Deprecate pg_extern on non-pub fn?

[workingjubilee]

In a very real sense, when you expose a function via pg_extern, you are making it public in every way that really matters: it is now a callable symbol in the binary with a predictable name! Admittedly, it is using a different ABI, so if we were truly writing these in Rust, they might be written

pub extern "Postgres" fn wrapper_around_rust_fn() -> Type {
    rust_fn()
}

( And we might be able to have people write that if we wanted, too, but we would have to consume the extern "Postgres" before rustc can complain about it. )

The real problem is that every proposed Rust compile-time reflection scheme I have seen, and even several macro-based solutions for problems in pgrx like schema generation, either inherently respects privacy or tends to respect it. So I have begun thinking that perhaps we should require these fn to be pub, so that we can use those solutions in the future (see also #1468).

However, this is a change that is likely to break a lot of code. Accordingly, it would be... extremely rude to land this outright. If this is done, it has to be incremental, with a deprecation warning first.

Even landing the deprecation warning, however, should not happen until 0.13, for two reasons:

• 0.12 is going to keep people busy enough fixing code due to the lifetime fixes.
• This needs more time to be thought over, for other considerations. For instance, the real problem is arguably that the arguments and return types are public information at this point: if these fn were written using their own real ABI string like above, then calling a private function would be fine, but using a non-public return type would cause reachability problems, so perhaps we want to do something more complicated and more precise?

[workingjubilee]

#1549 is another example of someone expecting pgrx to not change the symbol name.