I wanted to add the same thing.
SPIR-V as an IR is as straightforward as it can be. If you take a look at the specs, it cannot be too hard to generate, and the IR will provide the neccessary constraints on the language features. Rust could follow a similar path as what SYCL is taking. Ordinary host side code is compiled with some compiler (rustc), and there are special library classes which when encountered by another compiler (would be rustc also) trigger special compilation paths, generating host-side code in case of classes like cl::sycl::buffer and generating device-side SPIR-V in the body of kernel functions. The host side code could be either OpenCL 2.1, but to obtain a wider set of devices (mostly not lose those damned Nvidia devices), Vulkan API calls could be generated.