llvm error: export_name

[matthiaskrgr]

I tried this code:

#[unsafe(export_name = "\
")]
pub fn main() {}

This can cause a segfault within LLVM.

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rustc --version --verbose:

rustc 1.97.0-nightly (e9e32aca5 2026-04-17)
binary: rustc
commit-hash: e9e32aca5a4ffd08cbc29547b039d64b92a2c03b
commit-date: 2026-04-17
host: x86_64-unknown-linux-gnu
release: 1.97.0-nightly
LLVM version: 22.1.2

with debug assertions:

rust -Cprofile-generate=. a.rs

rustc: /checkout/src/llvm-project/llvm/include/llvm/ADT/StringRef.h:236: char llvm::StringRef::operator[](size_t) const: Assertion `Index < size() && "Invalid index!"' failed.

rustc -Cinstrument-coverage

rustc: /checkout/src/llvm-project/llvm/include/llvm/Support/Casting.h:572: decltype(auto) llvm::cast(From *) [To = llvm::ConstantDataArray, From = llvm::Constant]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

[folkertdev]

The issue reproduces just with #[unsafe(export_name = "")], no need for the newline.

An empty argument to export_name has some interesting behavior: it circumvents rust name mangling and instead has LLVM make up a symbol:

https://godbolt.org/z/555eKdjWe

I wonder if that has any practical uses, or whether we should just disallow the empty string as an argument.

[bjorn3]

We should disallow that. If the item is defined in a different crate than it is used, LLVM would make up entirely different symbols and you did get a linker error. Even within a crate there is no guarantee that the definition and the user end up in the same codegen unit. Basically the use case I can think of where it would not be UB is a #[used] static that is not referenced anywhere.

[folkertdev]

Basically the use case I can think of where it would not be UB is a #[used] static that is not referenced anywhere.

But even in this case, is there an advantage to LLVM generating that name? You could just use #[used] without export_name or no_mangle for the same effect right?

[bjorn3]

I don't think there is an advantage to having LLVM generate the name. It is just the only case I can think of where it would not be UB.