resolve should have a single spec with resolved features

Error message

resolve should have a single spec with resolved features

What it means

`resolve_std` pops the single resolved spec-and-features entry via `.pop().expect("resolve should have a single spec with resolved features")`, guarded by a `debug_assert_eq!(resolve.specs_and_features.len(), 1)`. The standard library resolve is constructed for exactly one spec (the std workspace), so the vector must be non-empty. The panic indicates the resolver returned zero specs for std.

Source

Thrown at src/compiler/standard_lib.rs:111

    let dry_run = false;
    let mut resolve = ops::resolve_ws_with_opts(
        &std_ws,
        target_data,
        &build_config.requested_kinds,
        &cli_features,
        &specs,
        HasDevUnits::No,
        crate::resolver::features::ForceAllTargets::No,
        dry_run,
    )?;
    debug_assert_eq!(resolve.specs_and_features.len(), 1);
    Ok((
        resolve.pkg_set,
        resolve.targeted_resolve,
        resolve
            .specs_and_features
            .pop()
            .expect("resolve should have a single spec with resolved features")
            .resolved_features,
    ))
}

/// Generates a map of root units for the standard library for each kind requested.
///
/// * `crates` is the arg value from `-Zbuild-std`.
/// * `units` is the root units of the build.
pub fn generate_std_roots(
    crates: &[String],
    units: &[Unit],
    std_resolve: &Resolve,
    std_features: &ResolvedFeatures,
    kinds: &[CompileKind],
    package_set: &PackageSet<'_>,
    interner: &UnitInterner,
    profiles: &Profiles,
    target_data: &RustcTargetData<'_>,

View on GitHub (pinned to 0e07a15537)

Solutions

Re-check the `-Zbuild-std` argument (e.g. `-Zbuild-std=std,panic_abort`) and ensure the crate names are valid.
`rustup component add rust-src` so the std workspace resolves correctly.
Update nightly Cargo; `-Zbuild-std` semantics change frequently.

Example fix

// before
resolve
    .specs_and_features
    .pop()
    .expect("resolve should have a single spec with resolved features")
    .resolved_features
// after
resolve
    .specs_and_features
    .into_iter()
    .next()
    .ok_or_else(|| anyhow::anyhow!("-Zbuild-std resolve produced no specs; check the -Zbuild-std crate list and rust-src"))?
    .resolved_features

Defensive patterns

Strategy: validation

Validate before calling

// Validate the -Zbuild-std crate list against known std-workspace lib crates
const KNOWN_STD_CRATES: &[&str] = &["std","core","alloc","proc_macro","panic_abort","panic_unwind","compiler_builtins"];
fn valid_build_std(crates: &[String]) -> bool {
    crates.iter().all(|c| KNOWN_STD_CRATES.contains(&c.as_str()))
}

Prevention

Use the documented `-Zbuild-std=std,panic_abort` form.
Run `rustup component add rust-src`.
Match the nightly Cargo version to the rust-src you use.

When it happens

Trigger: Using `-Zbuild-std` with a crate list/config that yields no std packages; a resolver bug that returns an empty `specs_and_features`; a std workspace whose source is missing so resolution produces nothing (though normally that errors earlier).

Common situations: `-Zbuild-std` with a malformed `crates` argument (e.g. `-Zbuild-std=` with names that match no std crate); missing `rust-src` component combined with a resolver quirk; nightly Cargo with a `-Zbuild-std` regression.

Related errors

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/44d906890415256d.json. Report an issue: GitHub.