Binary build fails if two crates exported same function with #[no_mangle] and LTO enabled

[ValeryAntopol]

When linking crates which both export function with the same name with #[no_mangle], LTO fails to process it and emits a message that cannot be understood without looking into rustc sources. For simplicity, i put a minimal reproducible example into a repo https://github.com/ValeryAntopol/rust-lto-bug. The helper crate builds to a library with default settings, rust-lto-bug builds to a binary with default settings + LTO enabled, links with the helper crate.

I tried this code:

helper/lib.rs:

#[no_mangle]
pub fn foo() {}

main.rs:

#[allow(unused)]
use helper;

#[no_mangle]
pub fn foo() {}

fn main() {}

I expected to see this happen:
One of the following:

• successful compilation where LTO somehow handles it (compilation without LTO works successfully, why should one with LTO enabled fail?)
• compilation/linking error saying that it failed because of a function exported twice

I also expect that the behavior will be the same for both enabled and disabled LTO, but i understand that it could be too difficult to achieve.

Instead, this happened:

cargo build, LTO enabled:

   Compiling helper v0.1.0 (/Users/valeryantopol/work/rust-lto-bug/helper)
   Compiling rust-lto-bug v0.1.0 (/Users/valeryantopol/work/rust-lto-bug)
warning: Linking globals named 'foo': symbol multiply defined!

error: failed to load bitcode of module "sqkmfdx3qawf5f0": 

warning: `rust-lto-bug` (bin "rust-lto-bug") generated 1 warning
error: could not compile `rust-lto-bug` due to previous error; 1 warning emitted

cargo build --release, LTO enabled:

   Compiling helper v0.1.0 (/Users/valeryantopol/work/rust-lto-bug/helper)
   Compiling rust-lto-bug v0.1.0 (/Users/valeryantopol/work/rust-lto-bug)
warning: Linking globals named 'foo': symbol multiply defined!

error: failed to load bitcode of module "rust_lto_bug.db9ad83a-cgu.1": 

warning: `rust-lto-bug` (bin "rust-lto-bug") generated 1 warning
error: could not compile `rust-lto-bug` due to previous error; 1 warning emitted

cargo build, LTO disabled:

   Compiling helper v0.1.0 (/Users/valeryantopol/work/rust-lto-bug/helper)
   Compiling rust-lto-bug v0.1.0 (/Users/valeryantopol/work/rust-lto-bug)
    Finished dev [unoptimized + debuginfo] target(s) in 0.70s

Meta

Versions checked:

rustc 1.61.0 (fe5b13d68 2022-05-18)
rustc 1.64.0-nightly (830880640 2022-06-28)
rustc 1.63.0-beta.2 (6c1f14289 2022-06-28)

On all of them behavior is the same, only the module hash changes. But it is the same on different compilations with the same version.

RUST_BACKTACE=1 adds nothing to the output.

[PPakalns]

Issue still actual with rustc 1.69.0 (84c898d 2023-04-16)
with lto="fat"

With lto = "thin", at least in my case, issue was resolved.

[ToufeeqP]

Yes, with lto = "thin", it works in my case as well

[DianQK]

I'm sure it's not a bug. This is the behavior of Linker when FatLTO is not used. Linker will not add symbols from static libraries if the corresponding symbols exist in the already loaded object file.

[jieyouxu]

This is superseded by #123757 as #[no_mangle] will be considered an unsafe attribute going forward. cf. RFC 3325 for the no_mangle discussions.