> contain complete compilers that munge your code with a focus on speed over correctness

If you’re a developer, the right tools are available. Set D3D11_CREATE_DEVICE_DEBUG when creating the device, also run debug builds of your HLSL shaders. You’ll get huge amount of error checking and diagnostics this way, including very detailed and specific messages about incorrect API use.

> GPU-focused debuggers are incredibly buggy and feature-poor

I indeed found and reported bugs in GPUs and their drivers (mostly Intel GPUs), but it only was a few times over my career. The majority of times they were bugs in my code, not in GPU drivers, and renderdoc.org helped a lot fixing them.

I generally agree GPU debugging is harder than CPU debugging, but I don’t see how open source driver gonna help. It’s an unfortunate combination of execution model (many thousands hardware threads running your code in parallel), external piece of hardware completely separate and asynchronous from the CPU, and very limited ways to interact or even output data (you can’t printf messages, nor allocate dynamic memory, from GPU code).