Add support for inherent projections

compiler/rustc_middle/src/ty/sty.rs

@@ -1313,7 +1313,7 @@ impl<'tcx> AliasTy<'tcx> {
     ///     I_i impl subst
     ///     P_j GAT subst
     /// ```
-    pub fn rebase_args_onto_impl(
+    pub fn rebase_inherent_args_onto_impl(
         self,
         impl_args: ty::GenericArgsRef<'tcx>,
         tcx: TyCtxt<'tcx>,

compiler/rustc_trait_selection/src/solve/inherent_projection.rs

@@ -0,0 +1,42 @@
+use rustc_middle::traits::solve::{Certainty, Goal, QueryResult};
+use rustc_middle::ty;
+
+use super::EvalCtxt;
+
+impl<'tcx> EvalCtxt<'_, 'tcx> {
+    pub(super) fn normalize_inherent_associated_type(
+        &mut self,
+        goal: Goal<'tcx, ty::ProjectionPredicate<'tcx>>,
+    ) -> QueryResult<'tcx> {
+        let tcx = self.tcx();
+        let inherent = goal.predicate.projection_ty;
+        let expected = goal.predicate.term.ty().expect("inherent consts are treated separately");
+
+        let impl_def_id = tcx.parent(inherent.def_id);
+        let impl_substs = self.fresh_args_for_item(impl_def_id);
+
+        // Equate impl header and add impl where clauses
+        self.eq(
+            goal.param_env,
+            inherent.self_ty(),
+            tcx.type_of(impl_def_id).instantiate(tcx, impl_substs),
+        )?;
+        self.add_goals(
+            tcx.predicates_of(impl_def_id)
+                .instantiate(tcx, impl_substs)
+                .into_iter()
+                .map(|(pred, _)| goal.with(tcx, pred)),
+        );
+
+        // Equate IAT with the RHS of the project goal
+        let inherent_substs = inherent.rebase_inherent_args_onto_impl(impl_substs, tcx);
+        self.eq(
+            goal.param_env,
+            expected,
+            tcx.type_of(inherent.def_id).instantiate(tcx, inherent_substs),
+        )
+        .expect("expected goal term to be fully unconstrained");
+
+        self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+    }
+}

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -25,6 +25,7 @@
 mod canonicalize;
 mod eval_ctxt;
 mod fulfill;
+mod inherent_projection;
 pub mod inspect;
 mod normalize;
 mod opaques;

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -48,7 +48,7 @@ pub(super) fn compute_projection_goal(
                             self.merge_candidates(candidates)
                         }
                         ty::AssocItemContainer::ImplContainer => {
-                            bug!("IATs not supported here yet")
+                            self.normalize_inherent_associated_type(goal)
                         }
                     }
                 } else {

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1421,7 +1421,7 @@ pub fn compute_inherent_assoc_ty_args<'a, 'b, 'tcx>(
         }
     }
 
-    alias_ty.rebase_args_onto_impl(impl_args, tcx)
+    alias_ty.rebase_inherent_args_onto_impl(impl_args, tcx)
 }
 
 enum Projected<'tcx> {

tests/ui/associated-inherent-types/inference.rs

@@ -1,5 +1,7 @@
 // Testing inference capabilities.
 // check-pass
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
 
 #![feature(inherent_associated_types)]
 #![allow(incomplete_features)]