The topic of the Rust experiment was just discussed at the annual Maintainers Summit. The consensus among the assembled developers is that Rust in the kernel is no longer experimental — it is now a core part of the kernel and is here to stay. So the “experimental” tag will be coming off. Congratulations are in order for all of the Rust for Linux team.
I have never heard the licensing of Rust being raised as a concern for the Linux kernel.
As Charles Babbage would say, “I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.”
The distro I use builds the entire Linux kernel with Clang which uses the same license as Rust. Linux is bound by the same modified GPL license regardless of what compiler I use to build it.
The compiler has no impact on the license applied to the code you build with that compiler. You can use closed source tools to build open source software and vice versa.
And, of course, the Rust license is totally open source as it is offered as both MIT and Apache. Apache 2.0 even provides patent guarantees which can matter for something like a compiler.
If you prefer to use GPL tools yourself, you may want to keep an eye on gccrs.
https://rust-gcc.github.io/
A legitimate concern about Rust may be that LLVM (Rust) supports a different list of hardware than GCC does. The gccrs project addresses that.