use ManuallyDrop instead of Option for PendingWrites

wgpu-core/src/device/global.rs

@@ -2347,11 +2347,7 @@ impl Global {
             // need to wait for submissions or triage them. We know we were
             // just polled, so `life_tracker.free_resources` is empty.
             debug_assert!(device.lock_life().queue_empty());
-            {
-                let mut pending_writes = device.pending_writes.lock();
-                let pending_writes = pending_writes.as_mut().unwrap();
-                pending_writes.deactivate();
-            }
+            device.pending_writes.lock().deactivate();
 
             drop(device);
         }

wgpu-core/src/device/queue.rs

@@ -407,7 +407,6 @@ impl Global {
         // `device.pending_writes.consume`.
         let mut staging_buffer = StagingBuffer::new(device, data_size)?;
         let mut pending_writes = device.pending_writes.lock();
-        let pending_writes = pending_writes.as_mut().unwrap();
 
         let staging_buffer = {
             profiling::scope!("copy");
@@ -418,7 +417,7 @@ impl Global {
         let result = self.queue_write_staging_buffer_impl(
             &queue,
             device,
-            pending_writes,
+            &mut pending_writes,
             &staging_buffer,
             buffer_id,
             buffer_offset,
@@ -478,7 +477,6 @@ impl Global {
             .ok_or_else(|| QueueWriteError::Transfer(TransferError::InvalidBufferId(buffer_id)))?;
 
         let mut pending_writes = device.pending_writes.lock();
-        let pending_writes = pending_writes.as_mut().unwrap();
 
         // At this point, we have taken ownership of the staging_buffer from the
         // user. Platform validation requires that the staging buffer always
@@ -489,7 +487,7 @@ impl Global {
         let result = self.queue_write_staging_buffer_impl(
             &queue,
             device,
-            pending_writes,
+            &mut pending_writes,
             &staging_buffer,
             buffer_id,
             buffer_offset,
@@ -713,7 +711,6 @@ impl Global {
             wgt::BufferSize::new(stage_bytes_per_row as u64 * block_rows_in_copy as u64).unwrap();
 
         let mut pending_writes = device.pending_writes.lock();
-        let pending_writes = pending_writes.as_mut().unwrap();
         let encoder = pending_writes.activate();
 
         // If the copy does not fully cover the layers, we need to initialize to
@@ -967,7 +964,7 @@ impl Global {
             extract_texture_selector(&destination.to_untagged(), &size, &dst)?;
 
         let mut pending_writes = device.pending_writes.lock();
-        let encoder = pending_writes.as_mut().unwrap().activate();
+        let encoder = pending_writes.activate();
 
         // If the copy does not fully cover the layers, we need to initialize to
         // zero *first* as we don't keep track of partial texture layer inits.
@@ -1315,8 +1312,7 @@ impl Global {
                 }
             }
 
-            let mut pending_writes_guard = device.pending_writes.lock();
-            let pending_writes = pending_writes_guard.as_mut().unwrap();
+            let mut pending_writes = device.pending_writes.lock();
 
             {
                 used_surface_textures.set_size(hub.textures.read().len());
@@ -1402,17 +1398,12 @@ impl Global {
             profiling::scope!("cleanup");
 
             // this will register the new submission to the life time tracker
-            let mut pending_write_resources = mem::take(&mut pending_writes.temp_resources);
             device.lock_life().track_submission(
                 submit_index,
-                pending_write_resources.drain(..),
+                pending_writes.temp_resources.drain(..),
                 active_executions,
             );
-
-            // pending_write_resources has been drained, so it's empty, but we
-            // want to retain its heap allocation.
-            pending_writes.temp_resources = pending_write_resources;
-            drop(pending_writes_guard);
+            drop(pending_writes);
 
             // This will schedule destruction of all resources that are no longer needed
             // by the user but used in the command stream, among other things.

wgpu-core/src/device/resource.rs

@@ -46,6 +46,7 @@ use wgt::{DeviceLostReason, TextureFormat, TextureSampleType, TextureViewDimensi
 use std::{
     borrow::Cow,
     iter,
+    mem::ManuallyDrop,
     num::NonZeroU32,
     sync::{
         atomic::{AtomicBool, AtomicU64, Ordering},
@@ -142,7 +143,7 @@ pub struct Device<A: HalApi> {
     pub(crate) features: wgt::Features,
     pub(crate) downlevel: wgt::DownlevelCapabilities,
     pub(crate) instance_flags: wgt::InstanceFlags,
-    pub(crate) pending_writes: Mutex<Option<PendingWrites<A>>>,
+    pub(crate) pending_writes: Mutex<ManuallyDrop<PendingWrites<A>>>,
     pub(crate) deferred_destroy: Mutex<Vec<DeferredDestroy<A>>>,
     #[cfg(feature = "trace")]
     pub(crate) trace: Mutex<Option<trace::Trace>>,
@@ -169,7 +170,8 @@ impl<A: HalApi> Drop for Device<A> {
     fn drop(&mut self) {
         resource_log!("Drop {}", self.error_ident());
         let raw = self.raw.take().unwrap();
-        let pending_writes = self.pending_writes.lock().take().unwrap();
+        // SAFETY: We are in the Drop impl and we don't use self.pending_writes anymore after this point.
+        let pending_writes = unsafe { ManuallyDrop::take(&mut self.pending_writes.lock()) };
         pending_writes.dispose(&raw);
         self.command_allocator.dispose(&raw);
         unsafe {
@@ -307,7 +309,10 @@ impl<A: HalApi> Device<A> {
             features: desc.required_features,
             downlevel,
             instance_flags,
-            pending_writes: Mutex::new(rank::DEVICE_PENDING_WRITES, Some(pending_writes)),
+            pending_writes: Mutex::new(
+                rank::DEVICE_PENDING_WRITES,
+                ManuallyDrop::new(pending_writes),
+            ),
             deferred_destroy: Mutex::new(rank::DEVICE_DEFERRED_DESTROY, Vec::new()),
             usage_scopes: Mutex::new(rank::DEVICE_USAGE_SCOPES, Default::default()),
         })
@@ -3611,7 +3616,7 @@ impl<A: HalApi> Device<A> {
 
     /// Wait for idle and remove resources that we can, before we die.
     pub(crate) fn prepare_to_die(&self) {
-        self.pending_writes.lock().as_mut().unwrap().deactivate();
+        self.pending_writes.lock().deactivate();
         let current_index = self
             .last_successful_submission_index
             .load(Ordering::Acquire);

wgpu-core/src/resource.rs

@@ -657,7 +657,6 @@ impl<A: HalApi> Buffer<A> {
                 }
 
                 let mut pending_writes = device.pending_writes.lock();
-                let pending_writes = pending_writes.as_mut().unwrap();
 
                 let staging_buffer = staging_buffer.flush();
 
@@ -746,7 +745,6 @@ impl<A: HalApi> Buffer<A> {
         };
 
         let mut pending_writes = device.pending_writes.lock();
-        let pending_writes = pending_writes.as_mut().unwrap();
         if pending_writes.contains_buffer(self) {
             pending_writes.consume_temp(temp);
         } else {
@@ -1210,7 +1208,6 @@ impl<A: HalApi> Texture<A> {
         };
 
         let mut pending_writes = device.pending_writes.lock();
-        let pending_writes = pending_writes.as_mut().unwrap();
         if pending_writes.contains_texture(self) {
             pending_writes.consume_temp(temp);
         } else {