Regression: #[no_mangle] symbols duplicated during test compilation starting nightly-2025-08-15

[joseluis]

Code

I tried this code (simplified from my library):

#[no_mangle]
pub unsafe extern "C" fn test_no_mangle() {}

I expected to see this happen: Successful compilation during cargo test

Instead, this happened: Linker errors with duplicate symbols for #[no_mangle] functions

Version it worked on

It most recently worked on: nightly-2025-08-14

Version with regression

rustc +nightly -vV:

rustc 1.91.0-nightly (8e7795415 2025-08-13)
binary: rustc
commit-hash: 8e7795415acaa9471b5eeb2301915d6996d289ec
commit-date: 2025-08-13
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

Backtrace

Backtrace

error: linking with `cc` failed: exit status: 1
  = note:  "cc" "-m64" "/tmp/rustcflOThX/symbols.o" "<257 object files omitted>" "-Wl,--as-needed" "-Wl,-Bstatic" "/home/user/.cargo/target/debug/deps/libdevela-c196ee63d71a35ed.rlib" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib/{libtest-*,libgetopts-*,libunicode_width-*,librustc_std_workspace_std-*,libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,libcfg_if-*,liblibc-*,librustc_std_workspace_core-*}.rlib" "/home/user/.cargo/target/debug/deps/libdevela_base-2f30b157371083c5.rlib" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib/{liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-L" "/tmp/rustcflOThX/raw-dylibs" "-B<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/bin/gcc-ld" "-fuse-ld=lld" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "<sysroot>/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-o" "/home/user/.cargo/target/debug/deps/devela-ed63e5346b66e85e" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments

          rust-lld: error: duplicate symbol: test_no_mangle
          >>> defined at lib.rs:238 (src/lib.rs:238)
          >>>            /home/user/.cargo/target/debug/deps/devela-5c2b9106189a1e66.5rjh5wt2c596w464wpw7eaq0k.1nc9mqr.rcgu.o:(test_no_mangle)
          >>> defined at lib.rs:238 (src/lib.rs:238)
          >>>            devela-fb6a4d7e6cf82649.59cfjdnj2r4set7kaot6bkd9v.1npt1ar.rcgu.o:(.text.test_no_mangle+0x0) in archive /home/user/.cargo/target/debug/deps/libdevela-fb6a4d7e6cf82649.rlib

          rust-lld: error: duplicate symbol: wasm_callback
          >>> defined at web_api.rs:152 (src/lang/ffi/js/web/web_api.rs:152)
          >>>            /home/user/.cargo/target/debug/deps/devela-5c2b9106189a1e66.6khk1lajtpiy3q4448ifwrbe2.0n5b0qy.rcgu.o:(wasm_callback)
          >>> defined at web_api.rs:152 (src/lang/ffi/js/web/web_api.rs:152)
          >>>            devela-fb6a4d7e6cf82649.1u0u674ci58js15eh7fqpgz9u.02kvoz4.rcgu.o:(.text.wasm_callback+0x0) in archive /home/user/.cargo/target/debug/deps/libdevela-fb6a4d7e6cf82649.rlib
          collect2: error: ld returned 1 exit status

Additional Information

• Only fails during cargo test in debug mode, cargo build doesn't fail.
• Works with LTO enabled in dev profile.
• Requires alloc/std enabled.
• The error also seems to depend on adding or removing arbitrary safe code in unrelated modules.
• Bisected to commit: 8e77954 (Auto merge of resolve: Split extern prelude into two scopes #144793)
• Workaround: Adding #[cfg(not(test))] to #[no_mangle] functions

[lqd]

Seeing the dependencies and wasm in the command line, this seems not to be an MCVE? But it’s another strange one @petrochenkov.

[joseluis]

Apart from the callbacks intended for wasm I have a linux signal restorer in asm that errors the same as well. But I only showed a couple in the backtrace since they're all the same.

I've tried making a MCVE without success. I also began stripping my library of functionality in order to find if some change made a difference, and what I found makes no sense:

For example, removing the implementation of safe constructor over an unrelated safe struct in other module prevents the error from happening on the mock extern fn test_no_mangle at the root. But re-enabling another no_mangled fn makes the errors appear for both.

This looks to me like UB related to extern functions scope calculations.

[bjorn3]

I think what happens is that your devela when being compiled as test executable now depends on the devela library, with both distinct crates containing a conflicting test_no_mangle. As for why you are depending on the devela library, that is not clear at all form your description. Please write a minimal complete verifiable example. https://blog.pnkfx.org/blog/2019/11/18/rust-bug-minimization-patterns/ may be helpful.

[joseluis]

Thank you for that key insight. I was indeed including the same library as a dev-dependency of itself in order to be able to run all the examples with the same default base set of features. Removing it as a dev-dependency no longer triggers the errors.

My doubt is why didn't it fail before that commit? I'm not sure if this means the change would be an unexpected bug-fix or a breaking regression.

I'll keep working on trying to make a MCVE, but I'm don't high hopes since as I said before removing completely unrelated parts seems to affect the triggering of those errors, and I suspect may only appear after a certain threshold of complexity.