I think this is more of a social problem with C++. Regardless whether the feature is implemented in the standard library or the core language, the broader C++ c

I think this is more of a social problem with C++. Regardless whether the feature is implemented in the standard library or the core language, the broader C++ community is not going to approve of enabling bounds checks on the bracket operator. And whether this operator is implemented as a builtin or as an overload on an stdlib type is basically transparent to the user. FWIW you can enable bounds checks for the bracket operator by default for STL types in GCC and Clang. So for interfacing with Rust it is both obvious how to translate between slices in Rust and std::span in C++, and you can verify to a degree that this std::span isn’t corrupted on C++ side (to a degree).