Report unused features

compiler/rustc_driver_impl/src/lib.rs

@@ -338,7 +338,11 @@ pub fn run_compiler(at_args: &[String], callbacks: &mut (dyn Callbacks + Send))
                 }
             }
 
-            Some(Linker::codegen_and_build_linker(tcx, &*compiler.codegen_backend))
+            let linker = Linker::codegen_and_build_linker(tcx, &*compiler.codegen_backend);
+
+            tcx.report_unused_features();
+
+            Some(linker)
         });
 
         // Linking is done outside the `compiler.enter()` so that the

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1330,7 +1330,7 @@ pub struct BuiltinAttribute {
         safety: AttributeSafety::Normal,
         template: template!(NameValueStr: "name"),
         duplicates: ErrorFollowing,
-        gate: Gated{
+        gate: Gated {
             feature: sym::rustc_attrs,
             message: "use of an internal attribute",
             check: Features::rustc_attrs,

compiler/rustc_feature/src/lib.rs

@@ -137,5 +137,5 @@ pub fn find_feature_issue(feature: Symbol, issue: GateIssue) -> Option<NonZero<u
 pub use removed::REMOVED_LANG_FEATURES;
 pub use unstable::{
     DEPENDENT_FEATURES, EnabledLangFeature, EnabledLibFeature, Features, INCOMPATIBLE_FEATURES,
-    UNSTABLE_LANG_FEATURES,
+    TRACK_FEATURE, UNSTABLE_LANG_FEATURES,
 };

compiler/rustc_feature/src/unstable.rs

@@ -3,11 +3,18 @@
 use std::path::PathBuf;
 use std::time::{SystemTime, UNIX_EPOCH};
 
+use rustc_data_structures::AtomicRef;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_span::{Span, Symbol, sym};
 
 use super::{Feature, to_nonzero};
 
+fn default_track_feature(_: Symbol) {}
+
+/// Recording used features in the dependency graph so incremental can
+/// replay used features when needed.
+pub static TRACK_FEATURE: AtomicRef<fn(Symbol)> = AtomicRef::new(&(default_track_feature as _));
+
 #[derive(PartialEq)]
 enum FeatureStatus {
     Default,
@@ -103,7 +110,12 @@ pub fn enabled_features_iter_stable_order(
 
     /// Is the given feature enabled (via `#[feature(...)]`)?
     pub fn enabled(&self, feature: Symbol) -> bool {
-        self.enabled_features.contains(&feature)
+        if self.enabled_features.contains(&feature) {
+            TRACK_FEATURE(feature);
+            true
+        } else {
+            false
+        }
     }
 }
 
@@ -124,7 +136,7 @@ macro_rules! declare_features {
         impl Features {
             $(
                 pub fn $feature(&self) -> bool {
-                    self.enabled_features.contains(&sym::$feature)
+                    self.enabled(sym::$feature)
                 }
             )*
 

compiler/rustc_interface/src/callbacks.rs

@@ -12,8 +12,9 @@
 use std::fmt;
 
 use rustc_errors::DiagInner;
-use rustc_middle::dep_graph::TaskDepsRef;
+use rustc_middle::dep_graph::{DepNodeIndex, QuerySideEffect, TaskDepsRef};
 use rustc_middle::ty::tls;
+use rustc_span::Symbol;
 
 fn track_span_parent(def_id: rustc_span::def_id::LocalDefId) {
     tls::with_context_opt(|icx| {
@@ -51,6 +52,25 @@ fn track_diagnostic<R>(diagnostic: DiagInner, f: &mut dyn FnMut(DiagInner) -> R)
     })
 }
 
