rust-lang/cargo · warning

ignoring `resolver.feature-unification` without `-Zfeature-u

Error message

ignoring `resolver.feature-unification` without `-Zfeature-unification`

What it means

In `Workspace::set_resolve_behavior`, the `resolver.feature-unification` config key is only honored under the unstable `-Zfeature-unification` flag. When the key is set in config but the flag is absent, Cargo emits this warning and ignores the setting (falling back to default unification behavior).

Source

Thrown at src/workspace/workspace.rs:358

            ResolveBehavior::V3 => {
                if self.resolve_behavior == ResolveBehavior::V3 {
                    self.resolve_honors_rust_version = true;
                }
            }
        }
        let config = self.gctx().get::<CargoResolverConfig>("resolver")?;
        if let Some(incompatible_rust_versions) = config.incompatible_rust_versions {
            self.resolve_honors_rust_version =
                incompatible_rust_versions == IncompatibleRustVersions::Fallback;
        }
        if self.gctx().cli_unstable().feature_unification {
            self.resolve_feature_unification = config
                .feature_unification
                .unwrap_or(FeatureUnification::Selected);
        } else if config.feature_unification.is_some() {
            self.gctx()
                .shell()
                .warn("ignoring `resolver.feature-unification` without `-Zfeature-unification`")?;
        };

        if let Some(lockfile_path) = config.lockfile_path {
            // Reserve the ability to add templates in the future.
            let replacements: [(&str, &str); 0] = [];
            let path = lockfile_path
                    .resolve_templated_path(self.gctx(), replacements)
                    .map_err(|e| match e {
                        context::ResolveTemplateError::UnexpectedVariable {
                            variable,
                            raw_template,
                        } => {
                            anyhow!(
                                "unexpected variable `{variable}` in resolver.lockfile-path `{raw_template}`"
                            )
                        }
                        context::ResolveTemplateError::UnexpectedBracket { bracket_type, raw_template } => {
                            let (btype, literal) = match bracket_type {

View on GitHub (pinned to 42eee92bc9)

Solutions

1. Run with nightly and pass `-Zfeature-unification` (e.g. `cargo +nightly -Zfeature-unification build`) so the setting takes effect.
2. Alternatively remove `resolver.feature-unification` from .cargo/config.toml to silence the warning if default behavior is acceptable.
3. Set `CARGO_UNSTABLE_FEATURE_UNIFICATION=true` in environments where passing -Z on the CLI is impractical.
4. Verify cargo version supports the feature (check `cargo -Z help`).

Example fix

// before (.cargo/config.toml, stable build)
[resolver]
feature-unification = "selected"
// after: run with
// cargo +nightly -Zfeature-unification build
// or remove the [resolver] section

// in CI, before invoking cargo
if grep -q 'feature-unification' .cargo/config.toml; then
  case "$CARGO_CHANNEL" in *nightly*) ;; *) echo "requires -Zfeature-unification on nightly"; exit 1;; esac
fi

When it happens

Trigger: Configuring `resolver.feature-unification = "selected"` (or similar) in .cargo/config.toml while building with stable Rust or without `-Zfeature-unification` on nightly.

Common situations: Teams adopting the new feature-unification setting from nightly docs, then running stable CI builds; copy-pasted config from projects using nightly features; forgetting to add `-Z feature-unification` or `cargo +nightly -Z ...`.

Related errors

• could not parse `{j}`. Number of parallel jobs should be `de
• failed to merge config value from `{}` into `{}`: expected {
• `{}` expected {}, but found a {}
• missing config key `{}`
• config.json not found

AI-assisted analysis of rust-lang/cargo@42eee92bc9 (2026-09-08). Data as JSON: /api/errors/2535c89a7aee9a57. Report an issue: GitHub.