feat(manifest)!: implement feature-metadata RFC3416

crates/cargo-util-schemas/manifest.schema.json

@@ -51,10 +51,7 @@
         "null"
       ],
       "additionalProperties": {
-        "type": "array",
-        "items": {
-          "type": "string"
-        }
+        "$ref": "#/$defs/FeatureDefinition"
       }
     },
     "lib": {
@@ -597,6 +594,38 @@
       ]
     },
     "TomlValue": true,
+    "FeatureDefinition": {
+      "description": "Definition of a feature.",
+      "anyOf": [
+        {
+          "description": "Features that this feature enables.",
+          "type": "array",
+          "items": {
+            "type": "string"
+          }
+        },
+        {
+          "description": "Unstable feature `feature-metadata`. Metadata of this feature.",
+          "$ref": "#/$defs/FeatureMetadata"
+        }
+      ]
+    },
+    "FeatureMetadata": {
+      "description": "Unstable feature `feature-metadata`. Metadata of a feature.",
+      "type": "object",
+      "properties": {
+        "enables": {
+          "description": "Features that this feature enables.",
+          "type": "array",
+          "items": {
+            "type": "string"
+          }
+        }
+      },
+      "required": [
+        "enables"
+      ]
+    },
     "TomlTarget": {
       "type": "object",
       "properties": {

crates/cargo-util-schemas/src/manifest/mod.rs

@@ -41,7 +41,7 @@ pub struct TomlManifest {
     pub package: Option<Box<TomlPackage>>,
     pub project: Option<Box<TomlPackage>>,
     pub badges: Option<BTreeMap<String, BTreeMap<String, String>>>,
-    pub features: Option<BTreeMap<FeatureName, Vec<String>>>,
+    pub features: Option<BTreeMap<FeatureName, FeatureDefinition>>,
     pub lib: Option<TomlLibTarget>,
     pub bin: Option<Vec<TomlBinTarget>>,
     pub example: Option<Vec<TomlExampleTarget>>,
@@ -110,7 +110,7 @@ impl TomlManifest {
             .or(self.build_dependencies2.as_ref())
     }
 
-    pub fn features(&self) -> Option<&BTreeMap<FeatureName, Vec<String>>> {
+    pub fn features(&self) -> Option<&BTreeMap<FeatureName, FeatureDefinition>> {
         self.features.as_ref()
     }
 
@@ -1521,6 +1521,63 @@ impl TomlPlatform {
     }
 }
 
+/// Definition of a feature.
+#[derive(Clone, Debug, Serialize)]
+#[cfg_attr(feature = "unstable-schema", derive(schemars::JsonSchema))]
+#[serde(untagged)]
+pub enum FeatureDefinition {
+    /// Features that this feature enables.
+    Array(Vec<String>),
+    /// Unstable feature `feature-metadata`. Metadata of this feature.
+    Metadata(FeatureMetadata),
+}
+
+// Implementing `Deserialize` manually allows for a better error message when the `enables` key is
+// missing.
+impl<'de> de::Deserialize<'de> for FeatureDefinition {
+    fn deserialize<D>(d: D) -> Result<FeatureDefinition, D::Error>
+    where
+        D: de::Deserializer<'de>,
+    {
+        UntaggedEnumVisitor::new()
+            .seq(|seq| {
+                seq.deserialize::<Vec<String>>()
+                    .map(FeatureDefinition::Array)
+            })
+            .map(|seq| {
+                seq.deserialize::<FeatureMetadata>()
+                    .map(FeatureDefinition::Metadata)
+            })
+            .deserialize(d)
+    }
+}
+
+impl FeatureDefinition {
+    /// Returns the features that this feature enables.
+    pub fn enables(&self) -> &[String] {
+        match self {
+            Self::Array(features) => features,
+            Self::Metadata(FeatureMetadata {
+                enables: features, ..
+            }) => features,
+        }
+    }
+}
+
+/// Unstable feature `feature-metadata`. Metadata of a feature.
+#[derive(Clone, Debug, Deserialize, Serialize)]
+#[cfg_attr(feature = "unstable-schema", derive(schemars::JsonSchema))]
+pub struct FeatureMetadata {
+    /// Features that this feature enables.
+    pub enables: Vec<String>,
+
+    /// This is here to provide a way to see the "unused manifest keys" when deserializing
+    #[serde(skip_serializing)]
+    #[serde(flatten)]
+    #[cfg_attr(feature = "unstable-schema", schemars(skip))]
+    pub _unused_keys: BTreeMap<String, toml::Value>,
+}
+
 #[derive(Serialize, Debug, Clone)]
 #[cfg_attr(feature = "unstable-schema", derive(schemars::JsonSchema))]
 pub struct InheritableLints {

src/cargo/core/features.rs

@@ -519,6 +519,9 @@ features! {
 
     /// Allow paths that resolve relatively to a base specified in the config.
     (unstable, path_bases, "", "reference/unstable.html#path-bases"),
+
+    /// Allow to use a table for defining features.
+    (unstable, feature_metadata, "", "reference/unstable.html#feature_metadata"),
 }
 
 /// Status and metadata for a single unstable feature.

