I wrote ampli-Fe, which is the world's first fully cross-platform graphical VST plugin written in Rust. It's just a volume knob, but the code is well-documented and freely-licensed, so it's a great starting point for any other kind of Rust VST plugin.
ampli-Fe uses the vst_window crate to create a wgpu-compatible drawing surface.
vst_window handles all platform-specific code, so ampli-Fe doesn't need any #[cfg(...)] attributes to gate by platform!
There's only a single statement of unsafe code required to transfer the window handle to wgpu, but it looks like this is being worked on upstream.
If you're interested in learning more, check out the README and doc comments in the repository.