PointeeSized + [const] Destruct>(to_drop: *mut T) {
     // Code here does not matter - this is replaced by the
     // real drop glue by the compiler.
 

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

@@ -1,8 +1,8 @@
 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 + [const] Destruct>(to_drop: *mut T) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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 +1,8 @@
 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 + [const] Destruct>(to_drop: *mut T) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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