[core] Use ManuallyDrop for wgpu_core::device::Queue::raw.

Change the field `wgpu_core::device::Queue::raw` from an
`Option<A::Queue>` to a `std::mem::ManuallyDrop<A::Queue>`. Replace
various `.as_ref().unwrap()` chains with calls to a new accessor
function `Queue::raw`.

An `Option` is misleading, as this field is always populated during
the lifetime of a `Queue`. Instead, we simply have a field whose value
needs to be moved in `<Queue as Drop>::drop`; `ManuallyDrop` is the
Rust idiom for this situation.

wgpu-core/src/device/queue.rs

@@ -39,10 +39,23 @@ use thiserror::Error;
 use super::Device;
 
 pub struct Queue<A: HalApi> {
-    pub(crate) raw: Option<A::Queue>,
+    raw: std::mem::ManuallyDrop<A::Queue>,
     pub(crate) device: Arc<Device<A>>,
 }
 
+impl<A: HalApi> Queue<A> {
+    pub(crate) fn new(device: Arc<Device<A>>, raw: A::Queue) -> Self {
+        Queue {
+            raw: std::mem::ManuallyDrop::new(raw),
+            device,
+        }
+    }
+
+    pub(crate) fn raw(&self) -> &A::Queue {
+        &self.raw
+    }
+}
+
 crate::impl_resource_type!(Queue);
 // TODO: https://github.com/gfx-rs/wgpu/issues/4014
 impl<A: HalApi> Labeled for Queue<A> {
@@ -56,7 +69,8 @@ crate::impl_storage_item!(Queue);
 impl<A: HalApi> Drop for Queue<A> {
     fn drop(&mut self) {
         resource_log!("Drop {}", self.error_ident());
-        let queue = self.raw.take().unwrap();
+        // Safety: we never access `self.raw` beyond this point.
+        let queue = unsafe { std::mem::ManuallyDrop::take(&mut self.raw) };
         self.device.release_queue(queue);
     }
 }
@@ -1272,11 +1286,9 @@ impl Global {
                 }
             }
 
-            if let Some(pending_execution) = pending_writes.pre_submit(
-                &device.command_allocator,
-                device.raw(),
-                queue.raw.as_ref().unwrap(),
-            )? {
+            if let Some(pending_execution) =
+                pending_writes.pre_submit(&device.command_allocator, device.raw(), queue.raw())?
+            {
                 active_executions.insert(0, pending_execution);
             }
 
@@ -1298,9 +1310,7 @@ impl Global {
 
                 unsafe {
                     queue
-                        .raw
-                        .as_ref()
-                        .unwrap()
+                        .raw()
                         .submit(
                             &hal_command_buffers,
                             &submit_surface_textures,
@@ -1356,7 +1366,7 @@ impl Global {
     ) -> Result<f32, InvalidQueue> {
         let hub = A::hub(self);
         match hub.queues.get(queue_id) {
-            Ok(queue) => Ok(unsafe { queue.raw.as_ref().unwrap().get_timestamp_period() }),
+            Ok(queue) => Ok(unsafe { queue.raw().get_timestamp_period() }),
             Err(_) => Err(InvalidQueue),
         }
     }

wgpu-core/src/device/resource.rs

@@ -1601,7 +1601,7 @@ impl<A: HalApi> Device<A> {
 
         let encoder = self
             .command_allocator
-            .acquire_encoder(self.raw(), queue.raw.as_ref().unwrap())?;
+            .acquire_encoder(self.raw(), queue.raw())?;
 
         Ok(command::CommandBuffer::new(
             encoder,

wgpu-core/src/instance.rs

@@ -293,11 +293,7 @@ impl<A: HalApi> Adapter<A> {
             instance_flags,
         ) {
             let device = Arc::new(device);
-            let queue = Queue {
-                device: device.clone(),
-                raw: Some(hal_device.queue),
-            };
-            let queue = Arc::new(queue);
+            let queue = Arc::new(Queue::new(device.clone(), hal_device.queue));
             device.set_queue(&queue);
             return Ok((device, queue));
         }

wgpu-core/src/present.rs

@@ -326,13 +326,7 @@ impl Global {
                             log::error!("Presented frame is from a different surface");
                             Err(hal::SurfaceError::Lost)
                         } else {
-                            unsafe {
-                                queue
-                                    .raw
-                                    .as_ref()
-                                    .unwrap()
-                                    .present(suf.unwrap(), raw.take().unwrap())
-                            }
+                            unsafe { queue.raw().present(suf.unwrap(), raw.take().unwrap()) }
                         }
                     }
                     _ => unreachable!(),