Make Box drop through Drop trait

compiler/rustc_hir/src/lang_items.rs

@@ -287,7 +287,6 @@ language_item_table! {
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn,             GenericRequirement::None;
 
     ExchangeMalloc,          sym::exchange_malloc,     exchange_malloc_fn,         Target::Fn,             GenericRequirement::None;
-    BoxFree,                 sym::box_free,            box_free_fn,                Target::Fn,             GenericRequirement::Minimum(1);
     DropInPlace,             sym::drop_in_place,       drop_in_place_fn,           Target::Fn,             GenericRequirement::Minimum(1);
     Oom,                     sym::oom,                 oom,                        Target::Fn,             GenericRequirement::None;
     AllocLayout,             sym::alloc_layout,        alloc_layout,               Target::Struct,         GenericRequirement::None;

compiler/rustc_mir_dataflow/src/elaborate_drops.rs

@@ -408,19 +408,6 @@ where
         self.drop_ladder(fields, succ, unwind).0
     }
 
-    fn open_drop_for_box(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
-        debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
-
-        let interior = self.tcx().mk_place_deref(self.place);
-        let interior_path = self.elaborator.deref_subpath(self.path);
-
-        let succ = self.box_free_block(adt, substs, self.succ, self.unwind);
-        let unwind_succ =
-            self.unwind.map(|unwind| self.box_free_block(adt, substs, unwind, Unwind::InCleanup));
-
-        self.drop_subpath(interior, interior_path, succ, unwind_succ)
-    }
-
     fn open_drop_for_adt(&mut self, adt: &'tcx ty::AdtDef, substs: SubstsRef<'tcx>) -> BasicBlock {
         debug!("open_drop_for_adt({:?}, {:?}, {:?})", self, adt, substs);
         if adt.variants.is_empty() {
@@ -872,11 +859,7 @@ where
                 self.open_drop_for_tuple(&tys)
             }
             ty::Adt(def, substs) => {
-                if def.is_box() {
-                    self.open_drop_for_box(def, substs)
-                } else {
-                    self.open_drop_for_adt(def, substs)
-                }
+                self.open_drop_for_adt(def, substs)
             }
             ty::Dynamic(..) => self.complete_drop(self.succ, self.unwind),
             ty::Array(ety, size) => {
@@ -925,59 +908,6 @@ where
         blk
     }
 
-    /// Creates a block that frees the backing memory of a `Box` if its drop is required (either
-    /// statically or by checking its drop flag).
-    ///
-    /// The contained value will not be dropped.
-    fn box_free_block(
-        &mut self,
-        adt: &'tcx ty::AdtDef,
-        substs: SubstsRef<'tcx>,
-        target: BasicBlock,
-        unwind: Unwind,
-    ) -> BasicBlock {
-        let block = self.unelaborated_free_block(adt, substs, target, unwind);
-        self.drop_flag_test_block(block, target, unwind)
-    }
-
-    /// Creates a block that frees the backing memory of a `Box` (without dropping the contained
-    /// value).
-    fn unelaborated_free_block(
-        &mut self,
-        adt: &'tcx ty::AdtDef,
-        substs: SubstsRef<'tcx>,
-        target: BasicBlock,
-        unwind: Unwind,
-    ) -> BasicBlock {
-        let tcx = self.tcx();
-        let unit_temp = Place::from(self.new_temp(tcx.mk_unit()));
-        let free_func = tcx.require_lang_item(LangItem::BoxFree, Some(self.source_info.span));
-        let args = adt.variants[VariantIdx::new(0)]
-            .fields
-            .iter()
-            .enumerate()
-            .map(|(i, f)| {
-                let field = Field::new(i);
-                let field_ty = f.ty(tcx, substs);
-                Operand::Move(tcx.mk_place_field(self.place, field, field_ty))
-            })
-            .collect();
-
-        let call = TerminatorKind::Call {
-            func: Operand::function_handle(tcx, free_func, substs, self.source_info.span),
-            args,
-            destination: Some((unit_temp, target)),
-            cleanup: None,
-            from_hir_call: false,
-            fn_span: self.source_info.span,
-        }; // FIXME(#43234)
-        let free_block = self.new_block(unwind, call);
-
-        let block_start = Location { block: free_block, statement_index: 0 };
-        self.elaborator.clear_drop_flag(block_start, self.path, DropFlagMode::Shallow);
-        free_block
-    }
-
     fn drop_block(&mut self, target: BasicBlock, unwind: Unwind) -> BasicBlock {
         let block =
             TerminatorKind::Drop { place: self.place, target, unwind: unwind.into_option() };

