Add check to ensure vulkan::CommandEncoder::discard_encoding is not called multiple times in a row

CHANGELOG.md

@@ -137,6 +137,10 @@ Bottom level categories:
 - Implement the `device_set_device_lost_callback` method for `ContextWebGpu`. By @suti in [#5438](https://github.com/gfx-rs/wgpu/pull/5438)
 - Add support for storage texture access modes `ReadOnly` and `ReadWrite`. By @JolifantoBambla in [#5434](https://github.com/gfx-rs/wgpu/pull/5434)
 
+#### Vulkan
+
+- Make `CommandEncoder::discard_encoding` do nothing if called multiple times, and implement `Drop` for `CommandEncoder` to call `discard_encoding`. By @villuna
+
 ### Bug Fixes
 
 #### General

wgpu-hal/src/lib.rs

@@ -617,6 +617,10 @@ pub trait Queue: WasmNotSendSync {
 ///   live `CommandBuffers` built from it. All the `CommandBuffer`s
 ///   are destroyed, and their resources are freed.
 ///
+/// You may want to implement the [core::ops::Drop] trait to discard
+/// the current commands before the encoder is dropped (e.g. using
+/// [CommandEncoder::discard_encoding]).
+///
 /// # Safety
 ///
 /// - The `CommandEncoder` must be in the states described above to
@@ -652,6 +656,10 @@ pub trait CommandEncoder: WasmNotSendSync + fmt::Debug {
     ///
     /// This puts this `CommandEncoder` in the "closed" state.
     ///
+    /// Implementations of this function must be idempotent, i.e.
+    /// if the function has just been called, calling it again should
+    /// not do anything.
+    ///
     /// # Safety
     ///
     /// This `CommandEncoder` must be in the "recording" state.

wgpu-hal/src/vulkan/command.rs

@@ -45,6 +45,13 @@ impl super::DeviceShared {
     }
 }
 
+impl Drop for super::CommandEncoder {
+    fn drop(&mut self) {
+        use crate::CommandEncoder;
+        unsafe { self.discard_encoding() }
+    }
+}
+
 impl super::CommandEncoder {
     fn write_pass_end_timestamp_if_requested(&mut self) {
         if let Some((query_set, index)) = self.end_of_pass_timer_query.take() {
@@ -104,8 +111,10 @@ impl crate::CommandEncoder for super::CommandEncoder {
     }
 
     unsafe fn discard_encoding(&mut self) {
-        self.discarded.push(self.active);
-        self.active = vk::CommandBuffer::null();
+        if self.active != vk::CommandBuffer::null() {
+            self.discarded.push(self.active);
+            self.active = vk::CommandBuffer::null();
+        }
     }
 
     unsafe fn reset_all<I>(&mut self, cmd_bufs: I)