Download precompiled artifacts

[LegNeato]

We know what version of rustc we have, we should be able to download a matching version of the dylib rather than building locally. We'll probably have to publish these to github releases.

[tombh]

I actually tried to do this and it's mostly possible: tombh/rust-gpu-cli@9099a4e

However there is a limitation: cross-compiling doesn't work for anything that depends on rustc-dev, namely rustc-codegen-spirv. Of course compiling natively always works no matter the host. The problem is that Github doesn't have native machines for all the classic targets, I think it's currently missing; Windows ARM, Darwin Intel, and Linux ARM.

• rustup thread that found the bug: "Can't find crate for rustc_*" when cross-compiling a crate with rustc_private on rust-lang/rustup#3255 (comment)
• rustc bug report: Nightly rustc-dev can't be used in cross-compilation rust-lang/rust#132728

[LegNeato]

Ah, ok! Should have known you tried 😅.

[tombh]

Oh! I've just seen that these 6 platform runners are available on Github Actoins:

- target: aarch64-unknown-linux-gnu
  os: ubuntu-22.04-arm
- target: x86_64-unknown-linux-gnu
  os: ubuntu-22.04

- target: aarch64-apple-darwin
  os: macos-13
- target: x86_64-apple-darwin
  os: macos-13

- target: aarch64-pc-windows-msvc
  os: windows-2022
- target: x86_64-pc-windows-msvc
  os: windows-2022

So no cross-compiling is needed for those architectures at least.

[LegNeato]

Those look like the major ones 🥇