no rust-std component available for

Error message

no rust-std component available for {}: must use nightly

What it means

`setup_components` refuses to proceed when a non-nightly toolchain is combined with `-Z build-std`, because only nightly rustc ships the `rust-src`/`rust-std` sources needed to compile the standard library from source. The error names the target triple that lacks the component. It is an upfront configuration check, not a build failure.

Solutions

Switch to a nightly toolchain (e.g. `nightly-2023-05-01`) — build-std requires nightly.
Remove the build-std requirement if a prebuilt `rust-std` for the target exists (use `rustup target add <triple>` on nightly instead).
Set the channel explicitly in rust-toolchain.toml to `nightly` when the target has no distributed std component.
Use a custom-built toolchain (`is_custom == true`) that already includes rust-std/rust-src if nightly is not acceptable.

Example fix

// before
# rust-toolchain.toml
channel = "stable"
build-std = true  # stable + build-std -> error
// after
# rust-toolchain.toml
channel = "nightly"
build-std = true

Defensive patterns

Strategy: validation

Validate before calling

fn build_std_ok(channel: &str, uses_build_std: bool) -> Result<(), String> {
    if uses_build_std && channel != "nightly" {
        Err(format!("-Z build-std requires a nightly toolchain (got {channel})"))
    } else { Ok(()) }
}

Try / catch

match setup_components(...) {
    Err(e) if e.to_string().contains("must use nightly") =>
        eprintln!("Switch toolchain to nightly (rustup install nightly) or drop build-std."),
    other => other?,
}

Prevention

Enable build-std only with nightly toolchains; pin nightly in rust-toolchain.toml when build-std is on.
Prefer `rustup target add <triple>` when a prebuilt std exists for the target.
Audit .cargo/config.toml for stale build-std settings after channel changes.

When it happens

Trigger: Calling `setup_components` with `is_custom == false`, a toolchain that is not nightly, and `uses_build_std == true` (e.g. `--build-std` requested for a target while using stable/beta toolchain).

Common situations: Cross-compiling for a target without prebuilt std using `-Zbuild-std` while pinned to stable; old config enabling build-std but toolchain channel later switched from nightly to stable; cargo config in .cargo/config.toml enabling build-std unconditionally.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of cross-rs/cross@8c1a8aa4b6 (2026-09-13). Data as JSON: /api/errors/7e7d99a87d066607. Report an issue: GitHub.

Appendix: source

Thrown at src/rustup.rs:281

    toolchain: &QualifiedToolchain,
    msg_info: &mut MessageInfo,
) -> Result<bool> {
    Ok(check_component(component, toolchain, msg_info)?.is_installed())
}

#[allow(clippy::too_many_arguments)]
pub fn setup_components(
    target: &Target,
    uses_build_std: bool,
    toolchain: &QualifiedToolchain,
    is_nightly: bool,
    available_targets: AvailableTargets,
    args: &crate::cli::Args,
    msg_info: &mut MessageInfo,
) -> Result<(), color_eyre::Report> {
    if !toolchain.is_custom {
        if !is_nightly && uses_build_std {
            eyre::bail!(
                "no rust-std component available for {}: must use nightly",
                target.triple()
            );
        }

        if !uses_build_std
            && !available_targets.is_installed(target)
            && available_targets.contains(target)
        {
            install(target, toolchain, msg_info)?;
        } else if !component_is_installed("rust-src", toolchain, msg_info)? {
            install_component("rust-src", toolchain, msg_info)?;
        }
        if args
            .subcommand
            .clone()
            .is_some_and(|sc| sc == crate::Subcommand::Clippy)
            && !component_is_installed("clippy", toolchain, msg_info)?

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