Here's a repro (of at least one way to get this error; I don't know if Gecko is hitting the same case I am, but I'm mentioning this here to hopefully avoid filing duplicate issues):

At wgpu commit 0b61a19
Take wgpu/examples/hello-triangle, perhaps.
Comment out rpass.set_pipeline(&render_pipeline); — i.e. submit a render pass without having specified the pipeline.

The results I get are:

[2022-04-23T22:20:09Z ERROR wgpu::backend::direct] Handling wgpu errors as fatal by default
thread 'main' panicked at 'wgpu error: Validation Error

Caused by:
    In a RenderPass
      note: encoder = `<CommandBuffer-(0, 1, Metal)>`
    In a draw command, indexed:false indirect:false
    render pipeline must be set

', wgpu/src/backend/direct.rs:2286:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
-[_MTLCommandEncoder dealloc]:131: failed assertion `Command encoder released without endEncoding'
Abort trap: 6

By inserting logging, the assertion appears to occur while Rust is unwinding, unsurprisingly when wgpu_hal::metal::CommandState is dropping its render field of type Option<mtl::RenderCommandEncoder>. Experimentally adding the code

impl Drop for CommandEncoder {
    fn drop(&mut self) {
        if let Some(render) = self.state.render.take() {
            render.end_encoding();
        }
    }
}

to wgpu_hal::metal::CommandEncoder causes the program to finish panicking and exit without the assertion, but I haven't studied the code architecture and certainly can't say this is a correct general fix. (Presumably it at least should treat blit and compute similarly?)

Command encoder released without endEncoding #2077

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