the #[track_caller] shim should not inherit #[no_mangle]

[folkertdev]

fixes #143162

builds on #143293 which introduced a mechanism to strip attributes from shims.

I'm now stripping specifically the attributes that modify the symbol from the #[track_caller] shim. Maybe I missed some though, and maybe we really should be stripping more things from either the #[track_caller] shim, or shims in general.

cc @Jules-Bertholet @workingjubilee @bjorn3

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Some changes occurred in compiler/rustc_codegen_ssa

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Some changes occurred to MIR optimizations

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[Jules-Bertholet]

I think this may be the exact opposite of what we want to do—the #[no_mangle] should probably apply only to the shim. (I think we actually need the lang team to weigh in on this… #143162 (comment))

[folkertdev]

Having the shim "steal" the attribute breaks this code:

https://github.com/rust-lang/rust/blob/master/tests/ui/rfcs/rfc-2091-track-caller/track-caller-ffi.rs

extern "Rust" {
    #[track_caller]
    fn rust_track_caller_ffi_test_tracked() -> &'static Location<'static>;
    fn rust_track_caller_ffi_test_untracked() -> &'static Location<'static>;
}

mod provides {
    use std::panic::Location;
    #[track_caller] // UB if we did not have this!
    #[no_mangle]
    fn rust_track_caller_ffi_test_tracked() -> &'static Location<'static> {
        Location::caller()
    }
    #[no_mangle]
    fn rust_track_caller_ffi_test_untracked() -> &'static Location<'static> {
        Location::caller()
    }
}

in other words, it is current behavior that you are able to export a symbol that takes the implicit location argument.

The #[track_caller] attribute is only allowed on functions that use the (unstable) rust ABI, so only interop with other rust code is relevant, and then it does make sense to me that you'd want to be able to export the "tracked" version. Users can always write their own shim if they want to export a symbol that does not have the implicit location argument.

edit: so basically I implemented what Björn suggests at #143162 (comment).

edit2: also Jubilee did not think that T-lang needed to be involved #143162 (comment). Given the above that seems right to me: if we want to keep existing code working, we need the shim to drop these attributes. That seems fine, because only extern "Rust" can be used in combination with #[track_caller].

[Jules-Bertholet]

in other words, it is current behavior that you are able to export a symbol that takes the implicit location argument.

Ah, I didn’t know that was actually intended to work and that we had a test for it.

also Jubilee did not think that T-lang needed to be involved #143162 (comment).

Yes, but she was advocating for the exact opposite resolution than you are proposing here! So I think it would actually have confirmed that they did need to be involved. Except that, at the time, neither she nor I knew that the lang team had already made this decision. So yes, in light of this new-to-me information, there’s no need for lang involvement, and we should just merge this PR.

[Jules-Bertholet]

You should also add a test that casting a #[track_caller] fn from an extern block to a function pointer works:

mod inner {
    #[unsafe(no_mangle)]
    extern "Rust" fn foo() {
        println!("hello world");
    }
}

unsafe extern "Rust" {
    #[track_caller]
    safe fn foo();
}

fn main() {
    foo();
    let fnptr = foo as fn();
    fnptr();
}