WIP: Lint unused features#152164

mu001999 · 2026-02-05T11:39:56Z

jyn514

this is neat! in the future, it would be easier to review if you split the rename into a separate commit from the change in behavior.

could you add some tests for this please?

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compiler/rustc_interface/src/passes.rs

mu001999 · 2026-02-05T15:33:26Z

Initial implementation completed. I plan to add more tests and check whether features in rustc are reported as unused correctly or not.

jyn514 · 2026-02-05T15:55:01Z

compiler/rustc_feature/src/unstable.rs

+    pub fn unused_features(&self) -> Vec<(Symbol, Span)> {
+        let used_features = USED_FEATURES.lock().unwrap();
+        self.enabled_features_iter_stable_order()
+            .filter(|(f, _)| !used_features.contains(f))
+            .collect()
+    }
+
+    /// Is the given feature enabled (via `#[feature(...)]`)?
+    pub fn enabled(&self, feature: Symbol) -> bool {
+        USED_FEATURES.lock().unwrap().insert(feature);
+        self.features.enabled(feature)
+    }
+
+    /// Returns a list of [`EnabledLangFeature`] with info about:
+    ///
+    /// - Feature gate name.
+    /// - The span of the `#[feature]` attribute.
+    /// - For stable language features, version info for when it was stabilized.
+    pub fn enabled_lang_features(&self) -> &Vec<EnabledLangFeature> {
+        self.features.enabled_lang_features()
+    }
+
+    pub fn enabled_lib_features(&self) -> &Vec<EnabledLibFeature> {
+        self.features.enabled_lib_features()
+    }
+
+    pub fn enabled_features(&self) -> &FxHashSet<Symbol> {
+        &self.features.enabled_features()
+    }
+
+    /// Returns a iterator of enabled features in stable order.
+    pub fn enabled_features_iter_stable_order(
+        &self,
+    ) -> impl Iterator<Item = (Symbol, Span)> + Clone {
+        self.features.enabled_features_iter_stable_order()
+    }
+
+    pub fn dump_feature_usage_metrics(
+        &self,
+        metrics_path: PathBuf,
+    ) -> Result<(), Box<dyn std::error::Error>> {
+        self.features.dump_feature_usage_metrics(metrics_path)
+    }
+
+    /// Some features are known to be incomplete and using them is likely to have
+    /// unanticipated results, such as compiler crashes. We warn the user about these
+    /// to alert them.
+    pub fn incomplete(&self, feature: Symbol) -> bool {
+        self.features.incomplete(feature)
+    }
+
+    /// Some features are internal to the compiler and standard library and should not
+    /// be used in normal projects. We warn the user about these to alert them.
+    pub fn internal(&self, feature: Symbol) -> bool {
+        self.features.internal(feature)
+    }
+}


i think for incremental soundness, none of these can be used outside the query system, right? maybe document that?

rust-log-analyzer · 2026-02-05T15:56:45Z

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

##[group]Runner Image Provisioner
Hosted Compute Agent
Version: 20260123.484
Commit: 6bd6555ca37d84114959e1c76d2c01448ff61c5d
Build Date: 2026-01-23T19:41:17Z
Worker ID: {afd8edb8-0350-47b3-b837-eef7332e6984}
Azure Region: southcentralus
##[endgroup]
##[group]VM Image
- OS: Linux (arm64)
- Source: Partner
- Name: Ubuntu 24.04 by Arm Limited
---
    |
206 | #![feature(riscv_target_feature)]
    |            ^^^^^^^^^^^^^^^^^^^^

warning: feature `rtm_target_feature` is declared but not used
   --> library/core/src/lib.rs:207:12
    |
207 | #![feature(rtm_target_feature)]
    |            ^^^^^^^^^^^^^^^^^^

warning: feature `s390x_target_feature` is declared but not used
   --> library/core/src/lib.rs:208:12
    |
---
   Compiling unwind v0.0.0 (/checkout/library/unwind)
warning: feature `libc` is declared but not used
 --> library/unwind/src/lib.rs:7:47
  |
7 | #![cfg_attr(not(target_env = "msvc"), feature(libc))]
  |                                               ^^^^
  |
  = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default

[RUSTC-TIMING] unwind test:false 0.070
---

warning: feature `string_replace_in_place` is declared but not used
  --> library/alloc/src/lib.rs:88:50
   |
88 | #![cfg_attr(not(no_global_oom_handling), feature(string_replace_in_place))]
   |                                                  ^^^^^^^^^^^^^^^^^^^^^^^

warning: feature `assert_matches` is declared but not used
  --> library/alloc/src/lib.rs:92:12
   |

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jyn514 reviewed Feb 5, 2026

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compiler/rustc_interface/src/passes.rs Outdated Show resolved Hide resolved

mu001999 force-pushed the lint/unused_features branch from a3c0247 to cb9b84c Compare February 5, 2026 15:25

mu001999 added 3 commits February 5, 2026 23:30

Use TrackedFeatures to track used features when calling enabled

fe3ee49

Replace Features usages with TrackedFeatures

46111ff

Add tests for unused-features

cd83e12

mu001999 force-pushed the lint/unused_features branch from cb9b84c to cd83e12 Compare February 5, 2026 15:31

jyn514 reviewed Feb 5, 2026

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WIP: Lint unused features#152164