rust-lang/cargo · error

not implemented

Error message

not implemented

What it means

Cargo's new feature resolver (enabled via -Z features) supports several sub-flags (build_dep, host_dep, dev_dep, itarget, all, compare). The 'ws' (workspace-unified feature resolution) sub-flag is declared in the match but not yet implemented, so it triggers unimplemented!(). This is nightly-only unstable functionality.

Solutions

1. Remove 'ws' from the -Z features value (use combinations of build_dep/host_dep/dev_dep/itarget/all/compare instead).
2. Use the stabilized resolver v2/v3 (resolver = "2"/"3" in Cargo.toml) which enables 'all' automatically, instead of the unstable 'ws' flag.
3. Upgrade or downgrade nightly to a version where 'ws' is implemented, if applicable.

Example fix

// before
"ws" => unimplemented!(),
// after (user side)
// cargo build -Z features=ws   -> remove
// cargo build -Z features=all

// Validate the -Z features value before running Cargo
let allowed = ["build_dep","host_dep","dev_dep","itarget","all","compare"];
for f in features_value.split(',') {
    if !allowed.contains(&f) {
        eprintln!("flag `{f}` is not implemented; remove it (notably 'ws')");
    }
}

null

null

When it happens

Trigger: Invoking Cargo with `-Z features=ws` on a nightly toolchain. FeatureOpts::new matches the 'ws' token and panics. Note: because this runs inside the resolver, the panic surfaces as a crash rather than a clean error.

Common situations: Experimenting with unstable Cargo feature flags; copy-pasting a flag list that includes 'ws' from docs/PRs; CI on nightly where 'ws' was advertised but not stabilized.

Related errors

• parsing ` ` requires `-Zscript
• parsing ` ` requires `-Zscript
• artifact-dir was not locked during clean
• cannot publish with `open-namespaces`
• lock file version ` ` requires `-Znext-lockfile-bump

AI-assisted analysis of rust-lang/cargo@42eee92bc9 (2026-08-11). Data as JSON: /api/errors/e3f199f3bb3b1dbe. Report an issue: GitHub.

Appendix: source

Thrown at src/resolver/features.rs:189

        ws: &Workspace<'_>,
        has_dev_units: HasDevUnits,
        force_all_targets: ForceAllTargets,
    ) -> CargoResult<FeatureOpts> {
        let mut opts = FeatureOpts::default();
        let unstable_flags = ws.gctx().cli_unstable();
        let mut enable = |feat_opts: &Vec<String>| {
            for opt in feat_opts {
                match opt.as_ref() {
                    "build_dep" | "host_dep" => opts.decouple_host_deps = true,
                    "dev_dep" => opts.decouple_dev_deps = true,
                    "itarget" => opts.ignore_inactive_targets = true,
                    "all" => {
                        opts.decouple_host_deps = true;
                        opts.decouple_dev_deps = true;
                        opts.ignore_inactive_targets = true;
                    }
                    "compare" => opts.compare = true,
                    "ws" => unimplemented!(),
                    s => bail!("-Zfeatures flag `{}` is not supported", s),
                }
            }
            Ok(())
        };
        if let Some(feat_opts) = unstable_flags.features.as_ref() {
            enable(feat_opts)?;
        }
        match ws.resolve_behavior() {
            ResolveBehavior::V1 => {}
            ResolveBehavior::V2 | ResolveBehavior::V3 => {
                enable(&vec!["all".to_string()]).unwrap();
            }
        }
        if let HasDevUnits::Yes = has_dev_units {
            // Dev deps cannot be decoupled when they are in use.
            opts.decouple_dev_deps = false;
        }

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