does not exist, unable to build with the standard library…

Error message

{:?} does not exist, unable to build with the standard library, try:
        rustup component add rust-src --toolchain {}

What it means

Cargo's `-Zbuild-std` / standard-library build path resolves a `library/Cargo.lock` under the rust-src component, and it does not exist. Because `RUSTUP_TOOLCHAIN` is set in the environment, the error appends `--toolchain <id>` to the suggested `rustup component add rust-src` command.

Solutions

Install the source for the active toolchain: `rustup component add rust-src --toolchain <id>` (use the exact value from the message).
Verify with `rustup component list --toolchain <id> | grep rust-src`.
If the toolchain is custom (not rustup-managed), ensure `library/Cargo.lock` is vendored alongside the source.
Re-run `rustup update` to refresh components.

Example fix

// before
cargo +nightly build -Zbuild-std
# error: ... try: rustup component add rust-src --toolchain nightly-x86_64-...

// after
rustup component add rust-src --toolchain nightly-x86_64-unknown-linux-gnu
cargo +nightly build -Zbuild-std

Defensive patterns

Strategy: validation

Validate before calling

fn rust_src_installed(toolchain: &str) -> bool {
    let out = std::process::Command::new("rustup").args(["component", "list", "--toolchain", toolchain]).output();
    matches!(out, Ok(o) if String::from_utf8_lossy(&o.stdout).contains("rust-src (installed)"))
}

Type guard

null

Try / catch

null

Prevention

Add a CI step: `rustup component add rust-src --toolchain <pinned>`.
Pin the toolchain in `rust-toolchain.toml` with `components = ["rust-src"]`.
Re-verify after `rustup update`.

When it happens

Trigger: Invoking `cargo +nightly build -Zbuild-std` without having added `rust-src` for that toolchain; using a custom toolchain path that has `lib/rustlib/src/rust/library` but lacks `Cargo.lock`.

Common situations: Fresh CI runners; switching toolchains after adding rust-src to a different one; partial rustup installations where the `rust-src` component is missing for the active toolchain.

Related errors

AI-assisted analysis of rust-lang/cargo@42eee92bc9 (2026-09-08). Data as JSON: /api/errors/9ce6b123615f5e9b. Report an issue: GitHub.

Appendix: source

Thrown at src/compiler/standard_lib.rs:243

    // NOTE: This is temporary until we figure out how to acquire the source.
    let src_path = target_data
        .info(CompileKind::Host)
        .sysroot
        .join("lib")
        .join("rustlib")
        .join("src")
        .join("rust")
        .join("library");
    let lock = src_path.join("Cargo.lock");
    if !lock.exists() {
        let msg = format!(
            "{:?} does not exist, unable to build with the standard \
             library, try:\n        rustup component add rust-src",
            lock
        );
        match target_data.gctx.get_env("RUSTUP_TOOLCHAIN") {
            Ok(rustup_toolchain) => {
                anyhow::bail!("{} --toolchain {}", msg, rustup_toolchain);
            }
            Err(_) => {
                anyhow::bail!(msg);
            }
        }
    }
    Ok(src_path)
}

View on GitHub (pinned to 42eee92bc9)