src/cargo/ops/registry/publish.rs

@@ -539,7 +539,7 @@ pub(crate) fn prepare_transmit(
             .map(|(feat, values)| {
                 (
                     feat.to_string(),
-                    values.iter().map(|fv| fv.to_string()).collect(),
+                    values.enables().iter().map(|fv| fv.to_string()).collect(),
                 )
             })
             .collect::<BTreeMap<String, Vec<String>>>(),

src/cargo/util/toml/mod.rs

@@ -11,7 +11,8 @@ use anyhow::{anyhow, bail, Context as _};
 use cargo_platform::Platform;
 use cargo_util::paths;
 use cargo_util_schemas::manifest::{
-    self, PackageName, PathBaseName, TomlDependency, TomlDetailedDependency, TomlManifest,
+    self, FeatureDefinition, FeatureMetadata, FeatureName, PackageName, PathBaseName,
+    TomlDependency, TomlDetailedDependency, TomlManifest,
 };
 use cargo_util_schemas::manifest::{RustVersion, StringOrBool};
 use itertools::Itertools;
@@ -708,8 +709,8 @@ fn default_readme_from_package_root(package_root: &Path) -> Option<String> {
 
 #[tracing::instrument(skip_all)]
 fn normalize_features(
-    original_features: Option<&BTreeMap<manifest::FeatureName, Vec<String>>>,
-) -> CargoResult<Option<BTreeMap<manifest::FeatureName, Vec<String>>>> {
+    original_features: Option<&BTreeMap<manifest::FeatureName, FeatureDefinition>>,
+) -> CargoResult<Option<BTreeMap<manifest::FeatureName, FeatureDefinition>>> {
     let Some(normalized_features) = original_features.cloned() else {
         return Ok(None);
     };
@@ -1296,6 +1297,8 @@ pub fn to_real_manifest(
         }
     }
 
+    validate_feature_definitions(&features, original_toml.features.as_ref(), warnings)?;
+
     validate_dependencies(original_toml.dependencies.as_ref(), None, None, warnings)?;
     validate_dependencies(
         original_toml.dev_dependencies(),
@@ -1497,7 +1500,7 @@ pub fn to_real_manifest(
                 .map(|(k, v)| {
                     (
                         InternedString::new(k),
-                        v.iter().map(InternedString::from).collect(),
+                        v.enables().iter().map(InternedString::from).collect(),
                     )
                 })
                 .collect(),
@@ -1764,6 +1767,32 @@ fn to_virtual_manifest(
     Ok(manifest)
 }
 
+fn validate_feature_definitions(
+    cargo_features: &Features,
+    features: Option<&BTreeMap<FeatureName, FeatureDefinition>>,
+    warnings: &mut Vec<String>,
+) -> CargoResult<()> {
+    let Some(features) = features else {
+        return Ok(());
+    };
+
+    for (feature, feature_definition) in features {
+        match feature_definition {
+            FeatureDefinition::Array(..) => {}
+            FeatureDefinition::Metadata(FeatureMetadata { _unused_keys, .. }) => {
+                cargo_features.require(Feature::feature_metadata())?;
+                warnings.extend(
+                    _unused_keys
+                        .keys()
+                        .map(|k| format!("unused manifest key: `features.{feature}.{k}`")),
+                );
+            }
+        }
+    }
+
+    Ok(())
+}
+
 #[tracing::instrument(skip_all)]
 fn validate_dependencies(
     original_deps: Option<&BTreeMap<manifest::PackageName, manifest::InheritableDependency>>,
@@ -2902,16 +2931,23 @@ fn prepare_toml_for_publish(
         };
 
         features.values_mut().for_each(|feature_deps| {
-            feature_deps.retain(|feature_dep| {
-                let feature_value = FeatureValue::new(InternedString::new(feature_dep));
-                match feature_value {
-                    FeatureValue::Dep { dep_name } | FeatureValue::DepFeature { dep_name, .. } => {
-                        let k = &manifest::PackageName::new(dep_name.to_string()).unwrap();
-                        dep_name_set.contains(k)
+            let feature_array = feature_deps
+                .enables()
+                .iter()
+                .filter(|feature_dep| {
+                    let feature_value = FeatureValue::new(InternedString::new(feature_dep));
+                    match feature_value {
+                        FeatureValue::Dep { dep_name }
+                        | FeatureValue::DepFeature { dep_name, .. } => {
+                            let k = &manifest::PackageName::new(dep_name.to_string()).unwrap();
+                            dep_name_set.contains(k)
+                        }
+                        _ => true,
                     }
-                    _ => true,
-                }
-            });
+                })
+                .cloned()
+                .collect();
+            *feature_deps = FeatureDefinition::Array(feature_array);
         });
     }
 

src/doc/src/reference/unstable.md

@@ -103,6 +103,7 @@ Each new feature described below should explain how to use it.
     * [Profile `trim-paths` option](#profile-trim-paths-option) --- Control the sanitization of file paths in build outputs.
     * [`[lints.cargo]`](#lintscargo) --- Allows configuring lints for Cargo.
     * [path bases](#path-bases) --- Named base directories for path dependencies.
+    * [feature-metadata](#feature-metadata) --- Table syntax for feature definitions.
 * Information and metadata
     * [Build-plan](#build-plan) --- Emits JSON information on which commands will be run.
     * [unit-graph](#unit-graph) --- Emits JSON for Cargo's internal graph structure.
@@ -1619,6 +1620,21 @@ will prefer the value in the configuration. The allows Cargo to add new built-in
 path bases without compatibility issues (as existing uses will shadow the
 built-in name).
 
+## feature-metadata
+
+* Tracking Issue: [#14157](https://github.com/rust-lang/cargo/issues/14157)
+
+This allows to use a table when defining features, with a required `enables` key:
+
+```toml
+[features]
+# same as `foo = []`
+foo = { enables = [] }
+```
+
+This is equivalent to the array-of-strings syntax.
+Support for other keys should be added later.
+
 ## lockfile-path
 * Original Issue: [#5707](https://github.com/rust-lang/cargo/issues/5707)
 * Tracking Issue: [#14421](https://github.com/rust-lang/cargo/issues/14421)