but it is ! the only reason why ODR is UB in C++ is because the C++ language authors can't force the system linkers (again, whether at link time, load time or runtime, I'm not only referring to dlopen) to perform LTO which trivially makes ODR violations a diagnosticable error.

But as far as I know, neither can the Rust language authors do so - so either the behaviour in Rust is as defined as in C++, or Rust does not support creating standard platform object files that are linked by ld, gold, or whatever (such as D, ADA, Fortran etc all support) which would make a fair amount of use cases impossible - it's pretty common in some HPC circles to link C++ and Fortran directly in the same executable for instance. And then, of course it's easier to define behaviour when you use a reduced set of constraints, but it definitely does not makes something worth bragging about.