fix(core): replace ~const Drop with ~const Destruct

This change was driven by [rust-lang/rust#94901][1]. It's actually more than just renaming - it fixes the irregular meaning of `~const Drop` (“the type can be destructed in a const context”, instead of the regular meaning of “`<T as Drop>::drop is `const fn`”), which the compiler was unable to reconcile in `Drop` bounds and thus necessitated the work- around explained in `[ref:drop_const_bounds]`. This is no longer an issue because `~const Destruct` has a regular meaning. [1]: rust-lang/rust#94901
r3-os · Mar 29, 2022 · ff7e987 · ff7e987
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diff --git a/doc/toolchain_limitations.md b/doc/toolchain_limitations.md
@@ -224,80 +224,6 @@ impl<C> Cfg<C> {
 ```
 
 
-### `[tag:drop_const_bounds]` It's non-trivial for a `Drop` implementation to depend on `TypeParam: ~const Drop`
-
-The following code doesn't compile (which is okay) because `T` might not be `T: const Drop`.
-
-```rust,compile_fail,E0493
-#![feature(const_trait_impl)]
-#![feature(const_mut_refs)]
-#![feature(const_option)]
-struct Type<T>(Option<T>);
-impl<T> const Drop for Type<T> {
-    fn drop(&mut self) {
-        // error[E0493]: destructors cannot be evaluated at compile-time
-        let _ = self.0.take().unwrap();
-    }
-}
-```
-
-The obvious solution is to add `T: ~const Drop` to the `Drop` implementation as well as to the type definition. However, this doesn't work because `~const` is not allowed to appear in the type definition.
-
-```rust,compile_fail,E0367
-#![feature(const_trait_impl)]
-#![feature(const_mut_refs)]
-#![feature(const_option)]
-// error: `~const` is not allowed here
-struct Type<T: ~const Drop>(Option<T>);
-// error[E0367]: `Drop` impl requires `T: ~const Drop` but the struct it is
-// implemented for does not
-impl<T: ~const Drop> const Drop for Type<T> {
-    fn drop(&mut self) {
-        let _ = self.0.take().unwrap();
-    }
-}
-```
-
-According to [rust-lang/rust#93028](https://github.com/rust-lang/rust/pull/93028), we can actually remove `~const` from this type definition, and the compiler permits the `Drop` implementation to have an extra `~const`. Unfortunately, this leaves a `Drop` trait bound on the type, which actually cover different types than `~const Drop` does. That's because `T: ~const Drop` means that `T` can be dropped in a constant context (n.b. this is [a special case for `Drop`](https://internals.rust-lang.org/t/pre-rfc-revamped-const-trait-impl-aka-rfc-2632/15192#const-drop-in-generic-code-6) and doesn't apply to other traits), while `T: Drop` means that `T` has a user-defined `Drop` implementation.
-
-```rust,compile_fail,E0277
-#![feature(const_trait_impl)]
-#![feature(const_mut_refs)]
-#![feature(const_option)]
-struct Type<T: Drop>(Option<T>);
-impl<T: ~const Drop> const Drop for Type<T> {
-    fn drop(&mut self) {
-        let _ = self.0.take().unwrap();
-    }
-}
-// error[E0277]: the trait bound `(): Drop` is not satisfied
-let _ = Type(Some(()));
-```
-
-A work-around is to enclose `T` in a container that unconditionally implements `const Drop`.
-
-```rust
-#![feature(const_trait_impl)]
-#![feature(const_mut_refs)]
-#![feature(const_option)]
-struct UserDrop<T>(T);
-impl<T> const Drop for UserDrop<T> {
-    fn drop(&mut self) {}
-}
-struct Type<T>(Option<T>) where UserDrop<T>: Drop;
-impl<T> const Drop for Type<T>
-where
-    UserDrop<T>: ~const Drop,
-{
-    fn drop(&mut self) {
-        let _ = UserDrop(self.0.take().unwrap());
-    }
-}
-let _ = Type(Some(()));
-const _: () = { let _ = Type(Some(())); };
-```
-
-
 ### `[tag:const_closures]` Closures can't be `impl const Fn`
 
