From b414a915f2484b82593a201c05a5ff69b6a23454 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Sun, 12 Jan 2025 09:38:15 +0100
Subject: [PATCH 1/9] test(feature-metadata): test valid and invalid feature
 metadata

---
 tests/testsuite/features.rs | 173 ++++++++++++++++++++++++++++++++++++
 1 file changed, 173 insertions(+)

diff --git a/tests/testsuite/features.rs b/tests/testsuite/features.rs
index 94a69cc6e11..c4a7611ffcf 100644
--- a/tests/testsuite/features.rs
+++ b/tests/testsuite/features.rs
@@ -1,6 +1,9 @@
 //! Tests for `[features]` table.
 
+use std::fs::File;
+
 use cargo_test_support::prelude::*;
+use cargo_test_support::publish::validate_crate_contents;
 use cargo_test_support::registry::{Dependency, Package};
 use cargo_test_support::str;
 use cargo_test_support::{basic_manifest, project};
@@ -2282,3 +2285,173 @@ fn invalid_feature_name_slash_error() {
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn feature_metadata() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                edition = "2015"
+
+                [features]
+                a = []
+                b = []
+                c = { enables = ["a", "b"] }
+            "#,
+        )
+        .file(
+            "src/main.rs",
+            r#"
+                #[cfg(feature = "a")]
+                fn a() {}
+                #[cfg(feature = "b")]
+                fn b() {}
+                fn main() {
+                    a();
+                    b();
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("check --features c").run();
+}
+
+#[cargo_test]
+fn feature_metadata_missing_enables() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                edition = "2015"
+
+                [features]
+                foo = {}
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] missing field `enables`
+ --> Cargo.toml:7:23
+  |
+7 |                 foo = {}
+  |                       ^^
+  |
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn unused_keys_in_feature_metadata() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+               [package]
+               name = "foo"
+               edition = "2015"
+
+               [features]
+               foo = { enables = ["bar"], foobar = false }
+               bar = []
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check")
+        .with_stderr_data(str![[r#"
+[WARNING] unused manifest key: `features.foo.foobar`
+[CHECKING] foo v0.0.0 ([ROOT]/foo)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn normalize_feature_metadata() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                edition = "2015"
+
+                [features]
+                a = []
+                b = []
+                c = { enables = ["a", "b"] }
+            "#,
+        )
+        .file(
+            "src/main.rs",
+            r#"
+                #[cfg(feature = "a")]
+                fn a() {}
+                #[cfg(feature = "b")]
+                fn b() {}
+                fn main() {
+                    a();
+                    b();
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("package --no-verify -v").run();
+    let f = File::open(&p.root().join("target/package/foo-0.0.0.crate")).unwrap();
+    let normalized_manifest = str![[r#"
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2015"
+name = "foo"
+build = false
+autolib = false
+autobins = false
+autoexamples = false
+autotests = false
+autobenches = false
+readme = false
+
+[features]
+a = []
+b = []
+c = [
+    "a",
+    "b",
+]
+
+[[bin]]
+name = "foo"
+path = "src/main.rs"
+
+"#]];
+    validate_crate_contents(
+        f,
+        "foo-0.0.0.crate",
+        &["Cargo.lock", "Cargo.toml", "Cargo.toml.orig", "src/main.rs"],
+        [("Cargo.toml", normalized_manifest)],
+    );
+}

From af99addc4cf44114df407d8eb8a4a16a9dde4bc9 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Sun, 12 Jan 2025 09:51:18 +0100
Subject: [PATCH 2/9] feat(manifest)!: implement feature-metadata RFC3416

