The basic trick for backwards compatibility is that all arrays have sizes, but you get to specify the expression which represents the size and associate it with the array. So you don't need array descriptors and can keep many existing representations.
Also, if you have slices, you rarely need pointer arithmetic. Slices are pointer arithmetic with sane semantics.
I'm tired of seeing decade after decade of C/C++ buffer overflows. It speaks badly of software engineering as a profession.
[1] http://www.animats.com/papers/languages/safearraysforc43.pdf