 ```rust

diff --git a/src/r3_core/src/bind/sorter.rs b/src/r3_core/src/bind/sorter.rs
@@ -49,7 +49,10 @@
 //!
 // FIXME: We might be able to improve the interface when `ComptimeVec` is not
 // restricted to "comptime" anymore
-use core::ops::{Deref, DerefMut, Index, IndexMut};
+use core::{
+    marker::Destruct,
+    ops::{Deref, DerefMut, Index, IndexMut},
+};
 
 use super::{BindBorrowType, BindUsage};
 use crate::utils::{
@@ -538,12 +541,12 @@ trait MyIterator {
 }
 
 trait GraphAccess<VertexRef> {
-    type VertexIter<'a>: ~const MyIterator<Item = VertexRef> + ~const Drop + 'a
+    type VertexIter<'a>: ~const MyIterator<Item = VertexRef> + ~const Destruct + 'a
     where
         Self: 'a;
     fn vertices(&self) -> Self::VertexIter<'_>;
 
-    type SuccessorIter<'a>: ~const MyIterator<Item = VertexRef> + ~const Drop + 'a
+    type SuccessorIter<'a>: ~const MyIterator<Item = VertexRef> + ~const Destruct + 'a
     where
         Self: 'a;
     fn successors(&self, v: &VertexRef) -> Self::SuccessorIter<'_>;
@@ -597,9 +600,9 @@ const fn topological_sort<
 where
     Graph: ~const GraphAccess<VertexRef>,
     // [ref:const_trait_not_implied] necessitates `: ~const MyIterator`
-    Graph::VertexIter<'a>: ~const MyIterator + ~const Drop,
+    Graph::VertexIter<'a>: ~const MyIterator + ~const Destruct,
     // [ref:const_trait_not_implied] necessitates `: ~const MyIterator`
-    Graph::SuccessorIter<'a>: ~const MyIterator + ~const Drop,
+    Graph::SuccessorIter<'a>: ~const MyIterator + ~const Destruct,
     VertexRef: Copy,
     VertexRefLessThan: ~const FnMut(&VertexRef, &VertexRef) -> bool,
     // `~const Deref[Mut]` isn't implied because of

diff --git a/src/r3_core/src/utils/binary_heap/mod.rs b/src/r3_core/src/utils/binary_heap/mod.rs
@@ -2,6 +2,8 @@
 //!
 //! The implementation is mostly based on the Rust standard library's
 //! `BinaryHeap`.
+use core::marker::Destruct;
+
 mod helpers;
 #[cfg(test)]
 mod tests;
@@ -31,36 +33,36 @@ pub trait BinaryHeap: VecLike {
     /// Remove the least item from the heap and return it.
     fn heap_pop<Ctx>(&mut self, ctx: Ctx) -> Option<Self::Element>
     where
-        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Drop;
+        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Destruct;
 
     /// Remove the item at the specified position and return it.
     fn heap_remove<Ctx>(&mut self, i: usize, ctx: Ctx) -> Option<Self::Element>
     where
-        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Drop;
+        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Destruct;
 
     /// Push an item onto the heap and return its position.
     fn heap_push<Ctx>(&mut self, item: Self::Element, ctx: Ctx) -> usize
     where
-        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Drop;
+        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Destruct;
 }
 
 impl<T> const BinaryHeap for T
 where
     // `~const Deref` isn't implied because of
     // [ref:veclike_const_supertrait]
     T: ~const VecLike + ~const core::ops::Deref + ~const core::ops::DerefMut,
-    T::Element: ~const Drop,
+    T::Element: ~const Destruct,
 {
     fn heap_pop<Ctx>(&mut self, ctx: Ctx) -> Option<Self::Element>
     where
-        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Drop,
+        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Destruct,
     {
         self.heap_remove(0, ctx)
     }
 
     fn heap_remove<Ctx>(&mut self, i: usize, mut ctx: Ctx) -> Option<Self::Element>
     where
-        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Drop,
+        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Destruct,
     {
         if i >= self.len() {
             return None;
@@ -94,7 +96,7 @@ where
 
