Tracking issue for #[ffi_returns_twice]

[gnzlbg]

I'll update with the PR and RFC links when those are ready.

PR: #58315
RFC: rust-lang/rfcs#2633

[jonas-schievink]

Implemented in #58315

[JohnTitor]

Could we stabilize this feature now? I'm not sure if this is unstable because it's newly added or there's a blocker but it'd be great if we could de-deprecate vfork on the libc :) cc @Amanieu

[Amanieu]

I am very hesitant to do so since it would stabilize a horrible hack that C uses for two functions: vfork and setjmp. In particular:

• This basically works by disabling a bunch of optimizations in LLVM, but it would likely require additional work in rustc for MIR optimizations.
• Functions that return twice are a hack, you can't even implement such functions in C: they must be written in assembly.
• If you really want to use vfork then you should use a C or asm wrapper that hides the "returns twice" hack and provides a safe callback interface that allows you to run a function in the child process.

[JohnTitor]

Ah, fair enough! I just thought this was missed from stabilizing eyes since rust-lang/libc#1574's OP says "I propose deprecating it until #[returns_twice] is stable". With the above comment, I'm fine to leave this feature as-is, thanks for clarifying!

[joshtriplett]

Closing because rust-lang/rfcs#2633 was closed. (The ffi-unwind project is looking at this.)