[kibwen]

  > Compilers are free to add bounds checking at every 
  > point in the program; in most cases it would be just as 
  > cheap as in C++ or even Rust.

It would not be as cheap as in Rust because Rust uses an explicit standard library feature (iterators) to obviate the need for bounds checks in the vast majority of loops to begin with. But in C indexing is pervasive within loops, so you'd need to come up with much cleverer compilers that could manage to prove that bounds checks were unnecessary (compilers can already do this in some cases, for C/C++/Rust, but it's not perfect).

Likewise, one could make integer overflow in C well-defined, but this would also make C slower than Rust because the use of iterators means that Rust doesn't need to check for overflow on each loop iteration. Via language (or rather, library) features, Rust reclaims the performance that it otherwise would have lost to C by dint of being free of undefined behavior. I think you'd have a hard time doing this in C without rewriting every `for` loop in existence.