From 53f0672397695adf8cf37f61c2be0835a405e16c Mon Sep 17 00:00:00 2001
From: Chris Jones <cjfj@deepmind.com>
Date: Fri, 29 Apr 2022 20:37:43 +0100
Subject: [PATCH 1/2] Remove unnecessary `Default` constraint on
 `DeviceBox::as_host_value`.

---
 crates/cust/src/memory/device/device_box.rs | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/crates/cust/src/memory/device/device_box.rs b/crates/cust/src/memory/device/device_box.rs
index 3bead1ba..5635a9d8 100644
--- a/crates/cust/src/memory/device/device_box.rs
+++ b/crates/cust/src/memory/device/device_box.rs
@@ -7,7 +7,7 @@ use crate::memory::{cuda_free_async, cuda_malloc_async, DeviceCopy};
 use crate::stream::Stream;
 use crate::sys as cuda;
 use std::fmt::{self, Pointer};
-use std::mem::{self, ManuallyDrop};
+use std::mem::{self, ManuallyDrop, MaybeUninit};
 
 use std::os::raw::c_void;
 
@@ -131,12 +131,15 @@ impl<T: DeviceCopy> DeviceBox<T> {
     }
 }
 
-impl<T: DeviceCopy + Default> DeviceBox<T> {
+impl<T: DeviceCopy> DeviceBox<T> {
     /// Read the data back from the GPU into host memory.
     pub fn as_host_value(&self) -> CudaResult<T> {
-        let mut val = T::default();
-        self.copy_to(&mut val)?;
-        Ok(val)
+        let mut val = MaybeUninit::uninit();
+        // SAFETY: We do not read from the uninitialized reference.
+        unsafe {
+            self.copy_to(val.assume_init_mut())?;
+            Ok(val.assume_init())
+        }
     }
 }
 

From 3cac529f1cc6960145859e981a91196d7d52d522 Mon Sep 17 00:00:00 2001
From: Chris Jones <cjfj@deepmind.com>
Date: Mon, 2 May 2022 08:41:49 +0100
Subject: [PATCH 2/2] Merge `impl` sections.

---
 crates/cust/src/memory/device/device_box.rs | 2 --
 1 file changed, 2 deletions(-)

diff --git a/crates/cust/src/memory/device/device_box.rs b/crates/cust/src/memory/device/device_box.rs
index 5635a9d8..a08b441e 100644
--- a/crates/cust/src/memory/device/device_box.rs
+++ b/crates/cust/src/memory/device/device_box.rs
@@ -129,9 +129,7 @@ impl<T: DeviceCopy> DeviceBox<T> {
         // you keep around a pointer, but in that case, we cannot guarantee safety.
         unsafe { cuda_free_async(stream, me.ptr) }
     }
-}
 
-impl<T: DeviceCopy> DeviceBox<T> {
     /// Read the data back from the GPU into host memory.
     pub fn as_host_value(&self) -> CudaResult<T> {
         let mut val = MaybeUninit::uninit();