+fn track_feature(feature: Symbol) {
+    tls::with_context_opt(|icx| {
+        let Some(icx) = icx else {
+            return;
+        };
+        let tcx = icx.tcx;
+
+        if let Some(dep_node_index) = tcx.sess.used_features.lock().get(&feature).copied() {
+            tcx.dep_graph.read_index(DepNodeIndex::from_u32(dep_node_index));
+        } else {
+            let dep_node_index = tcx
+                .dep_graph
+                .encode_side_effect(tcx, QuerySideEffect::CheckFeature { symbol: feature });
+            tcx.sess.used_features.lock().insert(feature, dep_node_index.as_u32());
+            tcx.dep_graph.read_index(dep_node_index);
+        }
+    })
+}
+
 /// This is a callback from `rustc_hir` as it cannot access the implicit state
 /// in `rustc_middle` otherwise.
 fn def_id_debug(def_id: rustc_hir::def_id::DefId, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -70,4 +90,5 @@ pub fn setup_callbacks() {
     rustc_span::SPAN_TRACK.swap(&(track_span_parent as fn(_)));
     rustc_hir::def_id::DEF_ID_DEBUG.swap(&(def_id_debug as fn(_, &mut fmt::Formatter<'_>) -> _));
     rustc_errors::TRACK_DIAGNOSTIC.swap(&(track_diagnostic as _));
+    rustc_feature::TRACK_FEATURE.swap(&(track_feature as _));
 }

compiler/rustc_lint_defs/src/builtin.rs

@@ -1088,11 +1088,6 @@
     /// crate-level [`feature` attributes].
     ///
     /// [`feature` attributes]: https://doc.rust-lang.org/nightly/unstable-book/
-    ///
-    /// Note: This lint is currently not functional, see [issue #44232] for
-    /// more details.
-    ///
-    /// [issue #44232]: https://github.com/rust-lang/rust/issues/44232
     pub UNUSED_FEATURES,
     Warn,
     "unused features found in crate-level `#[feature]` directives"

compiler/rustc_middle/src/dep_graph/graph.rs

@@ -17,6 +17,7 @@
 use rustc_macros::{Decodable, Encodable};
 use rustc_serialize::opaque::{FileEncodeResult, FileEncoder};
 use rustc_session::Session;
+use rustc_span::Symbol;
 use tracing::{debug, instrument};
 #[cfg(debug_assertions)]
 use {super::debug::EdgeFilter, std::env};
@@ -45,6 +46,11 @@ pub enum QuerySideEffect {
     /// the query as green, as that query will have the side
     /// effect dep node as a dependency.
     Diagnostic(DiagInner),
+    /// Records the feature used during query execution.
+    /// This feature will be inserted into `sess.used_features`
+    /// if we mark the query as green, as that query will have
+    /// the side effect dep node as a dependency.
+    CheckFeature { symbol: Symbol },
 }
 #[derive(Clone)]
 pub struct DepGraph {
@@ -514,29 +520,40 @@ pub fn read_index(&self, dep_node_index: DepNodeIndex) {
         }
     }
 
-    /// This encodes a diagnostic by creating a node with an unique index and associating
-    /// `diagnostic` with it, for use in the next session.
+    /// This encodes a side effect by creating a node with an unique index and associating
+    /// it with the node, for use in the next session.
     #[inline]
     pub fn record_diagnostic<'tcx>(&self, tcx: TyCtxt<'tcx>, diagnostic: &DiagInner) {
         if let Some(ref data) = self.data {
             read_deps(|task_deps| match task_deps {
                 TaskDepsRef::EvalAlways | TaskDepsRef::Ignore => return,
                 TaskDepsRef::Forbid | TaskDepsRef::Allow(..) => {
-                    self.read_index(data.encode_diagnostic(tcx, diagnostic));
+                    let dep_node_index = data
+                        .encode_side_effect(tcx, QuerySideEffect::Diagnostic(diagnostic.clone()));
+                    self.read_index(dep_node_index);
                 }
             })
         }
     }
-    /// This forces a diagnostic node green by running its side effect. `prev_index` would
-    /// refer to a node created used `encode_diagnostic` in the previous session.
+    /// This forces a side effect node green by running its side effect. `prev_index` would
+    /// refer to a node created used `encode_side_effect` in the previous session.
     #[inline]
-    pub fn force_diagnostic_node<'tcx>(
+    pub fn force_side_effect<'tcx>(&self, tcx: TyCtxt<'tcx>, prev_index: SerializedDepNodeIndex) {
+        if let Some(ref data) = self.data {
+            data.force_side_effect(tcx, prev_index);
+        }
+    }
+
+    #[inline]
+    pub fn encode_side_effect<'tcx>(
         &self,
         tcx: TyCtxt<'tcx>,
-        prev_index: SerializedDepNodeIndex,
-    ) {
+        side_effect: QuerySideEffect,
+    ) -> DepNodeIndex {
         if let Some(ref data) = self.data {
-            data.force_diagnostic_node(tcx, prev_index);
+            data.encode_side_effect(tcx, side_effect)
+        } else {
+            self.next_virtual_depnode_index()
         }
     }
 
