prevent unwinding past FFI boundaries in code generation

[thestinger]

It's undefined to unwind past an FFI boundary such as a pub extern "C" fn. Code generation should automatically insert a landing pad doing an abort. This will eliminate the class of memory safety errors resulting from unwinding into C from Rust. LLVM will be able to optimize it out if it is being caught and handled explicitly, such as to translate into an error code for C.

EDIT: Mentoring instructions can be found here.

[pnkfelix]

Assigning P-high, not 1.0.

[steveklabnik]

Triage: I'm not aware of a code change here, I believe that we are expecting people to handle this themselves with catch_panic or whatever it's called.

/cc @rust-lang/libs @rust-lang/lang is that the only solution we are pursing here, or do we plan on doing it automatically eventually?

[aturon]

@steveklabnik IIRC, there's still work to do to ensure that you correctly get an abort if you don't recover from the panic before hitting the boundary. I believe that's what this issue is about.

[ranma42]

cc @ranma42