Python and Perl were not backwards compatible with anything when they were released.
Perl 5 (and older) was highly backwards compatible from a programmer standpoint. Much of Perl is an agglomeration of preexisting Unix command line tools and little languages into one language.
Well, it's closest semantically to awk, but awk is not syntactically compatible with Perl [1]. That's pretty similar to the situation with Rust and C++: semantically compatible but not syntactically compatible.
Definitely it'd be interesting. But I feel like such a thing is going to have the same issues I worry about with the ISO Core C++ lifetime checker: existing C++ code just isn't designed for lifetimes, so you will have to rework the logic a lot to get the static analysis to pass. When you have that much manual intervention, I wonder whether you're porting or really just rewriting—and if you're rewriting, syntactic compatibility doesn't matter so much.
Perl 5 (and older) was highly backwards compatible from a programmer standpoint. Much of Perl is an agglomeration of preexisting Unix command line tools and little languages into one language.