Make gen blocks implement the Iterator trait

compiler/rustc_hir_typeck/src/closure.rs

@@ -652,6 +652,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         },
                     )
                 }
+                Some(hir::CoroutineKind::Gen(hir::CoroutineSource::Fn)) => {
+                    todo!("gen closures do not exist yet")
+                }
 
                 _ => astconv.ty_infer(None, decl.output.span()),
             },

compiler/rustc_middle/src/traits/select.rs

@@ -144,6 +144,10 @@ pub enum SelectionCandidate<'tcx> {
     /// generated for an async construct.
     FutureCandidate,
 
+    /// Implementation of an `Iterator` trait by one of the generator types
+    /// generated for a gen construct.
+    IteratorCandidate,
+
     /// Implementation of a `Fn`-family trait by one of the anonymous
     /// types generated for a fn pointer type (e.g., `fn(int) -> int`)
     FnPointerCandidate {

compiler/rustc_middle/src/ty/context.rs

@@ -782,6 +782,17 @@ impl<'tcx> TyCtxt<'tcx> {
         matches!(self.coroutine_kind(def_id), Some(hir::CoroutineKind::Async(_)))
     }
 
+    /// Returns `true` if the node pointed to by `def_id` is a general coroutine that implements `Coroutine`.
+    /// This means it is neither an `async` or `gen` construct.
+    pub fn is_general_coroutine(self, def_id: DefId) -> bool {
+        matches!(self.coroutine_kind(def_id), Some(hir::CoroutineKind::Coroutine))
+    }
+
+    /// Returns `true` if the node pointed to by `def_id` is a coroutine for a gen construct.
+    pub fn coroutine_is_gen(self, def_id: DefId) -> bool {
+        matches!(self.coroutine_kind(def_id), Some(hir::CoroutineKind::Gen(_)))
+    }
+
     pub fn stability(self) -> &'tcx stability::Index {
         self.stability_index(())
     }

compiler/rustc_span/src/symbol.rs

@@ -226,6 +226,7 @@ symbols! {
         IpAddr,
         IrTyKind,
         Is,
+        Item,
         ItemContext,
         IterEmpty,
         IterOnce,

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);
         }
 

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1798,7 +1798,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                 let self_ty = selcx.infcx.shallow_resolve(obligation.predicate.self_ty());
 
                 let lang_items = selcx.tcx().lang_items();
-                if [lang_items.gen_trait(), lang_items.future_trait()].contains(&Some(trait_ref.def_id))
+                if [lang_items.gen_trait(), lang_items.future_trait(), lang_items.iterator_trait()].contains(&Some(trait_ref.def_id))
                     || selcx.tcx().fn_trait_kind_from_def_id(trait_ref.def_id).is_some()
                 {
                     true
@@ -2015,6 +2015,8 @@ fn confirm_select_candidate<'cx, 'tcx>(
                 confirm_coroutine_candidate(selcx, obligation, data)
             } else if lang_items.future_trait() == Some(trait_def_id) {
                 confirm_future_candidate(selcx, obligation, data)
+            } else if lang_items.iterator_trait() == Some(trait_def_id) {
+                confirm_iterator_candidate(selcx, obligation, data)
             } else if selcx.tcx().fn_trait_kind_from_def_id(trait_def_id).is_some() {
                 if obligation.predicate.self_ty().is_closure() {
                     confirm_closure_candidate(selcx, obligation, data)
@@ -2135,6 +2137,50 @@ fn confirm_future_candidate<'cx, 'tcx>(
         .with_addl_obligations(obligations)
 }
 
+fn confirm_iterator_candidate<'cx, 'tcx>(
+    selcx: &mut SelectionContext<'cx, 'tcx>,
+    obligation: &ProjectionTyObligation<'tcx>,
+    nested: Vec<PredicateObligation<'tcx>>,
+) -> Progress<'tcx> {
+    let ty::Coroutine(_, args, _) =
+        selcx.infcx.shallow_resolve(obligation.predicate.self_ty()).kind()
+    else {
+        unreachable!()
+    };
+    let gen_sig = args.as_coroutine().poly_sig();
+    let Normalized { value: gen_sig, obligations } = normalize_with_depth(
+        selcx,
+        obligation.param_env,
+        obligation.cause.clone(),
+        obligation.recursion_depth + 1,
+        gen_sig,
+    );
+
+    debug!(?obligation, ?gen_sig, ?obligations, "confirm_future_candidate");
+
+    let tcx = selcx.tcx();
+    let iter_def_id = tcx.require_lang_item(LangItem::Iterator, None);
+
+    let predicate = super::util::iterator_trait_ref_and_outputs(
+        tcx,
+        iter_def_id,
+        obligation.predicate.self_ty(),
+        gen_sig,
+    )
+    .map_bound(|(trait_ref, yield_ty)| {
+        debug_assert_eq!(tcx.associated_item(obligation.predicate.def_id).name, sym::Item);
+
+        ty::ProjectionPredicate {
+            projection_ty: ty::AliasTy::new(tcx, obligation.predicate.def_id, trait_ref.args),
+            term: yield_ty.into(),
+        }
+    });
+
+    confirm_param_env_candidate(selcx, obligation, predicate, false)
+        .with_addl_obligations(nested)
+        .with_addl_obligations(obligations)
+}
+
 fn confirm_builtin_candidate<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,

