Implement FusedIterator for gen block

compiler/rustc_hir/src/lang_items.rs

@@ -214,6 +214,7 @@ language_item_table! {
     FnOnceOutput,            sym::fn_once_output,      fn_once_output,             Target::AssocTy,        GenericRequirement::None;
 
     Iterator,                sym::iterator,            iterator_trait,             Target::Trait,          GenericRequirement::Exact(0);
+    FusedIterator,           sym::fused_iterator,      fused_iterator_trait,       Target::Trait,          GenericRequirement::Exact(0);
     Future,                  sym::future_trait,        future_trait,               Target::Trait,          GenericRequirement::Exact(0);
     AsyncIterator,           sym::async_iterator,      async_iterator_trait,       Target::Trait,          GenericRequirement::Exact(0);
 

compiler/rustc_span/src/symbol.rs

@@ -207,6 +207,7 @@ symbols! {
         FromResidual,
         FsOpenOptions,
         FsPermissions,
+        FusedIterator,
         Future,
         FutureOutput,
         GlobalAlloc,
@@ -885,6 +886,7 @@ symbols! {
         fsub_algebraic,
         fsub_fast,
         fundamental,
+        fused_iterator,
         future,
         future_trait,
         gdb_script_file,

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

@@ -215,6 +215,13 @@ pub(super) trait GoalKind<'tcx>:
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx>;
 
+    /// A coroutine (that comes from a `gen` desugaring) is known to implement
+    /// `FusedIterator`
+    fn consider_builtin_fused_iterator_candidate(
+        ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx>;
+
     fn consider_builtin_async_iterator_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
@@ -497,6 +504,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             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.fused_iterator_trait() == Some(trait_def_id) {
+            G::consider_builtin_fused_iterator_candidate(self, goal)
         } else if lang_items.async_iterator_trait() == Some(trait_def_id) {
             G::consider_builtin_async_iterator_candidate(self, goal)
         } else if lang_items.coroutine_trait() == Some(trait_def_id) {

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

@@ -647,6 +647,13 @@ impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
         )
     }
 
+    fn consider_builtin_fused_iterator_candidate(
+        _ecx: &mut EvalCtxt<'_, 'tcx>,
+        goal: Goal<'tcx, Self>,
+    ) -> QueryResult<'tcx> {
+        bug!("`FusedIterator` does not have an associated type: {:?}", goal);
+    }
+
     fn consider_builtin_async_iterator_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -456,6 +456,28 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
     }
 
+    fn consider_builtin_fused_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 `FusedIterator`
+        ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+    }
+
     fn consider_builtin_async_iterator_candidate(
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -118,6 +118,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     self.assemble_future_candidates(obligation, &mut candidates);
                 } else if lang_items.iterator_trait() == Some(def_id) {
                     self.assemble_iterator_candidates(obligation, &mut candidates);
+                } else if lang_items.fused_iterator_trait() == Some(def_id) {
+                    self.assemble_fused_iterator_candidates(obligation, &mut candidates);
                 } else if lang_items.async_iterator_trait() == Some(def_id) {
                     self.assemble_async_iterator_candidates(obligation, &mut candidates);
                 } else if lang_items.async_fn_kind_helper() == Some(def_id) {
@@ -302,15 +304,32 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         let self_ty = obligation.self_ty().skip_binder();
-        if let ty::Coroutine(did, ..) = self_ty.kind() {
         // gen constructs get lowered to a special kind of coroutine that
         // should directly `impl Iterator`.
-            if self.tcx().coroutine_is_gen(*did) {
+        if let ty::Coroutine(did, ..) = self_ty.kind()
+            && self.tcx().coroutine_is_gen(*did)
+        {
             debug!(?self_ty, ?obligation, "assemble_iterator_candidates",);
 
             candidates.vec.push(IteratorCandidate);
         }
     }
+
+    fn assemble_fused_iterator_candidates(
+        &mut self,
+        obligation: &PolyTraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) {
+        let self_ty = obligation.self_ty().skip_binder();
+        // gen constructs get lowered to a special kind of coroutine that
+        // should directly `impl FusedIterator`.
+        if let ty::Coroutine(did, ..) = self_ty.kind()
+            && self.tcx().coroutine_is_gen(*did)
+        {
+            debug!(?self_ty, ?obligation, "assemble_fused_iterator_candidates",);
+
+            candidates.vec.push(BuiltinCandidate { has_nested: false });
+        }
     }
 
     fn assemble_async_iterator_candidates(

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -267,6 +267,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 self.copy_clone_conditions(obligation)
             } else if Some(trait_def) == lang_items.clone_trait() {
                 self.copy_clone_conditions(obligation)
+            } else if Some(trait_def) == lang_items.fused_iterator_trait() {
+                self.fused_iterator_conditions(obligation)
             } else {
                 bug!("unexpected builtin trait {:?}", trait_def)
             };

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2259,6 +2259,20 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         }
     }
 
+    fn fused_iterator_conditions(
+        &mut self,
+        obligation: &PolyTraitObligation<'tcx>,
+    ) -> BuiltinImplConditions<'tcx> {
+        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
+        if let ty::Coroutine(did, ..) = *self_ty.kind()
+            && self.tcx().coroutine_is_gen(did)
+        {
+            BuiltinImplConditions::Where(ty::Binder::dummy(Vec::new()))
+        } else {
+            BuiltinImplConditions::None
+        }
+    }
+
     /// For default impls, we need to break apart a type into its
     /// "constituent types" -- meaning, the types that it contains.
     ///

library/core/src/iter/traits/marker.rs

@@ -28,6 +28,7 @@ pub unsafe trait TrustedFused {}
 #[rustc_unsafe_specialization_marker]
 // FIXME: this should be a #[marker] and have another blanket impl for T: TrustedFused
 // but that ICEs iter::Fuse specializations.
+#[cfg_attr(not(bootstrap), lang = "fused_iterator")]
 pub trait FusedIterator: Iterator {}
 
 #[stable(feature = "fused", since = "1.26.0")]

tests/ui/coroutine/gen_block_is_fused_iter.rs

@@ -0,0 +1,21 @@
+//@ revisions: next old
+//@compile-flags: --edition 2024 -Zunstable-options
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+#![feature(gen_blocks)]
+
+use std::iter::FusedIterator;
+
+fn foo() -> impl FusedIterator {
+    gen { yield 42 }
+}
+
+fn bar() -> impl FusedIterator<Item = u16> {
+    gen { yield 42 }
+}
+
+fn baz() -> impl FusedIterator + Iterator<Item = i64> {
+    gen { yield 42 }
+}
+
+fn main() {}