---
 crates/cargo-util-schemas/src/manifest/mod.rs | 61 ++++++++++++++++++-
 src/cargo/ops/registry/publish.rs             |  2 +-
 src/cargo/util/toml/mod.rs                    | 60 ++++++++++++++----
 tests/testsuite/features.rs                   | 21 ++-----
 4 files changed, 112 insertions(+), 32 deletions(-)

diff --git a/crates/cargo-util-schemas/src/manifest/mod.rs b/crates/cargo-util-schemas/src/manifest/mod.rs
index 906ff3d431f..06f7f881947 100644
--- a/crates/cargo-util-schemas/src/manifest/mod.rs
+++ b/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>),
+    /// 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,
+        }
+    }
+}
+
+/// 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 {
diff --git a/src/cargo/ops/registry/publish.rs b/src/cargo/ops/registry/publish.rs
index de0f9ac4e0a..5b4d5e01cff 100644
--- a/src/cargo/ops/registry/publish.rs
+++ b/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>>>(),
diff --git a/src/cargo/util/toml/mod.rs b/src/cargo/util/toml/mod.rs
index bc80097b705..0f7074e42c2 100644
--- a/src/cargo/util/toml/mod.rs
+++ b/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(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 +1767,30 @@ fn to_virtual_manifest(
     Ok(manifest)
 }
 
+fn validate_feature_definitions(
+    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, .. }) => {
+                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 +2929,23 @@ 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);
         });
     }
 
diff --git a/tests/testsuite/features.rs b/tests/testsuite/features.rs
index c4a7611ffcf..a0636d24a13 100644
--- a/tests/testsuite/features.rs
+++ b/tests/testsuite/features.rs
@@ -2362,7 +2362,7 @@ fn unused_keys_in_feature_metadata() {
                edition = "2015"
 
                [features]
-               foo = { enables = ["bar"], foobar = false }
+               foo = { enables = ["bar"], a = false, b = true }
                bar = []
             "#,
         )
@@ -2371,7 +2371,8 @@ fn unused_keys_in_feature_metadata() {
 
     p.cargo("check")
         .with_stderr_data(str![[r#"
-[WARNING] unused manifest key: `features.foo.foobar`
+[WARNING] unused manifest key: `features.foo.a`
+[WARNING] unused manifest key: `features.foo.b`
 [CHECKING] foo v0.0.0 ([ROOT]/foo)
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 
@@ -2395,22 +2396,10 @@ fn normalize_feature_metadata() {
                 c = { enables = ["a", "b"] }
             "#,
         )
-        .file(
-            "src/main.rs",
-            r#"
-                #[cfg(feature = "a")]
-                fn a() {}
-                #[cfg(feature = "b")]
-                fn b() {}
-                fn main() {
-                    a();
-                    b();
-                }
-            "#,
-        )
+        .file("src/main.rs", "")
         .build();
 
-    p.cargo("package --no-verify -v").run();
+    p.cargo("package --no-verify").run();
     let f = File::open(&p.root().join("target/package/foo-0.0.0.crate")).unwrap();
     let normalized_manifest = str![[r#"
 # THIS FILE IS AUTOMATICALLY GENERATED BY CARGO

From 338281e3a609675a418d6971ffecf5da51a06f84 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Sun, 12 Jan 2025 11:37:30 +0100
Subject: [PATCH 3/9] fixup! feat(manifest)!: implement feature-metadata
 RFC3416

---
 .../cargo-util-schemas/manifest.schema.json   | 37 +++++++++++++++++--
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/crates/cargo-util-schemas/manifest.schema.json b/crates/cargo-util-schemas/manifest.schema.json
index 5bc43e49d37..cd45723b17a 100644
--- a/crates/cargo-util-schemas/manifest.schema.json
+++ b/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": "Metadata of this feature.",
+          "$ref": "#/$defs/FeatureMetadata"
+        }
+      ]
+    },
+    "FeatureMetadata": {
+      "description": "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": {

From 1cdcd4a9b50b271b5ed78ae03845e96da4855965 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Sun, 12 Jan 2025 20:18:05 +0100
Subject: [PATCH 4/9] fixup! test(feature-metadata): test valid and invalid
 feature metadata

