C-variadic functions may not be extern "C-unwind"

[asomers]

Overview

Using the unstable c_variadic feature, functions may be variadic as long as they have an extern "C" ABI. That used to be a reasonable restriction. However, after #74990 , extern "C" isn't as useful as it used to be. Now, any function that might panic needs to be extern "C-unwind". This may not technically be a compiler regression. But it's certainly a regression in functionality. Previously, Mockall was able to mock C variadic functions. Now, it cannot. Would it be possible to relax that restriction?

Example

I tried this code:

#![feature(c_variadic)]

pub extern "C-unwind" fn foo(x: i32, y: i32, ...) -> i32 {
    unimplemented!()
}

I expected to see this happen: It should compile ok.

Instead, this happened: It fails with this error:

error: only foreign or `unsafe extern "C"` functions may be C-variadic

Meta

rustc --version --verbose:

rustc 1.81.0-nightly (c1b336cb6 2024-06-21)
binary: rustc
commit-hash: c1b336cb6b491b3be02cd821774f03af4992f413
commit-date: 2024-06-21
host: x86_64-unknown-freebsd
release: 1.81.0-nightly
LLVM version: 18.1.7

[workingjubilee]

Thanks for reporting this. It seems in several places, we just check for the literal "C" ABI, and those places are not checked for the "C-unwind" ABI as a possibility.

[workingjubilee]

Now, any function that might panic needs to be extern "C-unwind". This may not technically be a compiler regression. But it's certainly a regression in functionality. Previously, Mockall was able to mock C variadic functions. Now, it cannot. Would it be possible to relax that restriction?

Just for note: You were not supposed to be allowing panics to unwind into C. Unless I am misunderstanding something, if this is a regression for you in functionality, your code was UB. I don't mean to quibble much, though, as I literally already fixed this.

[asomers]

Just for note: You were not supposed to be allowing panics to unwind into C.

Yeah, I realize that now. But since it worked before, I never bothered to read any of the fine details about external C functions. And thanks for the amazingly fast PR!