Remove panics in Deref impl of QueueWriteBufferView and BufferViewMut (#3336)

CHANGELOG.md

@@ -124,6 +124,7 @@ Additionally `Surface::get_default_config` now returns an Option and returns Non
 - Add missing `DEPTH_BIAS_CLAMP` and `FULL_DRAW_INDEX_UINT32` downlevel flags. By @teoxoy in [#3316](https://github.com/gfx-rs/wgpu/pull/3316)
 - Make `ObjectId` structure and invariants idiomatic. By @teoxoy in [#3347](https://github.com/gfx-rs/wgpu/pull/3347)
 - Add validation in accordance with WebGPU `GPUSamplerDescriptor` valid usage for `lodMinClamp` and `lodMaxClamp`. By @James2022-rgb in [#3353](https://github.com/gfx-rs/wgpu/pull/3353)
+- Remove panics in `Deref` implementations for `QueueWriteBufferView` and `BufferViewMut`. Instead, warnings are logged, since reading from these types is not recommended. By @botahamec in [#3336]
 
 #### WebGPU
 

wgpu/src/lib.rs

@@ -20,7 +20,7 @@ use std::{
     future::Future,
     marker::PhantomData,
     num::{NonZeroU32, NonZeroU8},
-    ops::{Bound, Range, RangeBounds},
+    ops::{Bound, Deref, DerefMut, Range, RangeBounds},
     sync::Arc,
     thread,
 };
@@ -2297,6 +2297,9 @@ pub struct BufferView<'a> {
 }
 
 /// Write only view into mapped buffer.
+///
+/// It is possible to read the buffer using this view, but doing so is not
+/// recommended, as it is likely to be slow.
 #[derive(Debug)]
 pub struct BufferViewMut<'a> {
     slice: BufferSlice<'a>,
@@ -2313,27 +2316,6 @@ impl std::ops::Deref for BufferView<'_> {
     }
 }
 
-impl std::ops::Deref for BufferViewMut<'_> {
-    type Target = [u8];
-
-    #[inline]
-    fn deref(&self) -> &[u8] {
-        assert!(
-            self.readable,
-            "Attempting to read a write-only mapping for buffer {:?}",
-            self.slice.buffer.id
-        );
-        self.data.slice()
-    }
-}
-
-impl std::ops::DerefMut for BufferViewMut<'_> {
-    #[inline]
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        self.data.slice_mut()
-    }
-}
-
 impl AsRef<[u8]> for BufferView<'_> {
     #[inline]
     fn as_ref(&self) -> &[u8] {
@@ -2348,6 +2330,24 @@ impl AsMut<[u8]> for BufferViewMut<'_> {
     }
 }
 
+impl Deref for BufferViewMut<'_> {
+    type Target = [u8];
+
+    fn deref(&self) -> &Self::Target {
+        if !self.readable {
+            log::warn!("Reading from a BufferViewMut is slow and not recommended.");
+        }
+
+        self.data.slice()
+    }
+}
+
+impl DerefMut for BufferViewMut<'_> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.data.slice_mut()
+    }
+}
+
 impl Drop for BufferView<'_> {
     fn drop(&mut self) {
         self.slice
@@ -3768,7 +3768,11 @@ impl<'a> RenderBundleEncoder<'a> {
     }
 }
 
-/// A write-only view into a staging buffer
+/// A read-only view into a staging buffer.
+///
+/// Reading into this buffer won't yield the contents of the buffer from the
+/// GPU and is likely to be slow. Because of this, although [`AsMut`] is
+/// implemented for this type, [`AsRef`] is not.
 pub struct QueueWriteBufferView<'a> {
     queue: &'a Queue,
     buffer: &'a Buffer,
@@ -3777,21 +3781,27 @@ pub struct QueueWriteBufferView<'a> {
 }
 static_assertions::assert_impl_all!(QueueWriteBufferView: Send, Sync);
 
-impl<'a> std::ops::Deref for QueueWriteBufferView<'a> {
+impl Deref for QueueWriteBufferView<'_> {
     type Target = [u8];
 
     fn deref(&self) -> &Self::Target {
-        panic!("QueueWriteBufferView is write-only!");
+        log::warn!("Reading from a QueueWriteBufferView won't yield the contents of the buffer and may be slow.");
+        self.inner.slice()
     }
 }
 
-impl<'a> std::ops::DerefMut for QueueWriteBufferView<'a> {
-    #[inline]
+impl DerefMut for QueueWriteBufferView<'_> {
     fn deref_mut(&mut self) -> &mut Self::Target {
         self.inner.slice_mut()
     }
 }
 
+impl<'a> AsMut<[u8]> for QueueWriteBufferView<'a> {
+    fn as_mut(&mut self) -> &mut [u8] {
+        self.inner.slice_mut()
+    }
+}
+
 impl<'a> Drop for QueueWriteBufferView<'a> {
     fn drop(&mut self) {
         DynContext::queue_write_staging_buffer(
@@ -3827,12 +3837,9 @@ impl Queue {
     }
 
     /// Schedule a data write into `buffer` starting at `offset` via the returned
-    /// [QueueWriteBufferView].
+    /// [`QueueWriteBufferView`].
     ///
-    /// The returned value can be dereferenced to a `&mut [u8]`; dereferencing it to a
-    /// `&[u8]` panics!
-    /// (It is not unsound to read through the `&mut [u8]` anyway, but doing so will not
-    /// yield the existing contents of `buffer` from the GPU, and it is likely to be slow.)
+    /// Reading from this buffer is slow and will not yield the actual contents of the buffer.
     ///
     /// This method is intended to have low performance costs.
     /// As such, the write is not immediately submitted, and instead enqueued