---
 tests/testsuite/features.rs | 29 ++++-------------------------
 1 file changed, 4 insertions(+), 25 deletions(-)

diff --git a/tests/testsuite/features.rs b/tests/testsuite/features.rs
index a0636d24a13..41d2aecccc9 100644
--- a/tests/testsuite/features.rs
+++ b/tests/testsuite/features.rs
@@ -2402,27 +2402,9 @@ fn normalize_feature_metadata() {
     p.cargo("package --no-verify").run();
     let f = File::open(&p.root().join("target/package/foo-0.0.0.crate")).unwrap();
     let normalized_manifest = str![[r#"
-# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
-#
-# When uploading crates to the registry Cargo will automatically
-# "normalize" Cargo.toml files for maximal compatibility
-# with all versions of Cargo and also rewrite `path` dependencies
-# to registry (e.g., crates.io) dependencies.
-#
-# If you are reading this file be aware that the original Cargo.toml
-# will likely look very different (and much more reasonable).
-# See Cargo.toml.orig for the original contents.
-
-[package]
-edition = "2015"
-name = "foo"
-build = false
-autolib = false
-autobins = false
-autoexamples = false
-autotests = false
-autobenches = false
-readme = false
+...
+
+...
 
 [features]
 a = []
@@ -2432,10 +2414,7 @@ c = [
     "b",
 ]
 
-[[bin]]
-name = "foo"
-path = "src/main.rs"
-
+...
 "#]];
     validate_crate_contents(
         f,

From d67aee59ebea54cdb693eee7b06232fe04857979 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Mon, 13 Jan 2025 17:53:15 +0100
Subject: [PATCH 5/9] fixup! test(feature-metadata): test valid and invalid
 feature metadata

---
 tests/testsuite/features.rs | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/tests/testsuite/features.rs b/tests/testsuite/features.rs
index 41d2aecccc9..8b2b4ac7337 100644
--- a/tests/testsuite/features.rs
+++ b/tests/testsuite/features.rs
@@ -2357,13 +2357,14 @@ fn unused_keys_in_feature_metadata() {
         .file(
             "Cargo.toml",
             r#"
-               [package]
-               name = "foo"
-               edition = "2015"
 
-               [features]
-               foo = { enables = ["bar"], a = false, b = true }
-               bar = []
+                [package]
+                name = "foo"
+                edition = "2015"
+
+                [features]
+                foo = { enables = ["bar"], a = false, b = true }
+                bar = []
             "#,
         )
         .file("src/lib.rs", "")

From 567a1042c2cb8f044d53159719ed3aa99a4dc599 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Mon, 13 Jan 2025 18:07:46 +0100
Subject: [PATCH 6/9] feat(feature-metadata): feature-gate the unstable feature

---
 src/cargo/core/features.rs        |  3 ++
 src/cargo/util/toml/mod.rs        |  4 ++-
 src/doc/src/reference/unstable.md | 16 +++++++++
 tests/testsuite/features.rs       | 56 ++++++++++++++++++++++++++++---
 4 files changed, 74 insertions(+), 5 deletions(-)

diff --git a/src/cargo/core/features.rs b/src/cargo/core/features.rs
index 4e17898d6e0..ed8ba617369 100644
--- a/src/cargo/core/features.rs
+++ b/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.
diff --git a/src/cargo/util/toml/mod.rs b/src/cargo/util/toml/mod.rs
index 0f7074e42c2..92164a680e8 100644
--- a/src/cargo/util/toml/mod.rs
+++ b/src/cargo/util/toml/mod.rs
@@ -1297,7 +1297,7 @@ pub fn to_real_manifest(
         }
     }
 
