rust-lang/rust · critical

require llvm component {component} but wasn't found

Error message

require llvm component {component} but wasn't found

What it means

Thrown at build.rs:232-235 when `llvm-config --components` does not report one of the required LLVM components. REQUIRED_COMPONENTS (build.rs:33-34) is [ipo, bitreader, bitwriter, linker, asmparser, lto, coverage, instrumentation]; the loop at build.rs:232-236 verifies each is present in the components list (after filtering to known names at build.rs:230). A missing required component means the LLVM installation rustc is linking against was built without enabling that component, so rustc_llvm cannot function.

Source

Thrown at compiler/rustc_llvm/build.rs:234

    // report itself with a `--host-target` of `x86_64-pc-windows-gnu`. This
    // tricks us into thinking we're doing a cross build when we aren't, so
    // havoc ensues.
    //
    // In any case, if we're cross compiling, this generally just means that we
    // can't trust all the output of llvm-config because it might be targeted
    // for the host rather than the target. As a result a bunch of blocks below
    // are gated on `if !is_crossed`
    let target = env::var("TARGET").expect("TARGET was not set");
    let host = env::var("HOST").expect("HOST was not set");
    let is_crossed = target != host;

    let components = output(Command::new(&llvm_config).arg("--components"));
    let mut components = components.split_whitespace().collect::<Vec<_>>();
    components.retain(|c| OPTIONAL_COMPONENTS.contains(c) || REQUIRED_COMPONENTS.contains(c));

    for component in REQUIRED_COMPONENTS {
        if !components.contains(component) {
            panic!("require llvm component {component} but wasn't found");
        }
    }

    for component in components.iter() {
        println!("cargo:rustc-cfg=llvm_component=\"{component}\"");
    }

    // Link in our own LLVM shims, compiled with the same flags as LLVM
    let mut cmd = Command::new(&llvm_config);
    cmd.arg("--cxxflags");
    let cxxflags = quoted_split(cmd);
    let mut cfg = cc::Build::new();
    cfg.warnings(false);

    // Prevent critical warnings when we're compiling from rust-lang/rust CI,
    // except on MSVC, as the compiler throws warnings that are only reported
    // for this platform. See https://github.com/rust-lang/rust/pull/145031#issuecomment-3162677202.
    // Moreover, LLVM generally guarantees warning-freedom only when building with Clang, as other

View on GitHub (pinned to 22057b88b0)

Solutions

1. Rebuild LLVM with the full set of targets/components, or use rustc's in-tree LLVM via bootstrap (`./x.py build`), which enables everything required.
2. Install any missing distro LLVM subpackages corresponding to the named component (e.g. coverage/lto/dev packages).
3. Verify with `llvm-config --components` that all of [ipo, bitreader, bitwriter, linker, asmparser, lto, coverage, instrumentation] are listed; if not, the LLVM_CONFIG path is wrong or the LLVM build is incomplete.

# Ensure every LLVM component rustc_llvm needs is present before building
NEEDED="orcjit native webassembly target"
HAVE=$("$LLVM_CONFIG" --components)
for c in $NEEDED; do
  grep -qw "$c" <<<"$HAVE" || { echo "missing required llvm component: $c"; exit 1; }
done

When it happens

Trigger: Pointing LLVM_CONFIG at an LLVM build that was configured with `-DLLVM_TARGETS_TO_BUILD=...` only or with components disabled (e.g. a minimal/system LLVM lacking lto, coverage, or instrumentation); building rustc against a distro LLVM package split into subpackages where not all are installed.

Common situations: Using a stripped-down or vendor LLVM that omits components rustc needs; switching from in-tree LLVM (built by bootstrap with all components) to a system LLVM that is incomplete; outdated LLVM version where component names differ.

Related errors

• command did not execute successfully: {:?} expected success,
• REAL_LIBRARY_PATH_VAR
• LLVM_CONFIG was not set
• TARGET was not set
• HOST was not set

AI-assisted analysis of rust-lang/rust@22057b88b0 (2026-08-03). Data as JSON: /data/errors/c21404f0e888aff9.json. Report an issue: GitHub.