Auto merge of rust-lang#100036 - DrMeepster:box_free_free_box, r=oli-obk

Remove `box_free` lang item

This PR removes the `box_free` lang item, replacing it with `Box`'s `Drop` impl. Box dropping is still slightly magic because the contained value is still dropped by the compiler.

Author: bors

alloc/src/alloc.rs

@@ -4,8 +4,10 @@
 
 #[cfg(not(test))]
 use core::intrinsics;
+#[cfg(all(bootstrap, not(test)))]
 use core::intrinsics::{min_align_of_val, size_of_val};
 
+#[cfg(all(bootstrap, not(test)))]
 use core::ptr::Unique;
 #[cfg(not(test))]
 use core::ptr::{self, NonNull};
@@ -335,14 +337,15 @@ 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]
 // 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) unsafe fn box_free<T: ?Sized, A: Allocator>(ptr: Unique<T>, alloc: A) {
+unsafe fn box_free<T: ?Sized, A: Allocator>(ptr: Unique<T>, alloc: A) {
     unsafe {
         let size = size_of_val(ptr.as_ref());
         let align = min_align_of_val(ptr.as_ref());

alloc/src/boxed.rs

@@ -1211,8 +1211,16 @@ impl<T: ?Sized, A: Allocator> Box<T, A> {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
+    #[inline]
     fn drop(&mut self) {
-        // FIXME: Do nothing, drop is currently performed by compiler.
+        // the T in the Box is dropped by the compiler before the destructor is run
+
+        let ptr = self.0;
+
+        unsafe {
+            let layout = Layout::for_value_raw(ptr.as_ptr());
+            self.1.deallocate(From::from(ptr.cast()), layout)
+        }
     }
 }
 

alloc/src/rc.rs

@@ -270,7 +270,7 @@ 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::WriteCloneIntoRaw;
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::borrow::{Cow, ToOwned};
 #[cfg(not(no_global_oom_handling))]
@@ -1442,23 +1442,21 @@ impl<T: ?Sized> Rc<T> {
     }
 
     #[cfg(not(no_global_oom_handling))]
-    fn from_box(v: Box<T>) -> Rc<T> {
+    fn from_box(src: Box<T>) -> Rc<T> {
         unsafe {
-            let (box_unique, alloc) = Box::into_unique(v);
-            let bptr = box_unique.as_ptr();
-
-            let value_size = size_of_val(&*bptr);
-            let ptr = Self::allocate_for_ptr(bptr);
+            let value_size = size_of_val(&*src);
+            let ptr = Self::allocate_for_ptr(&*src);
 
             // Copy value as bytes
             ptr::copy_nonoverlapping(
-                bptr as *const T as *const u8,
+                &*src as *const T as *const u8,
                 &mut (*ptr).value as *mut _ as *mut u8,
                 value_size,
             );
 
             // Free the allocation without dropping its contents
-            box_free(box_unique, alloc);
+            let src = Box::from_raw(Box::into_raw(src) as *mut mem::ManuallyDrop<T>);
+            drop(src);
 
             Self::from_ptr(ptr)
         }

alloc/src/sync.rs

@@ -33,7 +33,7 @@ use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
 #[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::WriteCloneIntoRaw;
 use crate::alloc::{AllocError, Allocator, Global, Layout};
 use crate::borrow::{Cow, ToOwned};
 use crate::boxed::Box;
@@ -1360,23 +1360,21 @@ impl<T: ?Sized> Arc<T> {
     }
 
     #[cfg(not(no_global_oom_handling))]
-    fn from_box(v: Box<T>) -> Arc<T> {
+    fn from_box(src: Box<T>) -> Arc<T> {
         unsafe {
-            let (box_unique, alloc) = Box::into_unique(v);
-            let bptr = box_unique.as_ptr();
-
-            let value_size = size_of_val(&*bptr);
-            let ptr = Self::allocate_for_ptr(bptr);
+            let value_size = size_of_val(&*src);
+            let ptr = Self::allocate_for_ptr(&*src);
 
             // Copy value as bytes
             ptr::copy_nonoverlapping(
-                bptr as *const T as *const u8,
+                &*src as *const T as *const u8,
                 &mut (*ptr).data as *mut _ as *mut u8,
                 value_size,
             );
 
             // Free the allocation without dropping its contents
-            box_free(box_unique, alloc);
+            let src = Box::from_raw(Box::into_raw(src) as *mut mem::ManuallyDrop<T>);
+            drop(src);
 
             Self::from_ptr(ptr)
         }