rust-lang/rust · critical

unsupported cpu type {cpu}

Error message

unsupported cpu type {cpu}

What it means

Thrown while generating an MSVC import library via ar_archive_writer::write_import_library. The machine type is derived from the target arch, and the match only maps X86_64, X86, AArch64, Arm64EC and Arm to COFF MachineTypes values. Any other Arch reaching this MSVC import-library path causes a panic because no COFF machine code exists for it. This code runs only for MSVC-like Windows targets when bundling a staticlib that needs an import library.

Source

Thrown at compiler/rustc_codegen_ssa/src/back/archive.rs:131

            // contain substrings like " @" or "NONAME" that are keywords or otherwise reserved
            // in definition files.

            let mut file = match fs::File::create_new(&output_path) {
                Ok(file) => file,
                Err(error) => sess
                    .dcx()
                    .emit_fatal(ErrorCreatingImportLibrary { lib_name, error: error.to_string() }),
            };

            let exports =
                items.into_iter().map(|item| item.into_coff_short_export(sess)).collect::<Vec<_>>();
            let machine = match &sess.target.arch {
                Arch::X86_64 => MachineTypes::AMD64,
                Arch::X86 => MachineTypes::I386,
                Arch::AArch64 => MachineTypes::ARM64,
                Arch::Arm64EC => MachineTypes::ARM64EC,
                Arch::Arm => MachineTypes::ARMNT,
                cpu => panic!("unsupported cpu type {cpu}"),
            };

            if let Err(error) = ar_archive_writer::write_import_library(
                &mut file,
                lib_name,
                &exports,
                machine,
                !sess.target.is_like_msvc,
                // Enable compatibility with MSVC's `/WHOLEARCHIVE` flag.
                // Without this flag a duplicate symbol error would be emitted
                // when linking a rust staticlib using `/WHOLEARCHIVE`.
                // See #129020
                true,
                &[],
            ) {
                sess.dcx()
                    .emit_fatal(ErrorCreatingImportLibrary { lib_name, error: error.to_string() });
            }

View on GitHub (pinned to 22057b88b0)

Solutions

1. Switch to a supported MSVC arch triple: x86_64-pc-windows-msvc, i686-pc-windows-msvc, aarch64-pc-windows-msvc, or thumbv7em-pc-windows-msvc.
2. If the target is really GNU/MinGW, ensure is_like_msvc is false so the dlltool path (archive.rs:229) is used instead.
3. For a new Windows arch, add the Arch => MachineTypes mapping at compiler/rustc_codegen_ssa/src/back/archive.rs:125.

fn archive_cpu_supported(cpu: &str) -> bool {
    matches!(
        cpu,
        "x86_64" | "aarch64" | "arm" | "armv7" | "powerpc"
            | "powerpc64" | "s390x" | "riscv64" | "riscv32"
            | "mips" | "mips64" | "sparc" | "sparc64"
            | "i686" | "i586" | "msp430" | "wasm32"
    )
}

// caller: assert!(archive_cpu_supported(&arch_str));

fn is_known_archive_cpu(cpu: &str) -> bool {
    !cpu.is_empty() && !cpu.contains(' ') && archive_cpu_supported(cpu)
}

// Panic in compiler; detect via stderr of the build tool.
let s = String::from_utf8_lossy(&output.stderr);
if s.contains("unsupported cpu type") {
    return Err(LinkError::UnsupportedCpu(cpu.into()));
}

When it happens

Trigger: Compiling a staticlib/cdylib for an MSVC Windows target whose arch is not in {X86_64, X86, AArch64, Arm64EC, Arm} (e.g. a target spec marking is_like_msvc=true with an unsupported arch such as RISC-V or MIPS). Triggered when rustc needs to emit the .dll.lib import library.

Common situations: Custom target JSON that sets is_like_msvc without a matching arch. Mixing a non-Windows arch string into an msvc triple. Bleeding-edge arch support (e.g. Arm64EC variants) before the match is extended.

Related errors

• unsupported arch {arch}
• {context}: {err}
• LLVM error: {}
• wanted an rlib
• corrupt rlib

AI-assisted analysis of rust-lang/rust@22057b88b0 (2026-08-03). Data as JSON: /data/errors/afafd4ded3b2facf.json. Report an issue: GitHub.