-    validate_feature_definitions(original_toml.features.as_ref(), warnings)?;
+    validate_feature_definitions(&features, original_toml.features.as_ref(), warnings)?;
 
     validate_dependencies(original_toml.dependencies.as_ref(), None, None, warnings)?;
     validate_dependencies(
@@ -1768,6 +1768,7 @@ fn to_virtual_manifest(
 }
 
 fn validate_feature_definitions(
+    cargo_features: &Features,
     features: Option<&BTreeMap<FeatureName, FeatureDefinition>>,
     warnings: &mut Vec<String>,
 ) -> CargoResult<()> {
@@ -1779,6 +1780,7 @@ fn validate_feature_definitions(
         match feature_definition {
             FeatureDefinition::Array(..) => {}
             FeatureDefinition::Metadata(FeatureMetadata { _unused_keys, .. }) => {
+                cargo_features.require(Feature::feature_metadata())?;
                 warnings.extend(
                     _unused_keys
                         .keys()
diff --git a/src/doc/src/reference/unstable.md b/src/doc/src/reference/unstable.md
index 3eba4e45804..44e3a5fc13c 100644
--- a/src/doc/src/reference/unstable.md
+++ b/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)
diff --git a/tests/testsuite/features.rs b/tests/testsuite/features.rs
index 8b2b4ac7337..10f7dff818e 100644
--- a/tests/testsuite/features.rs
+++ b/tests/testsuite/features.rs
@@ -2292,6 +2292,8 @@ fn feature_metadata() {
         .file(
             "Cargo.toml",
             r#"
+                cargo-features = ["feature-metadata"]
+
                 [package]
                 name = "foo"
                 edition = "2015"
@@ -2317,7 +2319,42 @@ fn feature_metadata() {
         )
         .build();
 
-    p.cargo("check --features c").run();
+    p.cargo("check --features c")
+        .masquerade_as_nightly_cargo(&[])
+        .run();
+}
+
+#[cargo_test]
+fn feature_metadata_is_unstable() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                edition = "2015"
+
+                [features]
+                a = { enables = [] }
+            "#,
+        )
+        .file("src/main.rs", "")
+        .build();
+
+    p.cargo("check --features a")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
+
+Caused by:
+  feature `feature-metadata` is required
+
+  The package requires the Cargo feature called `feature-metadata`, but that feature is not stabilized in this version of Cargo ([..]).
+  Consider trying a newer version of Cargo (this may require the nightly release).
+  See https://doc.rust-lang.org/nightly/cargo/ for more information about the status of this feature.
+
+"#]])
+        .run();
 }
 
 #[cargo_test]
@@ -2326,6 +2363,8 @@ fn feature_metadata_missing_enables() {
         .file(
             "Cargo.toml",
             r#"
+                cargo-features = ["feature-metadata"]
+
                 [package]
                 name = "foo"
                 edition = "2015"
@@ -2338,12 +2377,13 @@ fn feature_metadata_missing_enables() {
         .build();
 
     p.cargo("check")
+        .masquerade_as_nightly_cargo(&[])
         .with_status(101)
         .with_stderr_data(str![[r#"
 [ERROR] missing field `enables`
- --> Cargo.toml:7:23
+ --> Cargo.toml:[..]:23
   |
-7 |                 foo = {}
+[..] |                 foo = {}
   |                       ^^
   |
 
@@ -2357,6 +2397,7 @@ fn unused_keys_in_feature_metadata() {
         .file(
             "Cargo.toml",
             r#"
+                cargo-features = ["feature-metadata"]
 
                 [package]
                 name = "foo"
@@ -2371,6 +2412,7 @@ fn unused_keys_in_feature_metadata() {
         .build();
 
     p.cargo("check")
+        .masquerade_as_nightly_cargo(&[])
         .with_stderr_data(str![[r#"
 [WARNING] unused manifest key: `features.foo.a`
 [WARNING] unused manifest key: `features.foo.b`
@@ -2387,6 +2429,8 @@ fn normalize_feature_metadata() {
         .file(
             "Cargo.toml",
             r#"
+                cargo-features = ["feature-metadata"]
+
                 [package]
                 name = "foo"
                 edition = "2015"
@@ -2400,13 +2444,17 @@ fn normalize_feature_metadata() {
         .file("src/main.rs", "")
         .build();
 
-    p.cargo("package --no-verify").run();
+    p.cargo("package --no-verify")
+        .masquerade_as_nightly_cargo(&[])
+        .run();
     let f = File::open(&p.root().join("target/package/foo-0.0.0.crate")).unwrap();
     let normalized_manifest = str![[r#"
 ...
 
