rust_doc_test requires a local C++ toolchain

[sayrer]

I ran into problems with running this locally, on a VM with no C++ toolchain available.

---- path/to/lib.rs - DemoStruct::demo_method (line 131) stdout ----
error: linker `cc` not found
  |
  = note: No such file or directory (os error 2)

[UebelAndre]

I think this is expected? rustc requires some information from the toolchain so since we're building a test here, we'd need that toolchain to run the compiler, right?

[sayrer]

I don't think it is expected. The other rules, rust_test, cargo_build_script, etc all work without a local C++ toolchain (and instead use the Bazel-supplied cc_toolchain). Maybe something similar to #664 is needed. Looks like it's only failing to find a linker.

[wt]

Can you please try this with Bazel 5.0 and see if this worksw with the C++ autoconfigured toolchain?