There is a new pull request by q66 against master on the void-packages repository

https://github.com/void-ppc/void-packages rust139
https://github.com/void-linux/void-packages/pull/16243

rust: update to 1.39.0, cargo: update to 0.40.0
This also switches to LLVM9.

Generally seems to work fine except Firefox fails to build now. Fix is probably to be done in Firefox. Some generated code from `style` fails to compile, with errors like

```
19:20.28 error[E0119]: conflicting implementations of trait `std::clone::Clone` for type `gecko_bindings::structs::root::mozilla::GeckoBackground`:
```

As the generated file is one line, it spams the console really badly, but turns out that is not a new issue, so we can probably ignore that, as long as FF can get fixed.

A patch file from https://github.com/void-linux/void-packages/pull/16243.patch is attached