Document unwinding ABI extern "C"

[gnzlbg]

The current documentation of the extern "C" ABI in the reference says:

extern "C" -- This is the same as extern fn foo(); whatever the default your C compiler supports.

When it comes to whether C functions can unwind, the C compiler defaults on most platforms is that these cannot unwind. An exception is MSVC, where AFAICT all functions can always unwind (e.g. even C++ noexcept functions are allowed to be unwound by specific exceptions like, e.g., the one thrown by longjmp).

If this is actually the intended behavior we probably should not emit nounwind for extern "C" functions on Windows. That would have fixed rust-lang/rust#48251 in a different way since that would have allowed longjmps from C into Rust. In the LLVM issue different strategies to allowing nounwind there but fixing things at the LLVM level are being discussed: https://bugs.llvm.org/show_bug.cgi?id=36508.

So as written, whether extern "C" functions can unwind is platform dependent. Is this the intended behavior ? If so, we should clarify this. The earliest I can trace this documentation is 3461ff1

cc @Centril @nagisa @eddyb

[Centril]

That's an awfully vague description in the reference....

I don't think emitting nounwind should be platform dependent.

[gnzlbg]

So I just checked and MSVC does enable unwinding by default for C code: https://gcc.godbolt.org/z/-1Yu7T

[gnzlbg]

I don't think emitting nounwind should be platform dependent.

Note that emitting nounwind might not need to be platform dependent to support what the reference currently says. Apparently, nounwind does not mean "does not unwind", but rather "does not unwind in certain specific ways that are platform dependent". So for MSVC targets, nounwind functions can actually unwind in some ways, and that's ok according to LLVM.