     fn heap_push<Ctx>(&mut self, item: Self::Element, ctx: Ctx) -> usize
     where
-        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Drop,
+        Ctx: ~const BinaryHeapCtx<Self::Element> + ~const Destruct,
     {
         let i = self.len();
         self.push(item);
@@ -127,8 +129,8 @@ const unsafe fn sift_up<Element, Ctx>(
     mut ctx: Ctx,
 ) -> usize
 where
-    Ctx: ~const BinaryHeapCtx<Element> + ~const Drop,
-    Element: ~const Drop,
+    Ctx: ~const BinaryHeapCtx<Element> + ~const Destruct,
+    Element: ~const Destruct,
 {
     unsafe {
         // Take out the value at `pos` and create a hole.
@@ -163,8 +165,8 @@ where
 /// `pos` must point to an element within `this`.
 const unsafe fn sift_down<Element, Ctx>(this: &mut [Element], pos: usize, mut ctx: Ctx)
 where
-    Ctx: ~const BinaryHeapCtx<Element> + ~const Drop,
-    Element: ~const Drop,
+    Ctx: ~const BinaryHeapCtx<Element> + ~const Destruct,
+    Element: ~const Destruct,
 {
     let end = this.len();
     unsafe {

diff --git a/src/r3_core/src/utils/vec.rs b/src/r3_core/src/utils/vec.rs
@@ -1,30 +1,17 @@
-use core::{alloc::Layout, ops, ptr::NonNull};
+use core::{alloc::Layout, marker::Destruct, ops, ptr::NonNull};
 
 use super::{AllocError, Allocator, ConstAllocator};
 
-#[doc(hidden)]
-pub struct UserDrop<T>(T);
-
-impl<T> const Drop for UserDrop<T> {
-    #[inline]
-    fn drop(&mut self) {}
-}
-
 /// `Vec` that can only be used in a constant context.
 #[doc(hidden)]
-#[allow(drop_bounds)] // Due to the work-around for [ref:drop_const_bounds]
-pub struct ComptimeVec<T>
-where
-    // Work-around for [ref:drop_const_bounds]
-    UserDrop<T>: Drop,
-{
+pub struct ComptimeVec<T: Destruct> {
     ptr: NonNull<T>,
     len: usize,
     capacity: usize,
     allocator: ConstAllocator,
 }
 
-impl<T: ~const Clone + ~const Drop> const Clone for ComptimeVec<T> {
+impl<T: ~const Clone + ~const Destruct> const Clone for ComptimeVec<T> {
     fn clone(&self) -> Self {
         // FIXME: Work-around for a mysterious error saying "the trait bound
         // `for<'r> fn(&'r T) -> T {<T as Clone>::clone}: ~const FnMut<(&T,)>`
@@ -37,16 +24,10 @@ impl<T: ~const Clone + ~const Drop> const Clone for ComptimeVec<T> {
     }
 }
 
-impl<T> const Drop for ComptimeVec<T>
-where
-    // Work-around for [ref:drop_const_bounds]
-    UserDrop<T>: ~const Drop,
-{
+impl<T: ~const Destruct> const Drop for ComptimeVec<T> {
     fn drop(&mut self) {
         if core::mem::needs_drop::<T>() {
-            // Wrap `T` with `UserDrop` before dropping to work around
-            // [ref:drop_const_bounds]
-            while self.pop().map(UserDrop).is_some() {}
+            while self.pop().is_some() {}
         }
 
         // Safety: The referent is a valid heap allocation from `self.allocator`,
@@ -133,7 +114,10 @@ impl<T> ComptimeVec<T> {
 
     /// Return a `ComptimeVec` of the same `len` as `self` with function `f`
     /// applied to each element in order.
-    pub const fn map<F: ~const FnMut(&T) -> U + ~const Drop, U>(&self, mut f: F) -> ComptimeVec<U> {
+    pub const fn map<F: ~const FnMut(&T) -> U + ~const Destruct, U>(
+        &self,
+        mut f: F,
+    ) -> ComptimeVec<U> {
         let mut out = ComptimeVec::with_capacity_in(self.len, self.allocator.clone());
         let mut i = 0;
         while i < self.len() {
@@ -146,7 +130,7 @@ impl<T> ComptimeVec<T> {
     /// Remove all elements.
     pub const fn clear(&mut self)
     where
-        T: ~const Drop,
+        T: ~const Destruct,
     {
         if core::mem::needs_drop::<T>() {
             while self.pop().is_some() {}
@@ -217,7 +201,7 @@ macro_rules! vec_position {
 }
 
 /// Unwrap `Result<T, AllocError>`.
-const fn unwrap_alloc_error<T: ~const Drop>(x: Result<T, AllocError>) -> T {
+const fn unwrap_alloc_error<T: ~const Destruct>(x: Result<T, AllocError>) -> T {
     match x {
         Ok(x) => x,
         Err(AllocError) => panic!("compile-time heap allocation failed"),