Only lint unused features if they are unstable

compiler/rustc_passes/src/stability.rs

@@ -964,6 +964,9 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
     let mut lang_features = UnordSet::default();
     for EnabledLangFeature { gate_name, attr_sp, stable_since } in enabled_lang_features {
         if let Some(version) = stable_since {
+            // Mark the feature as enabled, to ensure that it is not marked as unused.
+            let _ = tcx.features().enabled(*gate_name);
+
             // Warn if the user has enabled an already-stable lang feature.
             unnecessary_stable_feature_lint(tcx, *attr_sp, *gate_name, *version);
         }
@@ -1021,6 +1024,9 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
             if let FeatureStability::AcceptedSince(since) = stability
                 && let Some(span) = remaining_lib_features.get(&feature)
             {
+                // Mark the feature as enabled, to ensure that it is not marked as unused.
+                let _ = tcx.features().enabled(feature);
+
                 // Warn if the user has enabled an already-stable lib feature.
                 if let Some(implies) = all_implications.get(&feature) {
                     unnecessary_partially_stable_feature_lint(tcx, *span, feature, *implies, since);

tests/ui/lint/unused-features/stable-features.rs

@@ -0,0 +1,17 @@
+//@ check-pass
+
+#![deny(warnings)]
+#![allow(stable_features)]
+
+// Lang feature
+#![feature(lint_reasons)]
+
+// Lib feature
+#![feature(euclidean_division)]
+
+#[allow(unused_variables, reason = "my reason")]
+fn main() {
+    let x = ();
+
+    let _ = 42.0_f32.div_euclid(3.0);
+}

tests/ui/lint/unused-features/unused-library-features.rs

@@ -5,8 +5,7 @@
 #![feature(step_trait)]
 //~^ ERROR feature `step_trait` is declared but not used
 #![feature(is_sorted)]
-//~^ ERROR feature `is_sorted` is declared but not used
-//~^^ WARN the feature `is_sorted` has been stable since 1.82.0 and no longer requires an attribute to enable
+//~^ WARN the feature `is_sorted` has been stable since 1.82.0 and no longer requires an attribute to enable
 
 // Enabled via cfg_attr, unused
 #![cfg_attr(all(), feature(slice_ptr_get))]

tests/ui/lint/unused-features/unused-library-features.stderr

@@ -18,17 +18,11 @@ note: the lint level is defined here
 LL | #![deny(unused_features)]
    |         ^^^^^^^^^^^^^^^
 
-error: feature `is_sorted` is declared but not used
-  --> $DIR/unused-library-features.rs:7:12
-   |
-LL | #![feature(is_sorted)]
-   |            ^^^^^^^^^
-
 error: feature `slice_ptr_get` is declared but not used
-  --> $DIR/unused-library-features.rs:12:28
+  --> $DIR/unused-library-features.rs:11:28
    |
 LL | #![cfg_attr(all(), feature(slice_ptr_get))]
    |                            ^^^^^^^^^^^^^
 
-error: aborting due to 3 previous errors; 1 warning emitted
+error: aborting due to 2 previous errors; 1 warning emitted