rust-lang/rust · error

{:?}

Error message

{:?}

What it means

Thrown by rustc_codegen_cranelift while emitting unwind/debug info for a compiled function. After Cranelift produces UnwindInfo for a function, the backend matches on its variants; SystemV, WindowsX64, and WindowsArm64 are handled, and any other variant falls through to unimplemented!("{:?}", unwind_info). It signals that Cranelift emitted an unwind-info format the Rust backend does not yet know how to serialize into .eh_frame / Windows unwind tables.

Source

Thrown at compiler/rustc_codegen_cranelift/src/debuginfo/unwind.rs:238

                                        DataId::from_u32(id & !(1 << 31)),
                                        &mut data,
                                    );
                                    data.write_data_addr(reloc.offset, gv, 0);
                                }
                            }
                        };
                    }

                    module.define_data(lsda, &data).unwrap();
                    fde.lsda = Some(address_for_data(lsda));
                }

                self.frame_table.add_fde(self.cie_id.unwrap(), fde);
            }
            UnwindInfo::WindowsX64(_) | UnwindInfo::WindowsArm64(_) => {
                // Windows does not have debug info for its unwind info.
            }
            unwind_info => unimplemented!("{:?}", unwind_info),
        }
    }

    pub(crate) fn emit(self, product: &mut ObjectProduct) {
        let mut eh_frame = EhFrame::from(super::emit::WriterRelocate::new(self.endian));
        self.frame_table.write_eh_frame(&mut eh_frame).unwrap();

        if !eh_frame.0.writer.slice().is_empty() {
            let id = eh_frame.id();
            let section_id = product.add_debug_section(id, eh_frame.0.writer.into_vec());
            let mut section_map = FxHashMap::default();
            section_map.insert(id, section_id);

            let use_section_symbol = product.object.format() != object::BinaryFormat::MachO;
            for reloc in &eh_frame.0.relocs {
                product.add_debug_reloc(&section_map, &section_id, reloc, use_section_symbol);
            }
        }

View on GitHub (pinned to 22057b88b0)

Solutions

1. Disable unwind-table generation: build with -C panic=abort and/or -C force-unwind-tables=no to avoid the add_function unwind path entirely.
2. Pin the cranelift and rustc_codegen_cranelift versions to a mutually compatible pair (rebuild cg_clif against the cranelift version rustc expects).
3. Target a supported OS for unwinding (Linux/SystemV x86_64/aarch64, Windows x86_64/arm64); avoid macOS x86_64 which is already short-circuited.
4. Add a match arm for the new UnwindInfo variant in compiler/rustc_codegen_cranelift/src/debuginfo/unwind.rs and serialize it accordingly (upstream patch).

Example fix

# before (Cargo.toml / .cargo/config.toml forcing unwinding)
[profile.release]
panic = "unwind"
# RUSTFLAGS = "-C force-unwind-tables=yes"

# after - abort path skips unwind-info emission in add_function()
[profile.release]
panic = "abort"

// Lower debuginfo level to avoid the unimplemented unwind emitter path
// in cg_clif debuginfo.
// .cargo/config.toml or CLI:
//   -Cdebuginfo=0   (or 1 instead of 2)
let lvl = std::env::var("RUSTFLAGS").unwrap_or_default();
if lvl.contains("debuginfo=2") && std::env::var("RUSTC_CODEGEN").as_deref() == Ok("cranelift") {
    eprintln!("Lower debuginfo to 0/1 under cg_clif to avoid unwind.rs:238 panic.");
}

// Compiler panic — uncatchable. Fallback is to drop debuginfo or use LLVM.
// In CI, separate the debug build (LLVM) from the fast-check build (cranelift, debuginfo=0).

When it happens

Trigger: Compiling a crate with panic=unwind (or -Cforce-unwind-tables=yes) on a target whose Cranelift backend returns an UnwindInfo variant other than SystemV/WindowsX64/WindowsArm64. macOS x86_64 is explicitly skipped earlier in add_function; this panic is for any other unsupported variant - e.g. a new Cranelift release adding an unwind format, or an OS/ABI combination Cranelift emits non-standard info for.

Common situations: Upgrading the cranelift crate / cg_clif component to a version that introduces a new UnwindInfo variant the pinned Rust version doesn't match; targeting an unusual OS (bare-metal, custom RTOS) with unwind tables enabled; mismatched cranelift <-> rustc_codegen_cranelift versions in a source build.

Related errors

• prologue for {:?}
• epilogue for {:?}
• epilogue_noreturn for {:?}
• save_register for {:?}
• restore_register for {:?}

AI-assisted analysis of rust-lang/rust@22057b88b0 (2026-08-03). Data as JSON: /data/errors/3be41d2560b0cb5b.json. Report an issue: GitHub.