Make const trait aliases work in next solver

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -1031,9 +1031,11 @@ pub(super) fn const_conditions<'tcx>(
         Node::Item(item) => match item.kind {
             hir::ItemKind::Impl(impl_) => (impl_.generics, None, false),
             hir::ItemKind::Fn { generics, .. } => (generics, None, false),
-            hir::ItemKind::TraitAlias(_, _, generics, supertraits)
-            | hir::ItemKind::Trait(_, _, _, _, generics, supertraits, _) => {
-                (generics, Some((item.owner_id.def_id, supertraits)), false)
+            hir::ItemKind::Trait(_, _, _, _, generics, supertraits, _) => {
+                (generics, Some((Some(item.owner_id.def_id), supertraits)), false)
+            }
+            hir::ItemKind::TraitAlias(_, _, generics, supertraits) => {
+                (generics, Some((None, supertraits)), false)
             }
             _ => bug!("const_conditions called on wrong item: {def_id:?}"),
         },
@@ -1090,12 +1092,14 @@ pub(super) fn const_conditions<'tcx>(
     }
 
     if let Some((def_id, supertraits)) = trait_def_id_and_supertraits {
-        // We've checked above that the trait is conditionally const.
-        bounds.push((
-            ty::Binder::dummy(ty::TraitRef::identity(tcx, def_id.to_def_id()))
-                .to_host_effect_clause(tcx, ty::BoundConstness::Maybe),
-            DUMMY_SP,
-        ));
+        if let Some(def_id) = def_id {
+            // We've checked above that the trait is conditionally const.
+            bounds.push((
+                ty::Binder::dummy(ty::TraitRef::identity(tcx, def_id.to_def_id()))
+                    .to_host_effect_clause(tcx, ty::BoundConstness::Maybe),
+                DUMMY_SP,
+            ));
+        }
 
         icx.lowerer().lower_bounds(
             tcx.types.self_param,

compiler/rustc_next_trait_solver/src/solve/effect_goals.rs

@@ -196,10 +196,35 @@ where
     }
 
     fn consider_trait_alias_candidate(
-        _ecx: &mut EvalCtxt<'_, D>,
-        _goal: Goal<I, Self>,
+        ecx: &mut EvalCtxt<'_, D>,
+        goal: Goal<I, Self>,
     ) -> Result<Candidate<I>, NoSolution> {
-        unreachable!("trait aliases are never const")
+        let cx = ecx.cx();
+
+        ecx.probe_builtin_trait_candidate(BuiltinImplSource::Misc).enter(|ecx| {
+            let where_clause_bounds = cx
+                .predicates_of(goal.predicate.def_id().into())
+                .iter_instantiated(cx, goal.predicate.trait_ref.args)
+                .map(|p| goal.with(cx, p));
+
+            let const_conditions = cx
+                .const_conditions(goal.predicate.def_id().into())
+                .iter_instantiated(cx, goal.predicate.trait_ref.args)
+                .map(|bound_trait_ref| {
+                    goal.with(
+                        cx,
+                        bound_trait_ref.to_host_effect_clause(cx, goal.predicate.constness),
+                    )
+                });
+            // While you could think of trait aliases to have a single builtin impl
+            // which uses its implied trait bounds as where-clauses, using
+            // `GoalSource::ImplWhereClause` here would be incorrect, as we also
+            // impl them, which means we're "stepping out of the impl constructor"
+            // again. To handle this, we treat these cycles as ambiguous for now.
+            ecx.add_goals(GoalSource::Misc, where_clause_bounds);
+            ecx.add_goals(GoalSource::Misc, const_conditions);
+            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+        })
     }
 
     fn consider_builtin_sizedness_candidates(