functions marked with #[no_mangle] cannot be GC-ed on MacOS

[usamoi]

Code

I tried this code:

// src/main.rs

unsafe extern "C" {
    unsafe static UNDEFINED: usize;
}

#[unsafe(no_mangle)]
pub fn used() {
    println!("UNDEFINED = {}", unsafe { UNDEFINED });
}

pub fn marker() -> usize {
    0
}

fn main() {
    assert_eq!(marker(), 0_usize);
}

cargo build --target aarch64-apple-darwin

I expected to see this happen: it builds

Instead, this happened: it emits linker errors

Version it worked on

1.86.0

Version with regression

nightly-2025-02-26; bisects to #133832

[madsmtm]

For completeness, the error looks like:

error: linking with `cc` failed: exit status: 1
  |
  = note:  "cc" "/var/folders/0j/tk3sfgz540712zgqd1hrry0m0000gn/T/rustc7pYeFN/symbols.o" "<2 object files omitted>" "<sysroot>/lib/rustlib/aarch64-apple-darwin/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,libcfg_if-*,liblibc-*,liballoc-*,librustc_std_workspace_core-*,libcore-*,libcompiler_builtins-*}.rlib" "-lSystem" "-lc" "-lm" "-arch" "arm64" "-mmacosx-version-min=11.0.0" "-o" "foo" "-Wl,-dead_strip" "-nodefaultlibs"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: Undefined symbols for architecture arm64:
            "_UNDEFINED", referenced from:
                foo::used::h39a7b37900f3aa5a in foo.foo.d6039e2b53427ca7-cgu.0.rcgu.o
                foo::used::h39a7b37900f3aa5a in foo.foo.d6039e2b53427ca7-cgu.0.rcgu.o
          ld: symbol(s) not found for architecture arm64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

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