 ...
 
+...
+
 [features]
 a = []
 b = []

From d61ffb50613e29985e0056da63594fd42fbc2f68 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Mon, 13 Jan 2025 22:29:27 +0100
Subject: [PATCH 7/9] fixup! feat(manifest)!: implement feature-metadata
 RFC3416

---
 crates/cargo-util-schemas/src/manifest/mod.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/crates/cargo-util-schemas/src/manifest/mod.rs b/crates/cargo-util-schemas/src/manifest/mod.rs
index 06f7f881947..a0fc160abb0 100644
--- a/crates/cargo-util-schemas/src/manifest/mod.rs
+++ b/crates/cargo-util-schemas/src/manifest/mod.rs
@@ -1528,7 +1528,7 @@ impl TomlPlatform {
 pub enum FeatureDefinition {
     /// Features that this feature enables.
     Array(Vec<String>),
-    /// Metadata of this feature.
+    /// Unstable feature `feature-metadata`. Metadata of this feature.
     Metadata(FeatureMetadata),
 }
 
@@ -1564,7 +1564,7 @@ impl FeatureDefinition {
     }
 }
 
-/// Metadata of a feature.
+/// Unstable feature `feature-metadata`. Metadata of a feature.
 #[derive(Clone, Debug, Deserialize, Serialize)]
 #[cfg_attr(feature = "unstable-schema", derive(schemars::JsonSchema))]
 pub struct FeatureMetadata {

From 2faf5e03192191070da962494ba3b359af32fd80 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Mon, 13 Jan 2025 22:51:39 +0100
Subject: [PATCH 8/9] fixup! feat(manifest)!: implement feature-metadata
 RFC3416

---
 crates/cargo-util-schemas/manifest.schema.json | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/crates/cargo-util-schemas/manifest.schema.json b/crates/cargo-util-schemas/manifest.schema.json
index cd45723b17a..d5a7e16c8de 100644
--- a/crates/cargo-util-schemas/manifest.schema.json
+++ b/crates/cargo-util-schemas/manifest.schema.json
@@ -605,13 +605,13 @@
           }
         },
         {
-          "description": "Metadata of this feature.",
+          "description": "Unstable feature `feature-metadata`. Metadata of this feature.",
           "$ref": "#/$defs/FeatureMetadata"
         }
       ]
     },
     "FeatureMetadata": {
-      "description": "Metadata of a feature.",
+      "description": "Unstable feature `feature-metadata`. Metadata of a feature.",
       "type": "object",
       "properties": {
         "enables": {

From 905820c63998157b63f60e5dc5ef620a9b163558 Mon Sep 17 00:00:00 2001
From: AudaciousAxiom <179637270+AudaciousAxiom@users.noreply.github.com>
Date: Wed, 15 Jan 2025 19:20:06 +0100
Subject: [PATCH 9/9] fixup! feat(feature-metadata): feature-gate the unstable
 feature

---
 tests/testsuite/features.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/testsuite/features.rs b/tests/testsuite/features.rs
index 10f7dff818e..21a10fe5f29 100644
--- a/tests/testsuite/features.rs
+++ b/tests/testsuite/features.rs
@@ -2351,7 +2351,7 @@ Caused by:
 
   The package requires the Cargo feature called `feature-metadata`, but that feature is not stabilized in this version of Cargo ([..]).
   Consider trying a newer version of Cargo (this may require the nightly release).
-  See https://doc.rust-lang.org/nightly/cargo/ for more information about the status of this feature.
+  See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#feature_metadata for more information about the status of this feature.
 
 "#]])
         .run();