fix: don't emit `E0711` if `staged_api` not enabled

compiler/rustc_passes/src/lib_features.rs

@@ -137,6 +137,12 @@ impl<'tcx> Visitor<'tcx> for LibFeatureCollector<'tcx> {
 }
 
 fn lib_features(tcx: TyCtxt<'_>, (): ()) -> LibFeatures {
+    // If `staged_api` is not enabled then we aren't allowed to define lib
+    // features; there is no point collecting them.
+    if !tcx.features().staged_api {
+        return new_lib_features();
+    }
+
     let mut collector = LibFeatureCollector::new(tcx);
     tcx.hir().walk_attributes(&mut collector);
     collector.lib_features

tests/ui/stability-attribute/issue-106589.rs

@@ -0,0 +1,10 @@
+// #![feature(staged_api)] // note: `staged_api` not enabled
+
+#![stable(feature = "foo", since = "1.0.0")]
+//~^ ERROR stability attributes may not be used outside of the standard library
+
+#[unstable(feature = "foo", issue = "none")]
+//~^ ERROR stability attributes may not be used outside of the standard library
+fn foo_unstable() {}
+
+fn main() {}

tests/ui/stability-attribute/issue-106589.stderr

@@ -0,0 +1,15 @@
+error[E0734]: stability attributes may not be used outside of the standard library
+  --> $DIR/issue-106589.rs:6:1
+   |
+LL | #[unstable(feature = "foo", issue = "none")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0734]: stability attributes may not be used outside of the standard library
+  --> $DIR/issue-106589.rs:3:1
+   |
+LL | #![stable(feature = "foo", since = "1.0.0")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0734`.