Auto merge of rust-lang#94901 - fee1-dead:destructable, r=oli-obk

Rename `~const Drop` to `~const Destruct`

r? `@oli-obk`

Completely switching to `~const Destructible` would be rather complicated, so it seems best to add it for now and wait for it to be backported to beta in the next release.

The rationale is to prevent complications such as rust-lang#92149 and rust-lang#94803 by introducing an entirely new trait. And `~const Destructible` reads a bit better than `~const Drop`. Name Bikesheddable.

Author: bors

alloc/src/alloc.rs

@@ -14,6 +14,8 @@ use core::ptr::{self, NonNull};
 #[doc(inline)]
 pub use core::alloc::*;
 
+use core::marker::Destruct;
+
 #[cfg(test)]
 mod tests;
 
@@ -324,12 +326,16 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
 #[cfg_attr(not(test), lang = "box_free")]
 #[inline]
 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+#[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
 // This signature has to be the same as `Box`, otherwise an ICE will happen.
 // When an additional parameter to `Box` is added (like `A: Allocator`), this has to be added here as
 // well.
 // For example if `Box` is changed to  `struct Box<T: ?Sized, A: Allocator>(Unique<T>, A)`,
 // this function has to be changed to `fn box_free<T: ?Sized, A: Allocator>(Unique<T>, A)` as well.
-pub(crate) const unsafe fn box_free<T: ?Sized, A: ~const Allocator + ~const Drop>(
+pub(crate) const unsafe fn box_free<
+    T: ?Sized,
+    A: ~const Allocator + ~const Drop + ~const Destruct,
+>(
     ptr: Unique<T>,
     alloc: A,
 ) {

alloc/src/borrow.rs

@@ -331,6 +331,7 @@ impl<B: ?Sized + ToOwned> Cow<'_, B> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_deref", issue = "88955")]
+#[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
 impl<B: ?Sized + ToOwned> const Deref for Cow<'_, B>
 where
     B::Owned: ~const Borrow<B>,

alloc/src/boxed.rs

@@ -143,7 +143,7 @@ use core::hash::{Hash, Hasher};
 #[cfg(not(no_global_oom_handling))]
 use core::iter::FromIterator;
 use core::iter::{FusedIterator, Iterator};
-use core::marker::{Unpin, Unsize};
+use core::marker::{Destruct, Unpin, Unsize};
 use core::mem;
 use core::ops::{
     CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
@@ -349,9 +349,10 @@ impl<T, A: Allocator> Box<T, A> {
     #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[must_use]
     #[inline]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn new_in(x: T, alloc: A) -> Self
     where
-        A: ~const Allocator + ~const Drop,
+        A: ~const Allocator + ~const Drop + ~const Destruct,
     {
         let mut boxed = Self::new_uninit_in(alloc);
         unsafe {
@@ -378,10 +379,11 @@ impl<T, A: Allocator> Box<T, A> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn try_new_in(x: T, alloc: A) -> Result<Self, AllocError>
     where
-        T: ~const Drop,
-        A: ~const Allocator + ~const Drop,
+        T: ~const Drop + ~const Destruct,
+        A: ~const Allocator + ~const Drop + ~const Destruct,
     {
         let mut boxed = Self::try_new_uninit_in(alloc)?;
         unsafe {
@@ -415,9 +417,10 @@ impl<T, A: Allocator> Box<T, A> {
     #[cfg(not(no_global_oom_handling))]
     #[must_use]
     // #[unstable(feature = "new_uninit", issue = "63291")]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn new_uninit_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
     where
-        A: ~const Allocator + ~const Drop,
+        A: ~const Allocator + ~const Drop + ~const Destruct,
     {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
@@ -453,9 +456,10 @@ impl<T, A: Allocator> Box<T, A> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn try_new_uninit_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
     where
-        A: ~const Allocator + ~const Drop,
+        A: ~const Allocator + ~const Drop + ~const Destruct,
     {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         let ptr = alloc.allocate(layout)?.cast();
@@ -487,9 +491,10 @@ impl<T, A: Allocator> Box<T, A> {
     #[cfg(not(no_global_oom_handling))]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     #[must_use]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn new_zeroed_in(alloc: A) -> Box<mem::MaybeUninit<T>, A>
     where
-        A: ~const Allocator + ~const Drop,
+        A: ~const Allocator + ~const Drop + ~const Destruct,
     {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         // NOTE: Prefer match over unwrap_or_else since closure sometimes not inlineable.
@@ -525,9 +530,10 @@ impl<T, A: Allocator> Box<T, A> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     // #[unstable(feature = "new_uninit", issue = "63291")]
     #[rustc_const_unstable(feature = "const_box", issue = "92521")]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn try_new_zeroed_in(alloc: A) -> Result<Box<mem::MaybeUninit<T>, A>, AllocError>
     where
-        A: ~const Allocator + ~const Drop,
+        A: ~const Allocator + ~const Drop + ~const Destruct,
     {
         let layout = Layout::new::<mem::MaybeUninit<T>>();
         let ptr = alloc.allocate_zeroed(layout)?.cast();
@@ -541,9 +547,10 @@ impl<T, A: Allocator> Box<T, A> {
     #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[must_use]
     #[inline(always)]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn pin_in(x: T, alloc: A) -> Pin<Self>
     where
-        A: 'static + ~const Allocator + ~const Drop,
+        A: 'static + ~const Allocator + ~const Drop + ~const Destruct,
     {
         Self::into_pin(Self::new_in(x, alloc))
     }
@@ -572,9 +579,10 @@ impl<T, A: Allocator> Box<T, A> {
     #[unstable(feature = "box_into_inner", issue = "80437")]
     #[rustc_const_unstable(feature = "const_box", issue = "92521")]
     #[inline]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn into_inner(boxed: Self) -> T
     where
-        Self: ~const Drop,
+        Self: ~const Drop + ~const Destruct,
     {
         *boxed
     }

