Panic in LLVM when using #[linkage] even with supported targets

[mattico]

λ cargo new test-linkage
λ cd test-linkage
λ cat src/lib.rs
#![feature(linkage)]

#[linkage = "extern_weak"]
pub static TEST: bool = false;

λ env RUST_BACKTRACE=1 cargo build --verbose # OR
λ env RUST_BACKTRACE=1 cargo build --verbose --target x86_64-unknown-linux-gnu
   Compiling test-linkage v0.1.0 (file:///C:/Users/Matt%20Ickstadt/Code/Rust/test-linkage)
     Running `rustc src\lib.rs --crate-name test_linkage --crate-type lib -g --out-dir "C:\Users\Matt Ickstadt\Code\Rust\test-linkage\target\x86_64-unknown-linux-gnu\debug" --emit=dep-info,link --target x86_64-unknown-linux-gnu -L "dependency=C:\Users\Matt Ickstadt\Code\Rust\test-linkage\target\x86_64-unknown-linux-gnu\debug" -L "dependency=C:\Users\Matt Ickstadt\Code\Rust\test-linkage\target\x86_64-unknown-linux-gnu\debug\deps"`
Don't know how to emit these
UNREACHABLE executed at C:/bot/slave/nightly-dist-rustc-win-gnu-32/build/src/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp:336!

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
error: Could not compile `test-linkage`.

Caused by:
  Process didn't exit successfully: `rustc src\lib.rs --crate-name test_linkage --crate-type lib -g --out-dir C:\Users\Matt Ickstadt\Code\Rust\test-linkage\target\x86_64-unknown-linux-gnu\debug --emit=dep-info,link --target x86_64-unknown-linux-gnu -L dependency=C:\Users\Matt Ickstadt\Code\Rust\test-linkage\target\x86_64-unknown-linux-gnu\debug -L dependency=C:\Users\Matt Ickstadt\Code\Rust\test-linkage\target\x86_64-unknown-linux-gnu\debug\deps` (exit code: 3)

Nothing too unexpected, since most targets don't support weak linkage. The error message could be a bit better though, rather than panicing in LLVM.

I was a bit surprised that this problem persists when cross compiling to supported targets, but I suppose that its an LLVM limitation, and a very niche one at that.

cc #29603

[mattico]

Just ran this on Ubuntu, expecting it to work, and it didn't. I guess this isn't Windows issue after all.

Am I using this attribute incorrectly? Does it work?

[mattico]

Well it works with #[linkage = "weak"] so I'm starting to think that LLVM changed/removed "extern_weak".

[mattico]

If I'm reading https://github.com/rust-lang/llvm/blob/master/include/llvm/IR/GlobalValue.h#L32 correctly, ExternalWeakLinkage should equal 14 and CommonLinkage should equal 15. https://github.com/rust-lang/rust/blob/master/src/librustc_llvm/lib.rs#L102

[mattico]

I'm putting together a PR with the fix and some compile-pass tests. Might take a few hours to compile, though 😢