How does foo/bin use both when foo/a/* and foo/b/ use ABI-incompatible versions of stdlib types, perhaps in their public interfaces? This can easily lead to breakage in interop across foo/a/* and foo/b/ .
libc++ and glibc++ both break ABI less frequently than new versions of C++ come out. As far as I'm aware, libc++ has never released a breaking change to its ABI.
Also, Rust compiles the whole world at once, so any ABI breakage from mixing code from different compiler versions doesn't happen. (Editions are different thing from compiler versions, a single version of the compiler supports multiple editions.)
For normal libraries, an and b could depend on different versions, so this could be a problem. The name mangling scheme allows for a “disambiguator” to differentiate the two, I believe that the version is used here but the documentation for it does not say if there’s more than that.