@@ -673,10 +690,14 @@ pub fn mark_debug_loaded_from_disk(&self, dep_node: DepNode) {
         self.debug_loaded_from_disk.lock().insert(dep_node);
     }
 
-    /// This encodes a diagnostic by creating a node with an unique index and associating
-    /// `diagnostic` with it, for use in the next session.
+    /// This encodes a side effect by creating a node with an unique index and associating
+    /// it with the node, for use in the next session.
     #[inline]
-    fn encode_diagnostic<'tcx>(&self, tcx: TyCtxt<'tcx>, diagnostic: &DiagInner) -> DepNodeIndex {
+    fn encode_side_effect<'tcx>(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        side_effect: QuerySideEffect,
+    ) -> DepNodeIndex {
         // Use `send_new` so we get an unique index, even though the dep node is not.
         let dep_node_index = self.current.encoder.send_new(
             DepNode {
@@ -684,28 +705,21 @@ fn encode_diagnostic<'tcx>(&self, tcx: TyCtxt<'tcx>, diagnostic: &DiagInner) ->
                 key_fingerprint: PackedFingerprint::from(Fingerprint::ZERO),
             },
             Fingerprint::ZERO,
-            // We want the side effect node to always be red so it will be forced and emit the
-            // diagnostic.
+            // We want the side effect node to always be red so it will be forced and run the
+            // side effect.
             std::iter::once(DepNodeIndex::FOREVER_RED_NODE).collect(),
         );
-        let side_effect = QuerySideEffect::Diagnostic(diagnostic.clone());
         tcx.store_side_effect(dep_node_index, side_effect);
         dep_node_index
     }
 
-    /// This forces a diagnostic node green by running its side effect. `prev_index` would
-    /// refer to a node created used `encode_diagnostic` in the previous session.
+    /// This forces a side effect node green by running its side effect. `prev_index` would
+    /// refer to a node created used `encode_side_effect` in the previous session.
     #[inline]
-    fn force_diagnostic_node<'tcx>(&self, tcx: TyCtxt<'tcx>, prev_index: SerializedDepNodeIndex) {
+    fn force_side_effect<'tcx>(&self, tcx: TyCtxt<'tcx>, prev_index: SerializedDepNodeIndex) {
         with_deps(TaskDepsRef::Ignore, || {
             let side_effect = tcx.load_side_effect(prev_index).unwrap();
 
-            match &side_effect {
-                QuerySideEffect::Diagnostic(diagnostic) => {
-                    tcx.dcx().emit_diagnostic(diagnostic.clone());
-                }
-            }
-
             // Use `send_and_color` as `promote_node_and_deps_to_current` expects all
             // green dependencies. `send_and_color` will also prevent multiple nodes
             // being encoded for concurrent calls.
@@ -720,6 +734,16 @@ fn force_diagnostic_node<'tcx>(&self, tcx: TyCtxt<'tcx>, prev_index: SerializedD
                 std::iter::once(DepNodeIndex::FOREVER_RED_NODE).collect(),
                 true,
             );
+
+            match &side_effect {
+                QuerySideEffect::Diagnostic(diagnostic) => {
+                    tcx.dcx().emit_diagnostic(diagnostic.clone());
+                }
+                QuerySideEffect::CheckFeature { symbol } => {
+                    tcx.sess.used_features.lock().insert(*symbol, dep_node_index.as_u32());
+                }
+            }
+
             // This will just overwrite the same value for concurrent calls.
             tcx.store_side_effect(dep_node_index, side_effect);
         })

