Should `Vec::as_ptr()`/`Vec::as_mut_ptr()` return nullptr instead of the dangling value?

In Rust's stdlib, the only way to know if the return from as_mut_ptr() is to check the .capacity() of the Vec.

While this is fine on its own, it does create an interesting interaction with system's APIs which expect either NULL or a valid pointer.

I think updating .as_ptr() and .as_mut_ptr() to return nullptr in the case of capacity() == 0 could help marginally reduce errors when using external APIs written in C or C++ where this idiom is common. There's a lot of debug checking that will specifically check for NULL but this isn't possible when the pointer's value is 0x1, as an example.

Start by reviewing the Rust stdlib implementations and documentation for Vec::as_ptr(), Vec::as_mut_ptr(), and capacity()==0 behavior. Determine whether changing the returned pointer semantics is acceptable for existing users and C/C++ API interoperability; done requires a decided design plus corresponding implementation and tests.