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

@@ -114,6 +114,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     self.assemble_coroutine_candidates(obligation, &mut candidates);
                 } else if lang_items.future_trait() == Some(def_id) {
                     self.assemble_future_candidates(obligation, &mut candidates);
+                } else if lang_items.iterator_trait() == Some(def_id) {
+                    self.assemble_iterator_candidates(obligation, &mut candidates);
                 }
 
                 self.assemble_closure_candidates(obligation, &mut candidates);
@@ -211,9 +213,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // type/region parameters.
         let self_ty = obligation.self_ty().skip_binder();
         match self_ty.kind() {
-            // async constructs get lowered to a special kind of coroutine that
+            // `async`/`gen` constructs get lowered to a special kind of coroutine that
             // should *not* `impl Coroutine`.
-            ty::Coroutine(did, ..) if !self.tcx().coroutine_is_async(*did) => {
+            ty::Coroutine(did, ..) if self.tcx().is_general_coroutine(*did) => {
                 debug!(?self_ty, ?obligation, "assemble_coroutine_candidates",);
 
                 candidates.vec.push(CoroutineCandidate);
@@ -243,6 +245,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
+    fn assemble_iterator_candidates(
+        &mut self,
+        obligation: &PolyTraitObligation<'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) {
+                debug!(?self_ty, ?obligation, "assemble_iterator_candidates",);
+
+                candidates.vec.push(IteratorCandidate);
+            }
+        }
+    }
+
     /// Checks for the artificial impl that the compiler will create for an obligation like `X :
     /// FnMut<..>` where `X` is a closure type.
     ///

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

@@ -93,6 +93,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ImplSource::Builtin(BuiltinImplSource::Misc, vtable_future)
             }
 
+            IteratorCandidate => {
+                let vtable_iterator = self.confirm_iterator_candidate(obligation)?;
+                ImplSource::Builtin(BuiltinImplSource::Misc, vtable_iterator)
+            }
+
             FnPointerCandidate { is_const } => {
                 let data = self.confirm_fn_pointer_candidate(obligation, is_const)?;
                 ImplSource::Builtin(BuiltinImplSource::Misc, data)
@@ -780,6 +785,36 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(nested)
     }
 
+    fn confirm_iterator_candidate(
+        &mut self,
+        obligation: &PolyTraitObligation<'tcx>,
+    ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
+        // Okay to skip binder because the args on coroutine types never
+        // touch bound regions, they just capture the in-scope
+        // type/region parameters.
+        let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
+        let ty::Coroutine(coroutine_def_id, args, _) = *self_ty.kind() else {
+            bug!("closure candidate for non-closure {:?}", obligation);
+        };
+
+        debug!(?obligation, ?coroutine_def_id, ?args, "confirm_iterator_candidate");
+
+        let gen_sig = args.as_coroutine().poly_sig();
+
+        let trait_ref = super::util::iterator_trait_ref_and_outputs(
+            self.tcx(),
+            obligation.predicate.def_id(),
+            obligation.predicate.no_bound_vars().expect("iterator has no bound vars").self_ty(),
+            gen_sig,
+        )
+        .map_bound(|(trait_ref, ..)| trait_ref);
+
+        let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+        debug!(?trait_ref, ?nested, "iterator candidate obligations");
+
+        Ok(nested)
+    }
+
     #[instrument(skip(self), level = "debug")]
     fn confirm_closure_candidate(
         &mut self,

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

@@ -1888,6 +1888,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | ClosureCandidate { .. }
                 | CoroutineCandidate
                 | FutureCandidate
+                | IteratorCandidate
                 | FnPointerCandidate { .. }
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
@@ -1916,6 +1917,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | ClosureCandidate { .. }
                 | CoroutineCandidate
                 | FutureCandidate
+                | IteratorCandidate
                 | FnPointerCandidate { .. }
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
@@ -1950,6 +1952,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | ClosureCandidate { .. }
                 | CoroutineCandidate
                 | FutureCandidate
+                | IteratorCandidate
                 | FnPointerCandidate { .. }
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
@@ -1964,6 +1967,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | ClosureCandidate { .. }
                 | CoroutineCandidate
                 | FutureCandidate
+                | IteratorCandidate
                 | FnPointerCandidate { .. }
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
@@ -2070,6 +2074,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | ClosureCandidate { .. }
                 | CoroutineCandidate
                 | FutureCandidate
+                | IteratorCandidate
                 | FnPointerCandidate { .. }
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
@@ -2080,6 +2085,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 | ClosureCandidate { .. }
                 | CoroutineCandidate
                 | FutureCandidate
+                | IteratorCandidate
                 | FnPointerCandidate { .. }
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate

compiler/rustc_trait_selection/src/traits/util.rs

@@ -297,6 +297,17 @@ pub fn future_trait_ref_and_outputs<'tcx>(
     sig.map_bound(|sig| (trait_ref, sig.return_ty))
 }
 
