1 files changed, 72 insertions, 0 deletions
diff --git a/src/librustdoc/passes/propagate_stability.rs b/src/librustdoc/passes/propagate_stability.rs
new file mode 100644
index 00000000000..f51e993bfa5
--- /dev/null
+++ b/src/librustdoc/passes/propagate_stability.rs
@@ -0,0 +1,72 @@
+//! Propagates stability to child items.
+//!
+//! The purpose of this pass is to make items whose parents are "more unstable"
+//! than the item itself inherit the parent's stability.
+//! For example, [`core::error::Error`] is marked as stable since 1.0.0, but the
+//! [`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_hir::def_id::CRATE_DEF_ID;
+
+use crate::clean::{Crate, Item, ItemId};
+use crate::core::DocContext;
+use crate::fold::DocFolder;
+use crate::passes::Pass;
+
+pub(crate) const PROPAGATE_STABILITY: Pass = Pass {
+    name: "propagate-stability",
+    run: propagate_stability,
+    description: "propagates stability to child items",
+};
+
+pub(crate) fn propagate_stability(cr: Crate, cx: &mut DocContext<'_>) -> Crate {
+    let crate_stability = cx.tcx.lookup_stability(CRATE_DEF_ID);
+    StabilityPropagator { parent_stability: crate_stability, cx }.fold_crate(cr)
+}
+
+struct StabilityPropagator<'a, 'tcx> {
+    parent_stability: Option<Stability>,
+    cx: &'a mut DocContext<'tcx>,
+}
+
+impl<'a, 'tcx> DocFolder for StabilityPropagator<'a, 'tcx> {
+    fn fold_item(&mut self, mut item: Item) -> Option<Item> {
+        let parent_stability = self.parent_stability;
+
+        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(own_stab) = own_stability
+                    && let StabilityLevel::Stable {
+                        since: own_since,
+                        allowed_through_unstable_modules: false,
+                    } = own_stab.level
+                    && let Some(parent_stab) = parent_stability
+                    && (parent_stab.is_unstable()
+                        || parent_stab
+                            .stable_since()
+                            .is_some_and(|parent_since| parent_since > own_since))
+                {
+                    parent_stability
+                } else {
+                    own_stability
+                }
+            }
+            ItemId::Auto { .. } | ItemId::Blanket { .. } => {
+                // For now, we do now show stability for synthesized impls.
+                None
+            }
+        };
+
+        item.inner.stability = stability;
+        self.parent_stability = stability;
+        let item = self.fold_item_recur(item);
+        self.parent_stability = parent_stability;
+
+        Some(item)
+    }
+}