This is a tracking issue for the RFC "C-unwind ABI" (rust-lang/rfcs#2945).

The feature gate for the issue is #![feature(c_unwind)].

This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also uses as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the RFC
• Adjust behavior of panic extern "C" fn"
  • panic in a no-unwind function leads to not dropping local variables #123231
  • Make destructors on extern "C" frames to be executed #129582
• Adjust documentation
  • panic runtime and C-unwind documentation reference#1226
• Stabilization PR (see instructions on rustc-dev-guide)

Implementation notes

Major provisions in the RFC:

• Add a C-unwind ABI and system-unwind (I think), we may need more variants.
• For external functions with the C ABI, we already (I believe) add the "nounwind" LLVM attributes when we build them. We want to continue doing this.
• For external functions with the C-unwind ABI, we do not want any such attributes.
• For Rust functions defined with the C ABI, e.g., extern "C" fn foo() { ... }, we want to force them to abort if there is a panic. The MIR helper function should_abort_on_panic already exists and I think will modify the generated code to insert the required shims in such a case. They should also be marked as "nounwind" in LLVM, if they're not already.
• Rust functions with the "C-unwind" ABI should not abort on panic.
• Use ABI to guide the "nounwind" attribute on callsites as well.
• Write suitable codegen tests to check generated LLVM IR.

Unresolved Questions

None. The unresolved questions in the RFC were meant to be solved by future RFCs building on this one.

Implementation history

• Initial implementation: Implement RFC 2945: "C-unwind" ABI  #76570
• Additional work (removing #[unwind], adjusting panic handling): rustc: Fill out remaining parts of C-unwind ABI #86155