rust-lang/cargo · error

multiple slashes in feature `{feature}` is not allowed

Error message

multiple slashes in feature `{feature}` is not allowed

What it means

From parse_dependencies (src/bin/cargo/commands/add.rs:327-329). In the DepFeature branch it checks if dep_feature (the part after the slash) itself contains a `/`. Only one slash (dep/feature) is allowed; a second slash means a malformed nested feature spec, which Cargo rejects.

Source

Thrown at src/bin/cargo/commands/add.rs:328

                    .1
                    .get_or_insert_with(IndexSet::default)
                    .insert(feature.to_owned());
            }
            FeatureValue::Dep { .. } => {
                anyhow::bail!("feature `{feature}` is not allowed to use explicit `dep:` syntax",)
            }
            FeatureValue::DepFeature {
                dep_name,
                dep_feature,
                ..
            } => {
                if infer_crate_name {
                    anyhow::bail!(
                        "`{feature}` is unsupported when inferring the crate name, use `{dep_feature}`"
                    );
                }
                if dep_feature.contains('/') {
                    anyhow::bail!("multiple slashes in feature `{feature}` is not allowed");
                }
                crates.get_mut(&Some(dep_name.as_str().to_owned())).ok_or_else(|| {
                    anyhow::format_err!("feature `{dep_feature}` activated for crate `{dep_name}` but the crate wasn't specified")
                })?
                    .get_or_insert_with(IndexSet::default)
                    .insert(dep_feature.as_str().to_owned());
            }
        }
    }

    let mut deps: Vec<DepOp> = Vec::new();
    for (crate_spec, features) in crates {
        let dep = DepOp {
            crate_spec,
            rename: rename.map(String::from),
            features,
            default_features,
            optional,

View on GitHub (pinned to 0e07a15537)

Solutions

1. List each feature separately with its own dep/feature: `--features tokio/rt,tokio/macros`.
2. Activate additional features of the dependency directly under that dependency in Cargo.toml.
3. Verify there is exactly one `/` per feature token.

Example fix

// before
cargo add tokio --features tokio/rt/macros

// after
cargo add tokio --features tokio/rt,tokio/macros

// Reject feature tokens with more than one slash before invoking cargo add
fn valid_feature_token(f: &str) -> bool {
    f.matches('/').count() <= 1
}

if !features.iter().all(|f| valid_feature_token(f)) {
    eprintln!("each --features value may have at most one `/`");
}

fn has_at_most_one_slash(s: &str) -> bool {
    s.matches('/').count() <= 1
}

When it happens

Trigger: `cargo add tokio --features tokio/rt/macros` or any --features token that, after the first split, still contains a slash.

Common situations: Trying to chain features across nested dependencies through one token; copying a path-like string into --features; misunderstanding that only `dep/feature` (one level) is supported.

Related errors

• feature `{feature}` must be qualified by the dependency it's
• feature `{feature}` is not allowed to use explicit `dep:` sy
• `{feature}` is unsupported when inferring the crate name, us
• invalid character `+` in dependency name: `+{toolchain}`
• cannot specify multiple crates with `--rename`

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/b6e45d01d091e580.json. Report an issue: GitHub.