fix some uses of pointer intrinsics with invalid pointers

Author: RalfJung

src/liballoc/vec.rs

@@ -2410,9 +2410,8 @@ impl<T> Iterator for IntoIter<T> {
                     // same pointer.
                     self.ptr = arith_offset(self.ptr as *const i8, 1) as *mut T;
 
-                    // Use a non-null pointer value
-                    // (self.ptr might be null because of wrapping)
-                    Some(ptr::read(1 as *mut T))
+                    // Read from a properly aligned pointer to make up a value of this ZST.
+                    Some(ptr::read(NonNull::dangling().as_ptr()))
                 } else {
                     let old = self.ptr;
                     self.ptr = self.ptr.offset(1);
@@ -2451,9 +2450,8 @@ impl<T> DoubleEndedIterator for IntoIter<T> {
                     // See above for why 'ptr.offset' isn't used
                     self.end = arith_offset(self.end as *const i8, -1) as *mut T;
 
-                    // Use a non-null pointer value
-                    // (self.end might be null because of wrapping)
-                    Some(ptr::read(1 as *mut T))
+                    // Read from a properly aligned pointer to make up a value of this ZST.
+                    Some(ptr::read(NonNull::dangling().as_ptr()))
                 } else {
                     self.end = self.end.offset(-1);
 

src/libstd/collections/hash/table.rs

@@ -742,7 +742,9 @@ impl<K, V> RawTable<K, V> {
     ) -> Result<RawTable<K, V>, CollectionAllocErr> {
         unsafe {
             let ret = RawTable::new_uninitialized_internal(capacity, fallibility)?;
-            ptr::write_bytes(ret.hashes.ptr(), 0, capacity);
+            if capacity > 0 {
+                ptr::write_bytes(ret.hashes.ptr(), 0, capacity);
+            }
             Ok(ret)
         }
     }