EDIT: Apparently CHERI can handle this scenario, please ignore the rest of the comment

Memory safety is not a binary choice. Consider this typical structure:

    struct buffer {
      char *data;
      size_t capacity;
      size_t length;
    }

In fact it doesn't even have to be C, you could write it in Java, Python, etc. Lets say you decide to reuse such a buffer multiple times. From the C semantics point of view, there is nothing wrong with accessing elements past length. But it can easily lead to a heartbleed style vulnerability. There is probably some escape hatch for CHERI to mark memory as uninitialized, but I'm not very familiar with it. In Rust you can probably give out slices which will ensure bounds checking (or eliminate it if possible). The only correct way towards safety is languages which enable the programmer to add additional restrictions, like Rust or Frama-C.

Anyway, this is mostly just theoretical musing, in practice CHERI will catch lots of issues. But I don't think it is correct to call it "memory safety".