Auto merge of #50097 - glandium:box_free, r=nikomatsakis

Partial future-proofing for Box<T, A>

In some ways, this is similar to @eddyb's PR #47043 that went stale, but doesn't cover everything. Notably, this still leaves Box internalized as a pointer in places, so practically speaking, only ZSTs can be practically added to the Box type with the changes here (the compiler ICEs otherwise).

The Box type is not changed here, that's left for the future because I want to test that further first, but this puts things in place in a way that hopefully will make things easier.

3 files changed, 17 insertions, 7 deletions

diff --git a/src/liballoc/alloc.rs b/src/liballoc/alloc.rs
index c0372d24ed5..f59c9f7fd61 100644
--- a/src/liballoc/alloc.rs
+++ b/src/liballoc/alloc.rs
@@ -16,7 +16,7 @@
             issue = "32838")]
 
 use core::intrinsics::{min_align_of_val, size_of_val};
-use core::ptr::NonNull;
+use core::ptr::{NonNull, Unique};
 use core::usize;
 
 #[doc(inline)]
@@ -152,9 +152,17 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
     }
 }
 
-#[cfg_attr(not(test), lang = "box_free")]
+#[cfg(stage0)]
+#[lang = "box_free"]
+#[inline]
+unsafe fn old_box_free<T: ?Sized>(ptr: *mut T) {
+    box_free(Unique::new_unchecked(ptr))
+}
+
+#[cfg_attr(not(any(test, stage0)), lang = "box_free")]
 #[inline]
-pub(crate) unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
+pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
+    let ptr = ptr.as_ptr();
     let size = size_of_val(&*ptr);
     let align = min_align_of_val(&*ptr);
     // We do not allocate for Box<T> when T is ZST, so deallocation is also not necessary.
diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index e52a0216dd3..d0950bff9ce 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -566,7 +566,8 @@ impl<T: ?Sized> Arc<T> {
 
     fn from_box(v: Box<T>) -> Arc<T> {
         unsafe {
-            let bptr = Box::into_raw(v);
+            let box_unique = Box::into_unique(v);
+            let bptr = box_unique.as_ptr();
 
             let value_size = size_of_val(&*bptr);
             let ptr = Self::allocate_for_ptr(bptr);
@@ -578,7 +579,7 @@ impl<T: ?Sized> Arc<T> {
                 value_size);
 
             // Free the allocation without dropping its contents
-            box_free(bptr);
+            box_free(box_unique);
 
             Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
         }
diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 8fb8e111754..d0188c6e828 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -681,7 +681,8 @@ impl<T: ?Sized> Rc<T> {
 
     fn from_box(v: Box<T>) -> Rc<T> {
         unsafe {
-            let bptr = Box::into_raw(v);
+            let box_unique = Box::into_unique(v);
+            let bptr = box_unique.as_ptr();
 
             let value_size = size_of_val(&*bptr);
             let ptr = Self::allocate_for_ptr(bptr);
@@ -693,7 +694,7 @@ impl<T: ?Sized> Rc<T> {
                 value_size);
 
             // Free the allocation without dropping its contents
-            box_free(bptr);
+            box_free(box_unique);
 
             Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
         }