Include declared list of features in fingerprint for -Zcheck-cfg

Otherwise changing (add, removing, ...) features from the `[features]`
table would not cause rustc to be called with the new `--check-cfg`,
showing potentially outdated warnings.

src/cargo/core/compiler/fingerprint/dirty_reason.rs

@@ -15,6 +15,10 @@ pub enum DirtyReason {
         old: String,
         new: String,
     },
+    DeclaredFeaturesChanged {
+        old: String,
+        new: String,
+    },
     TargetConfigurationChanged,
     PathToSourceChanged,
     ProfileConfigurationChanged,
@@ -141,6 +145,9 @@ impl DirtyReason {
             DirtyReason::FeaturesChanged { .. } => {
                 s.dirty_because(unit, "the list of features changed")
             }
+            DirtyReason::DeclaredFeaturesChanged { .. } => {
+                s.dirty_because(unit, "the list of declared features changed")
+            }
             DirtyReason::TargetConfigurationChanged => {
                 s.dirty_because(unit, "the target configuration changed")
             }

src/cargo/core/compiler/fingerprint/mod.rs

@@ -65,6 +65,7 @@
 //! Target Name                                | ✓           | ✓
 //! TargetKind (bin/lib/etc.)                  | ✓           | ✓
 //! Enabled Features                           | ✓           | ✓
+//! Declared Features                          | ✓           | ?
 //! Immediate dependency’s hashes              | ✓[^1]       | ✓
 //! [`CompileKind`] (host/target)              | ✓           | ✓
 //! __CARGO_DEFAULT_LIB_METADATA[^4]           |             | ✓
@@ -572,6 +573,8 @@ pub struct Fingerprint {
     rustc: u64,
     /// Sorted list of cfg features enabled.
     features: String,
+    /// Sorted list of all the declared cfg features.
+    declared_features: String,
     /// Hash of the `Target` struct, including the target name,
     /// package-relative source path, edition, etc.
     target: u64,
@@ -876,6 +879,7 @@ impl Fingerprint {
             profile: 0,
             path: 0,
             features: String::new(),
+            declared_features: String::new(),
             deps: Vec::new(),
             local: Mutex::new(Vec::new()),
             memoized_hash: Mutex::new(None),
@@ -922,6 +926,12 @@ impl Fingerprint {
                 new: self.features.clone(),
             };
         }
+        if self.declared_features != old.declared_features {
+            return DirtyReason::DeclaredFeaturesChanged {
+                old: old.declared_features.clone(),
+                new: self.declared_features.clone(),
+            };
+        }
         if self.target != old.target {
             return DirtyReason::TargetConfigurationChanged;
         }
@@ -1200,6 +1210,7 @@ impl hash::Hash for Fingerprint {
         let Fingerprint {
             rustc,
             ref features,
+            ref declared_features,
             target,
             path,
             profile,
@@ -1215,6 +1226,7 @@ impl hash::Hash for Fingerprint {
         (
             rustc,
             features,
+            declared_features,
             target,
             path,
             profile,
@@ -1439,6 +1451,17 @@ fn calculate_normal(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Finger
         // actually affect the output artifact so there's no need to hash it.
         path: util::hash_u64(path_args(cx.bcx.ws, unit).0),
         features: format!("{:?}", unit.features),
+        // Note we curently only populate `declared_features` when `-Zcheck-cfg`
+        // is passed since it's the only user-facing toggle that will make this
+        // fingerprint relevant.
+        declared_features: if cx.bcx.config.cli_unstable().check_cfg {
+            format!(
+                "{:?}",
+                unit.pkg.summary().features().keys().collect::<Vec<_>>()
+            )
+        } else {
+            "".to_string()
+        },
         deps,
         local: Mutex::new(local),
         memoized_hash: Mutex::new(None),

tests/testsuite/check_cfg.rs

@@ -141,6 +141,53 @@ fn features_with_namespaced_features() {
         .run();
 }
 
+#[cargo_test(nightly, reason = "--check-cfg is unstable")]
+fn features_fingerprint() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+
+                [features]
+                f_a = []
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    p.cargo("check -v -Zcheck-cfg")
+        .masquerade_as_nightly_cargo(&["check-cfg"])
+        .with_stderr_contains(x!("rustc" => "cfg" of "feature" with "f_a"))
+        .run();
+
+    p.cargo("check -v -Zcheck-cfg")
+        .masquerade_as_nightly_cargo(&["check-cfg"])
+        .with_stderr_does_not_contain("rustc")
+        .run();
+
+    p.change_file(
+        "Cargo.toml",
+        r#"
+            [package]
+            name = "foo"
+            version = "0.1.0"
+
+            [features]
+            f_a = []
+            f_b = []
+        "#,
+    );
+
+    p.cargo("check -v -Zcheck-cfg")
+        .masquerade_as_nightly_cargo(&["check-cfg"])
+        .with_stderr_contains("[..]Dirty[..]the list of declared features changed")
+        .with_stderr_contains(x!("rustc" => "cfg" of "feature" with "f_a" "f_b"))
+        .run();
+}
+
 #[cargo_test(nightly, reason = "--check-cfg is unstable")]
 fn well_known_names_values() {
     let p = project()