passes: propagate unstability for traits

Unstability is propagated when staged API is not used and
`-Zforce-unstable-if-unmarked` is, but this only happened for const
functions not const traits, which ends up being an issue for some of the
minicore tests of codegen backends when introducing the sizedness traits.

compiler/rustc_passes/src/stability.rs

@@ -155,7 +155,10 @@ impl<'a, 'tcx> Annotator<'a, 'tcx> {
             if let Some(stab) = self.parent_stab {
                 if inherit_deprecation.yes() && stab.is_unstable() {
                     self.index.stab_map.insert(def_id, stab);
-                    if fn_sig.is_some_and(|s| s.header.is_const()) {
+                    let is_const = fn_sig.is_some_and(|s| s.header.is_const())
+                        || (self.tcx.def_kind(def_id) == DefKind::Trait
+                            && self.tcx.is_const_trait(def_id.to_def_id()));
+                    if is_const {
                         self.index.const_stab_map.insert(
                             def_id,
                             ConstStability::unmarked(const_stability_indirect, stab),