The invariant is that PtrAndLen may hold ptr+len values which are legal for _either_ C++ string_view or Rust slices: constructing one from either Rust or C++ permits the laxest constraints, and care must be taken when converting a PtrAndLen into either type.
For "into Rust slice" case to handle is that len=0 ptr must be non-null, so len=0 ptr=null gets turned into an arbitrary non-null pointer as provided by std::ptr::NonNull::dangling() (any preexisting non-null ptr is kept in that direction).
For the "into C++ slice" the risk is more obscure that a Rust non-null pointer could potentially be an illegal pointer (such that ptr+0 is not a legal operation in C++), so when going into C++ we map any len=0 cases into ptr=null to avoid this as a possible risk.
PiperOrigin-RevId: 704776210
Putting it into BUILD files unintentionally forces it on all our downstream users. Instead, we just want to enable this during testing and let them choose for themselves in their builds.
Note, that this expands the scope of -Werror to our entire repo for CI, so a bunch of fixes and opt-outs had to be applied to get this change passing.
Closed#14714
PiperOrigin-RevId: 666903224