library/alloc/src/alloc.rs

@@ -321,17 +321,24 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
     }
 }
 
-#[cfg_attr(not(test), lang = "box_free")]
+#[cfg(all(bootstrap, not(test)))]
+#[lang = "box_free"]
 #[inline]
 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
-// 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>(
+const unsafe fn box_free<T: ?Sized, A: ~const Allocator + ~const Drop>(
     ptr: Unique<T>,
     alloc: A,
+) {
+    unsafe {
+        deallocate_box(ptr, &alloc);
+    }
+}
+
+#[inline]
+#[rustc_const_unstable(feature = "const_box", issue = "92521")]
+pub(crate) const unsafe fn deallocate_box<T: ?Sized, A: ~const Allocator>(
+    ptr: Unique<T>,
+    alloc: &A,
 ) {
     unsafe {
         let size = size_of_val(ptr.as_ref());

library/alloc/src/boxed.rs

@@ -154,7 +154,7 @@ use core::task::{Context, Poll};
 
 #[cfg(not(no_global_oom_handling))]
 use crate::alloc::{handle_alloc_error, WriteCloneIntoRaw};
-use crate::alloc::{AllocError, Allocator, Global, Layout};
+use crate::alloc::{deallocate_box, AllocError, Allocator, Global, Layout};
 #[cfg(not(no_global_oom_handling))]
 use crate::borrow::Cow;
 use crate::raw_vec::RawVec;
@@ -1171,9 +1171,15 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_box", issue = "92521")]
-unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> const Drop for Box<T, A> {
+unsafe impl<
+    #[may_dangle] T: ?Sized,
+    A: ~const Allocator,
+> const Drop for Box<T, A> {
+    #[inline]
     fn drop(&mut self) {
-        // FIXME: Do nothing, drop is currently performed by compiler.
+        unsafe {
+            deallocate_box(self.0, &self.1)
+        }
     }
 }
 

library/alloc/src/rc.rs

@@ -272,7 +272,7 @@ use core::slice::from_raw_parts_mut;
 #[cfg(not(no_global_oom_handling))]
 use crate::alloc::handle_alloc_error;
 #[cfg(not(no_global_oom_handling))]
-use crate::alloc::{box_free, WriteCloneIntoRaw};
+use crate::alloc::{deallocate_box, WriteCloneIntoRaw};
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::borrow::{Cow, ToOwned};
 #[cfg(not(no_global_oom_handling))]
@@ -1315,7 +1315,7 @@ impl<T: ?Sized> Rc<T> {
             );
 
             // Free the allocation without dropping its contents
-            box_free(box_unique, alloc);
+            deallocate_box(box_unique, &alloc);
 
             Self::from_ptr(ptr)
         }

library/alloc/src/sync.rs

@@ -30,7 +30,7 @@ use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
 #[cfg(not(no_global_oom_handling))]
 use crate::alloc::handle_alloc_error;
 #[cfg(not(no_global_oom_handling))]
-use crate::alloc::{box_free, WriteCloneIntoRaw};
+use crate::alloc::{deallocate_box, WriteCloneIntoRaw};
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
@@ -1191,7 +1191,7 @@ impl<T: ?Sized> Arc<T> {
             );
 
             // Free the allocation without dropping its contents
-            box_free(box_unique, alloc);
+            deallocate_box(box_unique, &alloc);
 
             Self::from_ptr(ptr)
         }