rustdoc: report all unstable features needed for items

Author: dianne

src/librustdoc/passes/propagate_stability.rs

@@ -6,7 +6,7 @@
 //! [`core::error`] module is marked as stable since 1.81.0, so we want to show
 //! [`core::error::Error`] as stable since 1.81.0 as well.
 
-use rustc_attr::{Stability, StabilityLevel};
+use rustc_attr::Stability;
 use rustc_hir::def_id::CRATE_DEF_ID;
 
 use crate::clean::{Crate, Item, ItemId};
@@ -32,27 +32,46 @@ struct StabilityPropagator<'a, 'tcx> {
 
 impl<'a, 'tcx> DocFolder for StabilityPropagator<'a, 'tcx> {
     fn fold_item(&mut self, mut item: Item) -> Option<Item> {
+        use rustc_attr::StabilityLevel::*;
+
         let parent_stability = self.parent_stability.clone();
 
         let stability = match item.item_id {
             ItemId::DefId(def_id) => {
                 let own_stability = self.cx.tcx.lookup_stability(def_id);
 
-                // If any of the item's parents was stabilized later or is still unstable,
-                // then use the parent's stability instead.
-                if let Some(ref own_stab) = own_stability
-                    && let StabilityLevel::Stable {
-                        since: own_since,
-                        allowed_through_unstable_modules: false,
-                        ..
-                    } = own_stab.level
+                if let Some(own_stab) = own_stability
                     && let Some(ref parent_stab) = parent_stability
-                    && (parent_stab.is_unstable()
-                        || parent_stab
-                            .stable_since()
-                            .is_some_and(|parent_since| parent_since > own_since))
                 {
-                    parent_stability.clone()
+                    match own_stab.level {
+                        // If any of the item's parents was stabilized later or is still unstable,
+                        // then use the parent's stability instead.
+                        Stable {
+                            since: own_since,
+                            allowed_through_unstable_modules: false,
+                            ..
+                        } if parent_stab.is_unstable()
+                            || parent_stab
+                                .stable_since()
+                                .is_some_and(|parent_since| parent_since > own_since) =>
+                        {
+                            parent_stability.clone()
+                        }
+                        // If any of an unstable item's parents depend on other unstable features,
+                        // then use those as well.
+                        Unstable { unstables: ref own_gates, is_soft }
+                            if let Unstable { unstables: parent_gates, .. } =
+                                &parent_stab.level =>
+                        {
+                            let missing_unstables = parent_gates
+                                .iter()
+                                .filter(|p| !own_gates.iter().any(|u| u.feature == p.feature));
+                            let unstables =
+                                own_gates.iter().chain(missing_unstables).cloned().collect();
+                            Some(Stability { level: Unstable { unstables, is_soft } })
+                        }
+                        _ => own_stability.cloned(),
+                    }
                 } else {
                     own_stability.cloned()
                 }

tests/rustdoc/stability.rs

@@ -39,6 +39,12 @@ pub mod unstable {
         #[stable(feature = "rust1", since = "1.0.0")]
         pub struct Inner;
     }
+
+    //@ has stability/unstable/fn.nested_unstable.html \
+    //      '//span[@class="item-info"]//div[@class="stab unstable"]' \
+    //      'This is a nightly-only experimental API. (test, unstable)'
+    #[unstable(feature = "test", issue = "none")]
+    pub fn nested_unstable() {}
 }
 
 #[stable(feature = "rust2", since = "2.2.2")]