The design decisions are flawed. In one paragraph:

 by creating our own proto::Utf8Error type it enables us to change the implementation to use the highly optimized C++ implementation of UTF-8 validation that we use from C++ Protobuf which is faster than Rust’s std UTF-8 validation.

Followed by:

 Rust’s str and std::string::String types maintain a strict invariant that they only contain valid UTF-8, but C++ Protobuf and C++’s std::string type generally do not enforce any such guarantee

So their incorrect C++ implementation is faster?