1 files changed, 31 insertions, 17 deletions

diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index 8ef2bac92..96b93830f 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -157,12 +157,12 @@ use core::hash::{Hash, Hasher};
 use core::iter::FusedIterator;
 use core::marker::Tuple;
 use core::marker::Unsize;
-use core::mem;
+use core::mem::{self, SizedTypeProperties};
 use core::ops::{
     CoerceUnsized, Deref, DerefMut, DispatchFromDyn, Generator, GeneratorState, Receiver,
 };
 use core::pin::Pin;
-use core::ptr::{self, Unique};
+use core::ptr::{self, NonNull, Unique};
 use core::task::{Context, Poll};
 
 #[cfg(not(no_global_oom_handling))]
@@ -479,8 +479,12 @@ impl<T, A: Allocator> Box<T, A> {
     where
         A: Allocator,
     {
-        let layout = Layout::new::<mem::MaybeUninit<T>>();
-        let ptr = alloc.allocate(layout)?.cast();
+        let ptr = if T::IS_ZST {
+            NonNull::dangling()
+        } else {
+            let layout = Layout::new::<mem::MaybeUninit<T>>();
+            alloc.allocate(layout)?.cast()
+        };
         unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
     }
 
@@ -549,8 +553,12 @@ impl<T, A: Allocator> Box<T, A> {
     where
         A: Allocator,
     {
-        let layout = Layout::new::<mem::MaybeUninit<T>>();
-        let ptr = alloc.allocate_zeroed(layout)?.cast();
+        let ptr = if T::IS_ZST {
+            NonNull::dangling()
+        } else {
+            let layout = Layout::new::<mem::MaybeUninit<T>>();
+            alloc.allocate_zeroed(layout)?.cast()
+        };
         unsafe { Ok(Box::from_raw_in(ptr.as_ptr(), alloc)) }
     }
 
@@ -675,14 +683,16 @@ impl<T> Box<[T]> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[inline]
     pub fn try_new_uninit_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
-        unsafe {
+        let ptr = if T::IS_ZST || len == 0 {
+            NonNull::dangling()
+        } else {
             let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
                 Ok(l) => l,
                 Err(_) => return Err(AllocError),
             };
-            let ptr = Global.allocate(layout)?;
-            Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
-        }
+            Global.allocate(layout)?.cast()
+        };
+        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
     }
 
     /// Constructs a new boxed slice with uninitialized contents, with the memory
@@ -707,14 +717,16 @@ impl<T> Box<[T]> {
     #[unstable(feature = "allocator_api", issue = "32838")]
     #[inline]
     pub fn try_new_zeroed_slice(len: usize) -> Result<Box<[mem::MaybeUninit<T>]>, AllocError> {
-        unsafe {
+        let ptr = if T::IS_ZST || len == 0 {
+            NonNull::dangling()
+        } else {
             let layout = match Layout::array::<mem::MaybeUninit<T>>(len) {
                 Ok(l) => l,
                 Err(_) => return Err(AllocError),
             };
-            let ptr = Global.allocate_zeroed(layout)?;
-            Ok(RawVec::from_raw_parts_in(ptr.as_mut_ptr() as *mut _, len, Global).into_box(len))
-        }
+            Global.allocate_zeroed(layout)?.cast()
+        };
+        unsafe { Ok(RawVec::from_raw_parts_in(ptr.as_ptr(), len, Global).into_box(len)) }
     }
 }
 
@@ -1219,7 +1231,9 @@ unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Box<T, A> {
 
         unsafe {
             let layout = Layout::for_value_raw(ptr.as_ptr());
-            self.1.deallocate(From::from(ptr.cast()), layout)
+            if layout.size() != 0 {
+                self.1.deallocate(From::from(ptr.cast()), layout);
+            }
         }
     }
 }
@@ -2169,7 +2183,7 @@ impl dyn Error + Send {
         let err: Box<dyn Error> = self;
         <dyn Error>::downcast(err).map_err(|s| unsafe {
             // Reapply the `Send` marker.
-            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send>>(s)
+            Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send))
         })
     }
 }
@@ -2183,7 +2197,7 @@ impl dyn Error + Send + Sync {
         let err: Box<dyn Error> = self;
         <dyn Error>::downcast(err).map_err(|s| unsafe {
             // Reapply the `Send + Sync` marker.
-            mem::transmute::<Box<dyn Error>, Box<dyn Error + Send + Sync>>(s)
+            Box::from_raw(Box::into_raw(s) as *mut (dyn Error + Send + Sync))
         })
     }
 }