trait_goals: Implement support for trait aliases

Author: compiler-errors

compiler/rustc_middle/src/ty/mod.rs

@@ -1252,6 +1252,10 @@ impl<'tcx> InstantiatedPredicates<'tcx> {
     pub fn is_empty(&self) -> bool {
         self.predicates.is_empty()
     }
+
+    pub fn into_iter(self) -> impl Iterator<Item = (ty::Predicate<'tcx>, Span)> {
+        std::iter::zip(self.predicates, self.spans)
+    }
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable, TyEncodable, TyDecodable, Lift)]

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -47,6 +47,11 @@ pub(super) trait GoalKind<'tcx>: TypeFoldable<'tcx> + Copy {
         impl_def_id: DefId,
     );
 
+    fn consider_trait_alias_candidate(
+        acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
+        goal: Goal<'tcx, Self>,
+    );
+
     fn consider_alias_bound_candidates(
         acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
         goal: Goal<'tcx, Self>,
@@ -106,6 +111,8 @@ impl<'a, 'tcx, G: GoalKind<'tcx>> AssemblyCtxt<'a, 'tcx, G> {
 
         acx.assemble_auto_trait_candidates(goal);
 
+        acx.assemble_trait_alias_candidates(goal);
+
         acx.assemble_fn_like_candidates(goal);
 
         acx.candidates
@@ -207,6 +214,12 @@ impl<'a, 'tcx, G: GoalKind<'tcx>> AssemblyCtxt<'a, 'tcx, G> {
         }
     }
 
+    fn assemble_trait_alias_candidates(&mut self, goal: Goal<'tcx, G>) {
+        if self.cx.tcx.is_trait_alias(goal.predicate.trait_def_id(self.cx.tcx)) {
+            G::consider_trait_alias_candidate(self, goal);
+        }
+    }
+
     fn assemble_fn_like_candidates(&mut self, goal: Goal<'tcx, G>) {
         let tcx = self.cx.tcx;
         let trait_def_id = goal.predicate.trait_def_id(tcx);

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -212,6 +212,13 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         })
     }
 
+    fn consider_trait_alias_candidate(
+        _acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
+        _goal: Goal<'tcx, Self>,
+    ) {
+        // Trait aliases never have (their own) associated types
+    }
+
     fn consider_alias_bound_candidates(
         _acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
         _goal: Goal<'tcx, ProjectionPredicate<'tcx>>,

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -23,6 +23,8 @@ use rustc_target::spec::abi::Abi;
 pub(super) enum CandidateSource {
     /// Some user-defined impl with the given `DefId`.
     Impl(DefId),
+    /// The automatic implementation of a trait alias.
+    TraitAlias,
     /// The n-th caller bound in the `param_env` of our goal.
     ///
     /// This is pretty much always a bound from the `where`-clauses of the
@@ -122,6 +124,24 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         })
     }
 
+    fn consider_trait_alias_candidate(
+        acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
+        goal: Goal<'tcx, Self>,
+    ) {
+        let tcx = acx.cx.tcx;
+        acx.infcx.probe(|_| {
+            let nested_goals = tcx
+                .predicates_of(goal.predicate.def_id())
+                .instantiate_own(tcx, goal.predicate.trait_ref.substs)
+                .predicates
+                .into_iter()
+                .map(|pred| goal.with(tcx, pred))
+                .collect();
+            let Ok(certainty) = acx.cx.evaluate_all(acx.infcx, nested_goals) else { return };
+            acx.try_insert_candidate(CandidateSource::TraitAlias, certainty);
+        })
+    }
+
     fn consider_alias_bound_candidates(
         acx: &mut AssemblyCtxt<'_, 'tcx, Self>,
         goal: Goal<'tcx, Self>,
@@ -414,6 +434,7 @@ impl<'tcx> EvalCtxt<'tcx> {
         // FIXME: implement this
         match (candidate.source, other.source) {
             (CandidateSource::Impl(_), _)
+            | (CandidateSource::TraitAlias, _)
             | (CandidateSource::ParamEnv(_), _)
             | (CandidateSource::AliasBound(_), _)
             | (CandidateSource::ObjectBound(_), _)