rust-lang/rust · error

not implemented

Error message

not implemented

What it means

An `unimplemented!()` ICE in rustc_codegen_gcc's `handle_native`. When the user requests `target-cpu=native`, the backend must query libgccjit's target info for the host CPU arch. That query only exists under the crate's `master` feature (a recent libgccjit build). Without `master`, `handle_native` has no way to detect the native CPU and panics. The non-master build is a stripped-down fallback that intentionally lacks this capability.

Solutions

1. Stop passing `-Ctarget-cpu=native`; pass an explicit CPU name (e.g. `-Ctarget-cpu=x86-64`) so `handle_native` returns early via `arch_to_gcc`.
2. Rebuild rustc_codegen_gcc with the `master` cargo feature against a libgccjit that supports `get_target_info()`.
3. Switch to the LLVM backend, where `target-cpu=native` is fully supported.

Example fix

# before (non-master cg_gcc -> ICE on native)
rustc -Ctarget-cpu=native -Zcodegen-backend=gcc crate.rs

# after — explicit CPU
rustc -Ctarget-cpu=x86-64 -Zcodegen-backend=gcc crate.rs

# Do not pass target-cpu=native to a non-master cg_gcc build
if [[ "${RUSTFLAGS:-}" == *target-cpu=native* && "${RUSTFLAGS:-}" == *codegen-backend=gcc* ]]; then
  echo "avoid: pass an explicit -Ctarget-cpu=<name>, not 'native', unless cg_gcc is built with the master feature"
fi

When it happens

Trigger: Compiling with the GCC backend that was built without the `master` cargo feature, while passing `-Ctarget-cpu=native` (or a target spec whose CPU is `NATIVE_CPU`). `target_cpu(sess)` calls `handle_native`, which hits the `#[cfg(not(feature = "master"))] unimplemented!()`.

Common situations: A distro or local build of rustc_codegen_gcc linked against an older/stock libgccjit (no master feature), combined with a build script, profile-guided optimization pass, or user flag that sets `-Ctarget-cpu=native`.

Related errors

• unsupported LLVM intrinsic
• funnel shift on
• Kind
• simd
• Accessing live loans requires `-Zpolonius=next`

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/4a6740ec4efe5dbd. Report an issue: GitHub.

Appendix: source

Thrown at compiler/rustc_codegen_gcc/src/gcc_util.rs:130

        "M68000" => "68000",
        "M68020" => "68020",
        _ => name,
    }
}

fn handle_native(name: &str) -> &str {
    if name != NATIVE_CPU {
        return arch_to_gcc(name);
    }

    #[cfg(feature = "master")]
    {
        // Get the native arch.
        let context = Context::default();
        context.get_target_info().arch().unwrap().to_str().unwrap()
    }
    #[cfg(not(feature = "master"))]
    unimplemented!();
}

pub fn target_cpu(sess: &Session) -> &str {
    match sess.opts.cg.target_cpu {
        Some(ref name) => handle_native(name),
        None => handle_native(sess.target.cpu.as_ref()),
    }
}

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