> The one big question I can’t answer is what happens if your dependencies use a different version of std? Are std v1 Strings compatible with std v2 strings? Oof.

C++ went through that around C++11, where some types had to have ABI breaks (on some implementations, such as that of GCC's libstdc++). It has been a while, but as I remember it std::string was affected since it went from COW to SSO. Map might have been affected too (don't quite remember). It was a mess.

Rust can link multiple semver versions of the same dependency, but you can't pass data structures between those versions. For transitive dependencies that usually isn't a big deal if your direct dependencies use them internally rather than exposing the types in their own APIs. But std contain vocabulary types that everyone uses (Option, Result, String, etc) so that would really not work.