compiler/rustc_middle/src/ty/context.rs

@@ -36,7 +36,7 @@
 use rustc_hir::intravisit::VisitorExt;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::limit::Limit;
-use rustc_hir::{self as hir, HirId, Node, TraitCandidate, find_attr};
+use rustc_hir::{self as hir, CRATE_HIR_ID, HirId, Node, TraitCandidate, find_attr};
 use rustc_index::IndexVec;
 use rustc_serialize::opaque::{FileEncodeResult, FileEncoder};
 use rustc_session::Session;
@@ -1688,6 +1688,36 @@ pub fn finish(self) {
             self.sess.dcx().emit_fatal(crate::error::FailedWritingFile { path: &path, error });
         }
     }
+
+    pub fn report_unused_features(self) {
+        // Collect first to avoid holding the lock while linting.
+        let used_features = self.sess.used_features.lock();
+        let unused_features = self
+            .features()
+            .enabled_features_iter_stable_order()
+            .filter(|(f, _)| {
+                !used_features.contains_key(f)
+                // FIXME: `restricted_std` is used to tell a standard library built
+                // for a platform that it doesn't know how to support. But it
+                // could only gate a private mod (see `__restricted_std_workaround`)
+                // with `cfg(not(restricted_std))`, so it cannot be recorded as used
+                // in downstream crates. It should never be linted, but should we
+                // hack this in the linter to ignore it?
+                && f.as_str() != "restricted_std"
+            })
+            .collect::<Vec<_>>();
+
+        for (feature, span) in unused_features {
+            self.node_span_lint(
+                rustc_session::lint::builtin::UNUSED_FEATURES,
+                CRATE_HIR_ID,
+                span,
+                |lint| {
+                    lint.primary_message(format!("feature `{}` is declared but not used", feature));
+                },
+            );
+        }
+    }
 }
 
 macro_rules! nop_lift {

compiler/rustc_query_impl/src/dep_kind_vtables.rs

@@ -42,7 +42,7 @@ pub(crate) fn SideEffect<'tcx>() -> DepKindVTable<'tcx> {
             is_eval_always: false,
             key_fingerprint_style: KeyFingerprintStyle::Unit,
             force_from_dep_node_fn: Some(|tcx, _, prev_index| {
-                tcx.dep_graph.force_diagnostic_node(tcx, prev_index);
+                tcx.dep_graph.force_side_effect(tcx, prev_index);
                 true
             }),
             promote_from_disk_fn: None,

compiler/rustc_session/src/session.rs

@@ -166,6 +166,11 @@ pub struct Session {
     /// Used by `-Zmir-opt-bisect-limit` to assign an index to each
     /// optimization-pass execution candidate during this compilation.
     pub mir_opt_bisect_eval_count: AtomicUsize,
+
+    /// Enabled features that are used in the current compilation.
+    ///
+    /// The value is the `DepNodeIndex` of the node encodes the used feature.
+    pub used_features: Lock<FxHashMap<Symbol, u32>>,
 }
 
 #[derive(Clone, Copy)]
@@ -1096,6 +1101,7 @@ pub fn build_session(
         replaced_intrinsics: FxHashSet::default(), // filled by `run_compiler`
         thin_lto_supported: true,                  // filled by `run_compiler`
         mir_opt_bisect_eval_count: AtomicUsize::new(0),
+        used_features: Lock::default(),
     };
 
     validate_commandline_args_with_session_available(&sess);