This adds `#![deny(unsafe_op_in_unsafe_fn)]` which removes the
implicit `unsafe` block that `unsafe fn` does.
It also adds many more `SAFETY` docs, corrects some incomplete
ones, and catches a null pointer returned by `upb_Arena_New`.
PiperOrigin-RevId: 549067106
In this CL I'd like to call existing C++ Protobuf API from the V0 Rust API. Since parts of the C++ API are defined inline and using (obviously) C++ name mangling, we need to create a "thunks.cc" file that:
1) Generates code for C++ API function we use from Rust
2) Exposes these functions without any name mangling (meaning using `extern "C"`)
In this CL we add Bazel logic to generate "thunks" file, compile it, and propagate its object to linking. We also add logic to protoc to generate this "thunks" file.
The protoc logic is rather rudimentary still. I hope to focus on protoc code quality in my followup work on V0 Rust API using C++ kernel.
PiperOrigin-RevId: 523479839
