Linker error with wasmtime 38 on ARM64 macOS when using ThinLTO

[mkaput]

Wasmtime 38.0.3 produces a linker error on ARM64 macOS when building with ThinLTO enabled. The error indicates that wasmtime_fiber_switch symbol is undefined during linking.

error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "<426 object files omitted>" "/var/folders/8c/r_zrp9mj12xd414t08zrks940000gn/T/rustc2wYYXA/{libwasmtime-e843f7bbc21927d3,libzstd_sys-7eabaf414ee67d02,libwasmtime_internal_jit_debug-9043bb9a4e496da8}.rlib" "<sysroot>/lib/rustlib/aarch64-apple-darwin/lib/libcompiler_builtins-*.rlib" "-liconv" "-lSystem" "-lc" "-lm" "-arch" "arm64" "-mmacosx-version-min=11.0.0" "-L" "/Users/mk/Developer/software-mansion/scarb/linker-repro/wasmtime-only-test/target/release/build/wasmtime-15671f59b24bbe0f/out" "-L" "/Users/mk/Developer/software-mansion/scarb/linker-repro/wasmtime-only-test/target/release/build/zstd-sys-1ea01473940232bc/out" "-L" "/Users/mk/Developer/software-mansion/scarb/linker-repro/wasmtime-only-test/target/release/build/wasmtime-internal-jit-debug-2aaf81cce8508d62/out" "-o" "/Users/mk/Developer/software-mansion/scarb/linker-repro/wasmtime-only-test/target/release/deps/wasmtime_only_test-69cba5150eff8962" "-Wl,-dead_strip" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: ld: warning: object file (/private/var/folders/8c/r_zrp9mj12xd414t08zrks940000gn/T/rustc2wYYXA/libwasmtime-e843f7bbc21927d3.rlib[2](242b1992def1ef0b-helpers.o)) was built for newer 'macOS' version (26.0) than being linked (11.0)
          ld: warning: object file (/private/var/folders/8c/r_zrp9mj12xd414t08zrks940000gn/T/rustc2wYYXA/libwasmtime_internal_jit_debug-9043bb9a4e496da8.rlib[2](db3b6bfb95261072-gdbjit.o)) was built for newer 'macOS' version (26.0) than being linked (11.0)
          Undefined symbols for architecture arm64:
            "wasmtime_internal_fiber::stackswitch::aarch64::wasmtime_fiber_switch::h0b241ee887e10750", referenced from:
                wasmtime_internal_fiber::unix::Suspend::switch::hfd4514b4efef3e13 in wasmtime_only_test-69cba5150eff8962.wasmtime-e843f7bbc21927d3.wasmtime.7e3b8021754df0c4-cgu.05.rcgu.o.rcgu.o
          ld: symbol(s) not found for architecture arm64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)
          

This breaks our releases, as seen here: https://github.com/software-mansion/scarb/actions/runs/18924473454/job/54028423511.

Reverting to Wasmtime 37 workarounds this issue. The same happens when we disable LTO or use Fat LTO. This only happens with ThinLTO which is the only thing we can accept in our project due to other reasons.

Steps to Reproduce

• Architecture: ARM64 (Apple Silicon)
• OS: macOS 26.0
• Rust: 1.90.0
• Wasmtime: 38.0.3
• Build Profile: release with lto = "thin"

Cargo.toml:

[package]
name = "linker-repro"
version = "0.1.0"
edition = "2021"

[dependencies]
wasmtime = "38"

[profile.release]
lto = "thin"

src/main.rs:

fn main() {
    let engine = wasmtime::Engine::default();
    println!("Engine created: {:?}", engine);
}

Build command:

cargo build --release

This will fail with the linker error shown above.

Thoughts

• I think this regressed in 3e9eca8.
• I'm not well versed in these low-level naked functions-LLVM interactions, but prompting llms guided me that there are some issues when you try to combine naked fns with LTO.
• Getting into guessing mode: could it be that ThinLTO is attempting to inline wasmtime_fiber_switch in some places, but forgets in others?
  • Is it reasonable to attach #[inline(never)] to wasmtime_fiber_switch?

[bjorn3]

wasmtime_fiber_switch will never get inlined. Rustc implemented naked asm with global_asm!(). LLVM is not even aware there is a function called wasmtime_fiber_switch during optimizations. There is a fair chance this is a visibility bug in rustc. Can you open an issue at https://github.com/rust-lang/rust/issues/?

[mkaput]

@bjorn3 makes sense, created the issue there: rust-lang/rust#148307

I have also tried to find a more minimal reproduction, but no luck in reasonable time. Updated this issue description. Would appreciate any hints.

[alexcrichton]

Thanks for the report @mkaput! Mind testing out #11960 and seeing if that works for you? It works for me locally but I want to double-check.

[mkaput]

Yay! I can confirm #11960 works on both my reproduction and real project. Thank you