Hi!

A person on Reddit posted a strange build error when ThinLTO is enabled. I performed additional build tests in different configurations and I can confirm the error on current Rustc compilers (see the versions below). My environment is Fedora 41 + AMD Ryzen 5900x (x86-64). The test project is https://github.com/anza-xyz/agave on the master branch with 144925eda5eba98ef28a47a659be68b93211cdb2. The test command is cargo +stable test --profile release-with-debug/cargo +nightly test --profile release-with-debug.

The original report is when ThinLTO is enabled, some binaries fail to be built with the duplicate split compilation unit error. I performed the build in multiple configurations and here are my results:

Build ok:

[profile.release-with-debug] inherits = "release" debug = true #split-debuginfo = "packed" lto = "thin" 

Build ok:

[profile.release-with-debug] inherits = "release" debug = true split-debuginfo = "packed" #lto = "thin" 

Build ok:

[profile.release-with-debug] inherits = "release" debug = true split-debuginfo = "packed" lto = "fat" 

Build ok:

[profile.release-with-debug] inherits = "release" debug = true split-debuginfo = "unpacked" lto = "thin" 

Build fails:

[profile.release-with-debug] inherits = "release" debug = true split-debuginfo = "packed" lto = "thin" 

Build fails:

[profile.release-with-debug] inherits = "release" debug = true split-debuginfo = "packed" lto = "thin" codegen-units = 1 

According to the tests, the buggy is only the combination of ThinLTO and split-debuginfo = "packed". Disabling one of these options or enabling Fat LTO instead of ThinLTO resolves the issue.

I expected to see this happen: the build with lto = "thin" + split-debuginfo = "packed" is successful

Instead, this happened: the build fails with the duplicate split compilation unit error

Meta

The issue is reproduced on both Rustc versions: stable and current nightly

rustc --version --verbose:

rustc 1.82.0 (f6e511eec 2024-10-15) binary: rustc commit-hash: f6e511eec7342f59a25f7c0534f1dbea00d01b14 commit-date: 2024-10-15 host: x86_64-unknown-linux-gnu release: 1.82.0 LLVM version: 19.1.1 

rustc +nightly --version --verbose

rustc 1.84.0-nightly (fbab78289 2024-11-04) binary: rustc commit-hash: fbab78289dd8c6e8860034e0048cfb538f217700 commit-date: 2024-11-04 host: x86_64-unknown-linux-gnu release: 1.84.0-nightly LLVM version: 19.1.3 

Providing RUST_BACKTRACE=1 doesn't give more information so no backtraces here.