rust-lang/cargo · error

feature `{feature}` is not allowed to use explicit `dep:` sy

Error message

feature `{feature}` is not allowed to use explicit `dep:` syntax

What it means

From parse_dependencies (src/bin/cargo/commands/add.rs:314-316). Cargo's `add` command parses --features values via FeatureValue; the Dep { .. } variant corresponds to the explicit `dep:<name>` syntax (activate a dependency without its features). `cargo add` rejects this syntax entirely, because adding a dependency and simultaneously enabling a dep:-only reference is not meaningful in that flow.

Source

Thrown at src/bin/cargo/commands/add.rs:315

                                c.as_deref().expect("only none when there is 1"),
                                feature
                            )
                        })
                        .collect::<Vec<_>>();
                    anyhow::bail!(
                        "feature `{feature}` must be qualified by the dependency it's being activated for, like {}",
                        candidates.join(", ")
                    );
                }
                crates
                    .first_mut()
                    .expect("always at least one crate")
                    .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)

View on GitHub (pinned to 0e07a15537)

Solutions

1. Drop the `dep:` prefix; just add the crate normally (`cargo add serde`).
2. If you need to enable a feature, use the plain feature name or dep/feature syntax.
3. Edit Cargo.toml [features] by hand if you genuinely need a dep: entry.

Example fix

// before
cargo add serde --features dep:serde

// after
cargo add serde

// Strip any explicit `dep:` tokens before passing to `cargo add`
fn clean_add_features(features: &mut Vec<String>) {
    features.retain(|f| !f.starts_with("dep:"));
}

fn uses_dep_syntax(f: &str) -> bool {
    f.starts_with("dep:")
}

When it happens

Trigger: `cargo add serde --features dep:serde` or any `--features dep:<name>` token passed to `cargo add`.

Common situations: Copying a feature string from an existing Cargo.toml [features] table (where dep: is valid) into a `cargo add` command; misunderstanding that dep: is a manifest-level feature resolver construct, not an add flag.

Related errors

• feature `{feature}` must be qualified by the dependency it's
• `{feature}` is unsupported when inferring the crate name, us
• multiple slashes in feature `{feature}` is not allowed
• 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/8f5279bf27eac5dc.json. Report an issue: GitHub.