Make const trait aliases work in next solver

Author: oli-obk

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(

tests/ui/consts/trait_alias.fail.stderr

@@ -1,23 +1,9 @@
 error[E0277]: the trait bound `T: [const] Baz` is not satisfied
- --> $DIR/trait_alias.rs:20:11
+ --> $DIR/trait_alias.rs:22:11
  |
 LL | x.baz();
  | ^^^
 
-error[E0277]: the trait bound `(): const Foo` is not satisfied
- --> $DIR/trait_alias.rs:25:19
- |
-LL | const _: () = foo(&());
- | --- ^^^
- | |
- | required by a bound introduced by this call
- |
-note: required by a bound in `foo`
- --> $DIR/trait_alias.rs:16:17
- |
-LL | const fn foo<T: [const] Foo>(x: &T) {
- | ^^^^^^^^^^^ required by this bound in `foo`
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/consts/trait_alias.pass.stderr

@@ -1,5 +1,7 @@
 #![feature(trait_alias, const_trait_impl)]
 //@ revisions: pass fail
+//@ compile-flags: -Znext-solver
+//@[pass] check-pass
 
 const trait Bar {
  fn bar(&self) {}
@@ -23,6 +25,5 @@ const fn foo<T: [const] Foo>(x: &T) {
 }
 
 const _: () = foo(&());
-//~^ ERROR: `(): const Foo` is not satisfied
 
 fn main() {}