Support zero-length vertex/index buffers

[jimblandy]

The WebGPU specification says that setVertexBuffer and setIndexBuffer may be passed a size of zero, but the wgpu Rust API doesn't support this: the set_index_buffer and set_vertex_buffer methods take BufferSlice values, which are not allowed to be empty.

The wgpu-core APIs (used by Firefox and Deno) allow zero-size vertex/index buffers and bindings.

As explained in the documentation for hal::BufferBinding, some backends do not support zero-size bindings. Previously, this documentation stated that a zero-size hal::BufferBinding was not permitted, but this restriction was not honored elsewhere in the code. To handle zero-size bindings propertly, they need to be quashed or remapped to a non-zero size, either universally in wgpu-core, or in the specific backends that do not support them.

An incomplete attempt to clean up the handling of zero-length buffers was made in #7875 (later reverted in #7905) and #7911. The relevant code (in particular, the ShouldBeNonZeroExt trait and the type of the size member of hal::BufferBinding) should be cleaned up when this issue is addressed.

Note that although WebGPU allows zero-size vertex and index buffers, it does not allow zero-size buffer bindings in a bind group:

The bound part designated by bufferBinding.offset and bufferBinding.size resides inside the buffer and has non-zero size.

[CryptArchy]

After upgrading from v0.20 to v22.0 I'm running into a new bug around this. Where previously I had slices being passed into set_vertex_buffer without problem, now those same calls are failing due to slice offset 0 is out of range for buffer of size 0. The slice operation fails when copying the full range (.slice(..)) of an empty buffer.

I'm not even sure why the buffers are empty, perhaps an edge condition on startup, but I know it was working before so either something changed to cause the buffers to be empty or something changed to make empty slices fail.

[andyleiserson]

This case is exercised by the CTS test webgpu:api,validation,encoding,cmds,render,setIndexBuffer:offset_and_size_oob and the setVertexBuffer version of the same. (The test name is offset_and_size_oob, but they also cover cases where the offset and size are not OOB, including the case of a zero-size buffer.)