core/src/bool.rs

@@ -1,5 +1,7 @@
 //! impl bool {}
 
+use crate::marker::Destruct;
+
 #[lang = "bool"]
 impl bool {
     /// Returns `Some(t)` if the `bool` is [`true`](../std/keyword.true.html),
@@ -16,9 +18,10 @@ impl bool {
     #[unstable(feature = "bool_to_option", issue = "80967")]
     #[rustc_const_unstable(feature = "const_bool_to_option", issue = "91917")]
     #[inline]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn then_some<T>(self, t: T) -> Option<T>
     where
-        T: ~const Drop,
+        T: ~const Drop + ~const Destruct,
     {
         if self { Some(t) } else { None }
     }
@@ -35,10 +38,11 @@ impl bool {
     #[stable(feature = "lazy_bool_to_option", since = "1.50.0")]
     #[rustc_const_unstable(feature = "const_bool_to_option", issue = "91917")]
     #[inline]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     pub const fn then<T, F>(self, f: F) -> Option<T>
     where
         F: ~const FnOnce() -> T,
-        F: ~const Drop,
+        F: ~const Drop + ~const Destruct,
     {
         if self { Some(f()) } else { None }
     }

core/src/clone.rs

@@ -36,6 +36,8 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+use crate::marker::Destruct;
+
 /// A common trait for the ability to explicitly duplicate an object.
 ///
 /// Differs from [`Copy`] in that [`Copy`] is implicit and an inexpensive bit-wise copy, while
@@ -128,9 +130,10 @@ pub trait Clone: Sized {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[default_method_body_is_const]
+    #[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
     fn clone_from(&mut self, source: &Self)
     where
-        Self: ~const Drop,
+        Self: ~const Drop + ~const Destruct,
     {
         *self = source.clone()
     }

core/src/intrinsics.rs

@@ -54,7 +54,7 @@
 )]
 #![allow(missing_docs)]
 
-use crate::marker::DiscriminantKind;
+use crate::marker::{Destruct, DiscriminantKind};
 use crate::mem;
 
 // These imports are used for simplifying intra-doc links
@@ -2353,14 +2353,15 @@ pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
 #[rustc_const_unstable(feature = "const_eval_select", issue = "none")]
 #[lang = "const_eval_select"]
 #[rustc_do_not_const_check]
+#[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
 pub const unsafe fn const_eval_select<ARG, F, G, RET>(
     arg: ARG,
     _called_in_const: F,
     called_at_rt: G,
 ) -> RET
 where
     F: ~const FnOnce<ARG, Output = RET>,
-    G: FnOnce<ARG, Output = RET> + ~const Drop,
+    G: FnOnce<ARG, Output = RET> + ~const Drop + ~const Destruct,
 {
     called_at_rt.call_once(arg)
 }
@@ -2372,14 +2373,15 @@ where
 )]
 #[rustc_const_unstable(feature = "const_eval_select", issue = "none")]
 #[lang = "const_eval_select_ct"]
+#[cfg_attr(not(bootstrap), allow(drop_bounds))] // FIXME remove `~const Drop` and this attr when bumping
 pub const unsafe fn const_eval_select_ct<ARG, F, G, RET>(
     arg: ARG,
     called_in_const: F,
     _called_at_rt: G,
 ) -> RET
 where
     F: ~const FnOnce<ARG, Output = RET>,
-    G: FnOnce<ARG, Output = RET> + ~const Drop,
+    G: FnOnce<ARG, Output = RET> + ~const Drop + ~const Destruct,
 {
     called_in_const.call_once(arg)
 }

core/src/marker.rs

@@ -792,6 +792,22 @@ impl<T: ?Sized> Unpin for *const T {}
 #[stable(feature = "pin_raw", since = "1.38.0")]
 impl<T: ?Sized> Unpin for *mut T {}
 
+/// A marker for types that can be dropped.
+///
+/// This should be used for `~const` bounds,
+/// as non-const bounds will always hold for every type.
+#[unstable(feature = "const_trait_impl", issue = "67792")]
+#[cfg_attr(not(bootstrap), lang = "destruct")]
+#[cfg_attr(
+    not(bootstrap),
+    rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg,)
+)]
+pub trait Destruct {}
+
+#[cfg(bootstrap)]
+#[unstable(feature = "const_trait_impl", issue = "67792")]
+impl<T: ?Sized> const Destruct for T {}
+
 /// Implementations of `Copy` for primitive types.
 ///
 /// Implementations that cannot be described in Rust