Make gen blocks implement the Iterator trait

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -199,7 +199,15 @@ pub(super) trait GoalKind<'tcx>:
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
-    /// A coroutine (that doesn't come from an `async` desugaring) is known to
+    /// A coroutine (that comes from a `gen` desugaring) is known to implement
+    /// `Iterator<Item = O>`, where `O` is given by the generator's yield type
+    /// that was computed during type-checking.
+    fn consider_builtin_iterator_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx>;
+
+    /// A coroutine (that doesn't come from an `async` or `gen` desugaring) is known to
     /// implement `Coroutine<R, Yield = Y, Return = O>`, given the resume, yield,
     /// and return types of the coroutine computed during type-checking.
     fn consider_builtin_coroutine_candidate(
@@ -552,6 +560,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             G::consider_builtin_pointee_candidate(self, goal)
         } else if lang_items.future_trait() == Some(trait_def_id) {
             G::consider_builtin_future_candidate(self, goal)
+        } else if lang_items.iterator_trait() == Some(trait_def_id) {
+            G::consider_builtin_iterator_candidate(self, goal)
         } else if lang_items.gen_trait() == Some(trait_def_id) {
             G::consider_builtin_coroutine_candidate(self, goal)
         } else if lang_items.discriminant_kind_trait() == Some(trait_def_id) {

compiler/rustc_trait_selection/src/solve/project_goals/mod.rs

@@ -485,6 +485,37 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         )
     }
 
+    fn consider_builtin_iterator_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx> {
+        let self_ty = goal.predicate.self_ty();
+        let ty::Coroutine(def_id, args, _) = *self_ty.kind() else {
+            return Err(NoSolution);
+        };
+
+        // Coroutines are not Iterators unless they come from `gen` desugaring
+        let tcx = ecx.tcx();
+        if !tcx.coroutine_is_gen(def_id) {
+            return Err(NoSolution);
+        }
+
+        let term = args.as_coroutine().yield_ty().into();
+
+        Self::consider_implied_clause(
+            ecx,
+            goal,
+            ty::ProjectionPredicate {
+                projection_ty: ty::AliasTy::new(ecx.tcx(), goal.predicate.def_id(), [self_ty]),
+                term,
+            }
+            .to_predicate(tcx),
+            // Technically, we need to check that the iterator type is Sized,
+            // but that's already proven by the generator being WF.
+            [],
+        )
+    }
+
     fn consider_builtin_coroutine_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
@@ -496,7 +527,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
 
         // `async`-desugared coroutines do not implement the coroutine trait
         let tcx = ecx.tcx();
-        if tcx.coroutine_is_async(def_id) {
+        if !tcx.is_general_coroutine(def_id) {
             return Err(NoSolution);
         }
 
@@ -523,7 +554,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
                 term,
             }
             .to_predicate(tcx),
-            // Technically, we need to check that the future type is Sized,
+            // Technically, we need to check that the coroutine type is Sized,
             // but that's already proven by the coroutine being WF.
             [],
         )

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -335,6 +335,30 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
     }
 
+    fn consider_builtin_iterator_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx> {
+        if goal.predicate.polarity != ty::ImplPolarity::Positive {
+            return Err(NoSolution);
+        }
+
+        let ty::Coroutine(def_id, _, _) = *goal.predicate.self_ty().kind() else {
+            return Err(NoSolution);
+        };
+
+        // Coroutines are not iterators unless they come from `gen` desugaring
+        let tcx = ecx.tcx();
+        if !tcx.coroutine_is_gen(def_id) {
+            return Err(NoSolution);
+        }
+
+        // Gen coroutines unconditionally implement `Iterator`
+        // Technically, we need to check that the iterator output type is Sized,
+        // but that's already proven by the coroutines being WF.
+        ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+    }
+
     fn consider_builtin_coroutine_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
@@ -350,7 +374,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
 
         // `async`-desugared coroutines do not implement the coroutine trait
         let tcx = ecx.tcx();
-        if tcx.coroutine_is_async(def_id) {
+        if !tcx.is_general_coroutine(def_id) {
             return Err(NoSolution);
         }