What causes "LLVM does not have support for catchret" in rust-lang/rust?

Use the rustc toolchain's bundled LLVM.

LLVM does not have support for catchret

Error message

LLVM does not have support for catchret

What it means

This panic is raised by `GenericBuilder::catch_ret` (builder.rs:1807-1813) when the LLVM-C call `LLVMBuildCatchRet` returns None. catchret exits a SEH catch funclet and transfers control back to normal code on MSVC targets; a null return means the linked LLVM cannot produce it. The `.expect("LLVM does not have support for catchret")` guard converts the unsupported instruction into an immediate compiler panic because partial funclet IR is invalid.

Source

Thrown at compiler/rustc_codegen_llvm/src/builder.rs:1813

            &[self.val_ty(src)],
            &[src],
        )
    }
}
impl<'a, 'll, CX: Borrow<SCx<'ll>>> GenericBuilder<'a, 'll, CX> {
    pub(crate) fn add_clause(&mut self, landing_pad: &'ll Value, clause: &'ll Value) {
        unsafe {
            llvm::LLVMAddClause(landing_pad, clause);
        }
    }

    pub(crate) fn catch_ret(
        &mut self,
        funclet: &Funclet<'ll>,
        unwind: &'ll BasicBlock,
    ) -> &'ll Value {
        let ret = unsafe { llvm::LLVMBuildCatchRet(self.llbuilder, funclet.cleanuppad(), unwind) };
        ret.expect("LLVM does not have support for catchret")
    }

    pub(crate) fn check_call<'b>(
        &mut self,
        typ: &str,
        fn_ty: &'ll Type,
        llfn: &'ll Value,
        args: &'b [&'ll Value],
    ) -> Cow<'b, [&'ll Value]> {
        assert!(
            self.cx.type_kind(fn_ty) == TypeKind::Function,
            "builder::{typ} not passed a function, but {fn_ty:?}"
        );

        let param_tys = self.cx.func_params_types(fn_ty);

        let all_args_match = iter::zip(&param_tys, args.iter().map(|&v| self.cx.val_ty(v)))
            .all(|(expected_ty, actual_ty)| *expected_ty == actual_ty);

View on GitHub (pinned to 22057b88b0)

Solutions

Use the rustc toolchain's bundled LLVM.
Rebuild LLVM with EH (`-DLLVM_ENABLE_EH=ON`) and the X86 MSVC target.
Align rustc and LLVM versions (rebuild from source).
Avoid the MSVC target or compile with `-Cpanic=abort` to skip catchret emission.

Example fix

// before
rustc --target x86_64-pc-windows-msvc -Cpanic=unwind
// after
rustc --target x86_64-pc-windows-msvc -Cpanic=abort   # no catchret needed

Defensive patterns

Strategy: validation

Validate before calling

# catchret returns from a catch handler; emitted alongside catchswitch.
# Validate that panic=unwind is even meaningful for this LLVM before relying on it.
rustc -C panic=unwind --print=cfg 2>/dev/null | grep -q panic=unwind \
  || echo 'NOTE: unwind cfg not honoured; SEH catchret may be unlowerable on this LLVM'

Try / catch

out="$(cargo build 2>&1)"; rc=$?
if [ $rc -ne 0 ]; then
  case "$out" in
    *"LLVM does not have support for catchret"*)
      echo "catchret unlowerable; use a WinEH-capable LLVM or avoid catching across FFI" >&2 ;;
    *) echo "$out" >&2 ;;
  esac
  exit $rc
fi

Prevention

Keep SEH-catching code on the MSVC target and the rustup default LLVM.
Don't ship custom libLLVM.so alongside rustc unless EH is enabled at LLVM build time.
Lock the toolchain in CI with rust-toolchain.toml.
If you only need unwinding (not catching), prefer -Cpanic=unwind on a verified toolchain over SEH catch constructs.

When it happens

Trigger: Compiling any SEH catch block whose exit must be lowered to a `catchret` when targeting `*-pc-windows-msvc`, while LLVM returns null from `LLVMBuildCatchRet`. Triggers at builder.rs:1812-1813 inside the codegen of the catch handler's terminator.

Common situations: Custom/system LLVM lacking MSVC EH; rustc-vs-LLVM version skew; cross-compiling to MSVC with a host LLVM that does not implement catchret.

Related errors

AI-assisted analysis of rust-lang/rust@22057b88b0 (2026-08-03). Data as JSON: /data/errors/c587526409015e72.json. Report an issue: GitHub.