+pub fn iterator_trait_ref_and_outputs<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    iterator_def_id: DefId,
+    self_ty: Ty<'tcx>,
+    sig: ty::PolyGenSig<'tcx>,
+) -> ty::Binder<'tcx, (ty::TraitRef<'tcx>, Ty<'tcx>)> {
+    assert!(!self_ty.has_escaping_bound_vars());
+    let trait_ref = ty::TraitRef::new(tcx, iterator_def_id, [self_ty]);
+    sig.map_bound(|sig| (trait_ref, sig.yield_ty))
+}
+
 pub fn impl_item_is_final(tcx: TyCtxt<'_>, assoc_item: &ty::AssocItem) -> bool {
     assoc_item.defaultness(tcx).is_final()
         && tcx.defaultness(assoc_item.container_id(tcx)).is_final()

tests/ui/coroutine/gen_block_is_coro.rs

@@ -0,0 +1,18 @@
+//compile-flags: --edition 2024 -Zunstable-options
+#![feature(coroutines, coroutine_trait)]
+
+use std::ops::Coroutine;
+
+fn foo() -> impl Coroutine<Yield = u32, Return = ()> { //~ ERROR: Coroutine` is not satisfied
+    gen { yield 42 }
+}
+
+fn bar() -> impl Coroutine<Yield = i64, Return = ()> { //~ ERROR: Coroutine` is not satisfied
+    gen { yield 42 }
+}
+
+fn baz() -> impl Coroutine<Yield = i32, Return = ()> { //~ ERROR: Coroutine` is not satisfied
+    gen { yield 42 }
+}
+
+fn main() {}

tests/ui/coroutine/gen_block_is_coro.stderr

@@ -0,0 +1,21 @@
+error[E0277]: the trait bound `{gen block@$DIR/gen_block_is_coro.rs:7:5: 7:21}: Coroutine` is not satisfied
+  --> $DIR/gen_block_is_coro.rs:6:13
+   |
+LL | fn foo() -> impl Coroutine<Yield = u32, Return = ()> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Coroutine` is not implemented for `{gen block@$DIR/gen_block_is_coro.rs:7:5: 7:21}`
+
+error[E0277]: the trait bound `{gen block@$DIR/gen_block_is_coro.rs:11:5: 11:21}: Coroutine` is not satisfied
+  --> $DIR/gen_block_is_coro.rs:10:13
+   |
+LL | fn bar() -> impl Coroutine<Yield = i64, Return = ()> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Coroutine` is not implemented for `{gen block@$DIR/gen_block_is_coro.rs:11:5: 11:21}`
+
+error[E0277]: the trait bound `{gen block@$DIR/gen_block_is_coro.rs:15:5: 15:21}: Coroutine` is not satisfied
+  --> $DIR/gen_block_is_coro.rs:14:13
+   |
+LL | fn baz() -> impl Coroutine<Yield = i32, Return = ()> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Coroutine` is not implemented for `{gen block@$DIR/gen_block_is_coro.rs:15:5: 15:21}`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/coroutine/gen_block_is_iter.rs

@@ -0,0 +1,19 @@
+// revisions: next old
+//compile-flags: --edition 2024 -Zunstable-options
+//[next] compile-flags: -Ztrait-solver=next
+// check-pass
+#![feature(coroutines)]
+
+fn foo() -> impl Iterator<Item = u32> {
+    gen { yield 42 }
+}
+
+fn bar() -> impl Iterator<Item = i64> {
+    gen { yield 42 }
+}
+
+fn baz() -> impl Iterator<Item = i32> {
+    gen { yield 42 }
+}
+
+fn main() {}

tests/ui/coroutine/gen_block_is_no_future.rs

@@ -0,0 +1,8 @@
+//compile-flags: --edition 2024 -Zunstable-options
+#![feature(coroutines)]
+
+fn foo() -> impl std::future::Future { //~ ERROR is not a future
+    gen { yield 42 }
+}
+
+fn main() {}

tests/ui/coroutine/gen_block_is_no_future.stderr

@@ -0,0 +1,12 @@
+error[E0277]: `{gen block@$DIR/gen_block_is_no_future.rs:5:5: 5:21}` is not a future
+  --> $DIR/gen_block_is_no_future.rs:4:13
+   |
+LL | fn foo() -> impl std::future::Future {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^ `{gen block@$DIR/gen_block_is_no_future.rs:5:5: 5:21}` is not a future
+   |
+   = help: the trait `Future` is not implemented for `{gen block@$DIR/gen_block_is_no_future.rs:5:5: 5:21}`
+   = note: {gen block@$DIR/gen_block_is_no_future.rs:5:5: 5:21} must be a future or must implement `IntoFuture` to be awaited
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.