extern "C" functions are not internalized with -C lto

[Zoxc]

When marking public functions with #[no_mangle] and then linking to the crate with the functions using -C lto, the functions will be exported in the output.

This can result in errors about duplicate definitions (when linking things linked with -C lto together) and also prevents the removal of unused functions.

[Zoxc]

Some further investigation shows that this is due to extern "C" being on function definitions, not #[no_mangle]. It's also not due to IR differences in from adding extern "C".

[thestinger]

At some point, there was a decision to treat extern functions differently in the visibility rules. There may be no way to make these functions have internal visibility now, so it's a performance bug.

[huonw]

Why would there be no way to make them internally visible?

[thestinger]

I can't tell you the reasoning behind it because I strongly disagreed with it and made that clear when it happened. I can point you at the commit that implemented this. I didn't understand why the ABI is relevant to visibility then and I don't understand it now. IMO, it's an awful hack to work around broken code that could have just been fixed.

[thestinger]

Nominating since fixing this would be a backwards incompatible language change. It would begin causing errors due to missing symbols in previously valid code.

[alexcrichton]

@Zoxc can you provide a test case? I'm unable to reproduce this with a few different permutations

[thestinger]

Define a pub fn foo() and a pub extern "C" fn foo() in a crate, and link against it with LTO. The Rust ABI function is marked internal is the executable and the C ABI function is not.

[Zoxc]

etest.rs:

#![crate_name = "etest"]
#![crate_type = "rlib"]
#![no_std]

pub fn func1() {}
pub extern fn func2() {}

bare.rs:

#![no_std]
#![no_main]
#![feature(lang_items)]

extern crate etest;

#[lang = "begin_unwind"] extern fn begin_unwind() {}
#[lang = "stack_exhausted"] extern fn stack_exhausted() {}
#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "fail_fmt"] fn fail_fmt() {}

Buliding:
rustc etest.rs
rustc -L . -C lto bare.rs --emit=ir

func2 is exported in the bare create, but func1 is not.

[pnkfelix]

Assigning P-low, not 1.0.