The API accepts iterators in order to avoid forcing the user to store data in particular containers. The implementation doesn't guarantee that any of the iterators are drained, unless stated otherwise by the function documentation.
Most of the information in https://github.com/gfx-rs/wgpu/wiki/Debugging-wgpu-Applications still applies to this API, with an exception of API tracing/replay functionality, which is only available in *wgpu-core*.