rustdoc: treat allowed_through_unstable_modules as deprecation

This ensures `std::intrinsics::transmute` is deemphasized
in the search engine and other UI, by cleaning it into a deprecation
without propagating it through reexports when the parent module
is stable.

src/librustdoc/clean/types.rs

@@ -400,7 +400,27 @@ impl Item {
     }
 
     pub(crate) fn deprecation(&self, tcx: TyCtxt<'_>) -> Option<Deprecation> {
-        self.def_id().and_then(|did| tcx.lookup_deprecation(did))
+        self.def_id().and_then(|did| tcx.lookup_deprecation(did)).or_else(|| {
+            // `allowed_through_unstable_modules` is a bug-compatibility hack for old rustc
+            // versions; the paths that are exposed through it are "deprecated" because they
+            // were never supposed to work at all.
+            let stab = self.stability(tcx)?;
+            if let rustc_attr_parsing::StabilityLevel::Stable {
+                allowed_through_unstable_modules: true,
+                ..
+            } = stab.level
+            {
+                Some(Deprecation {
+                    // FIXME(#131676, #135003): when a note is added to this stability tag,
+                    // translate it here
+                    since: rustc_attr_parsing::DeprecatedSince::Unspecified,
+                    note: None,
+                    suggestion: None,
+                })
+            } else {
+                None
+            }
+        })
     }
 
     pub(crate) fn inner_docs(&self, tcx: TyCtxt<'_>) -> bool {

src/librustdoc/passes/propagate_stability.rs

@@ -107,6 +107,15 @@ fn merge_stability(
             || parent_stab.stable_since().is_some_and(|parent_since| parent_since > own_since))
     {
         parent_stability
+    } else if let Some(mut own_stab) = own_stability
+        && let StabilityLevel::Stable { since, allowed_through_unstable_modules: true } =
+            own_stab.level
+        && let Some(parent_stab) = parent_stability
+        && parent_stab.is_stable()
+    {
+        // this property does not apply transitively through re-exports
+        own_stab.level = StabilityLevel::Stable { since, allowed_through_unstable_modules: false };
+        Some(own_stab)
     } else {
         own_stability
     }