Make sure async constructs do not impl Generator

Async lowering turns async functions and blocks into generators internally.
Though these special kinds of generators should not `impl Generator` themselves.
The other way around, normal generators should not `impl Future`.

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

@@ -203,7 +203,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // type/region parameters.
         let self_ty = obligation.self_ty().skip_binder();
         match self_ty.kind() {
-            ty::Generator(..) => {
+            // async constructs get lowered to a special kind of generator that
+            // should *not* `impl Generator`.
+            ty::Generator(did, ..) if !self.tcx().generator_is_async(*did) => {
                 debug!(?self_ty, ?obligation, "assemble_generator_candidates",);
 
                 candidates.vec.push(GeneratorCandidate);
@@ -223,6 +225,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) {
         let self_ty = obligation.self_ty().skip_binder();
         if let ty::Generator(did, ..) = self_ty.kind() {
+            // async constructs get lowered to a special kind of generator that
+            // should directly `impl Future`.
             if self.tcx().generator_is_async(*did) {
                 debug!(?self_ty, ?obligation, "assemble_future_candidates",);
 

src/test/ui/async-await/generator-not-future.rs

@@ -0,0 +1,45 @@
+// edition:2018
+#![feature(generators, generator_trait)]
+
+use std::future::Future;
+use std::ops::Generator;
+
+async fn async_fn() {}
+fn returns_async_block() -> impl Future<Output = ()> {
+    async {}
+}
+fn returns_generator() -> impl Generator<(), Yield = (), Return = ()> {
+    || {
+        let _: () = yield ();
+    }
+}
+
+fn takes_future(_f: impl Future<Output = ()>) {}
+fn takes_generator<ResumeTy>(_g: impl Generator<ResumeTy, Yield = (), Return = ()>) {}
+
+fn main() {
+    // okay:
+    takes_future(async_fn());
+    takes_future(returns_async_block());
+    takes_future(async {});
+    takes_generator(returns_generator());
+    takes_generator(|| {
+        let _: () = yield ();
+    });
+
+    // async futures are not generators:
+    takes_generator(async_fn());
+    //~^ ERROR the trait bound
+    takes_generator(returns_async_block());
+    //~^ ERROR the trait bound
+    takes_generator(async {});
+    //~^ ERROR the trait bound
+
+    // generators are not futures:
+    takes_future(returns_generator());
+    //~^ ERROR is not a future
+    takes_future(|ctx| {
+        //~^ ERROR is not a future
+        ctx = yield ();
+    });
+}

src/test/ui/async-await/generator-not-future.stderr

@@ -0,0 +1,81 @@
+error[E0277]: the trait bound `impl Future<Output = ()>: Generator<_>` is not satisfied
+  --> $DIR/generator-not-future.rs:31:21
+   |
+LL |     takes_generator(async_fn());
+   |     --------------- ^^^^^^^^^^ the trait `Generator<_>` is not implemented for `impl Future<Output = ()>`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `takes_generator`
+  --> $DIR/generator-not-future.rs:18:39
+   |
+LL | fn takes_generator<ResumeTy>(_g: impl Generator<ResumeTy, Yield = (), Return = ()>) {}
+   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_generator`
+
+error[E0277]: the trait bound `impl Future<Output = ()>: Generator<_>` is not satisfied
+  --> $DIR/generator-not-future.rs:33:21
+   |
+LL |     takes_generator(returns_async_block());
+   |     --------------- ^^^^^^^^^^^^^^^^^^^^^ the trait `Generator<_>` is not implemented for `impl Future<Output = ()>`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `takes_generator`
+  --> $DIR/generator-not-future.rs:18:39
+   |
+LL | fn takes_generator<ResumeTy>(_g: impl Generator<ResumeTy, Yield = (), Return = ()>) {}
+   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_generator`
+
+error[E0277]: the trait bound `[async block@$DIR/generator-not-future.rs:35:21: 35:29]: Generator<_>` is not satisfied
+  --> $DIR/generator-not-future.rs:35:21
+   |
+LL |     takes_generator(async {});
+   |     --------------- ^^^^^^^^ the trait `Generator<_>` is not implemented for `[async block@$DIR/generator-not-future.rs:35:21: 35:29]`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `takes_generator`
+  --> $DIR/generator-not-future.rs:18:39
+   |
+LL | fn takes_generator<ResumeTy>(_g: impl Generator<ResumeTy, Yield = (), Return = ()>) {}
+   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_generator`
+
+error[E0277]: `impl Generator<Yield = (), Return = ()>` is not a future
+  --> $DIR/generator-not-future.rs:39:18
+   |
+LL |     takes_future(returns_generator());
+   |     ------------ ^^^^^^^^^^^^^^^^^^^ `impl Generator<Yield = (), Return = ()>` is not a future
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Future` is not implemented for `impl Generator<Yield = (), Return = ()>`
+   = note: impl Generator<Yield = (), Return = ()> must be a future or must implement `IntoFuture` to be awaited
+note: required by a bound in `takes_future`
+  --> $DIR/generator-not-future.rs:17:26
+   |
+LL | fn takes_future(_f: impl Future<Output = ()>) {}
+   |                          ^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_future`
+
+error[E0277]: `[generator@$DIR/generator-not-future.rs:41:18: 41:23]` is not a future
+  --> $DIR/generator-not-future.rs:41:18
+   |
+LL |       takes_future(|ctx| {
+   |  _____------------_^
+   | |     |
+   | |     required by a bound introduced by this call
+LL | |
+LL | |         ctx = yield ();
+LL | |     });
+   | |_____^ `[generator@$DIR/generator-not-future.rs:41:18: 41:23]` is not a future
+   |
+   = help: the trait `Future` is not implemented for `[generator@$DIR/generator-not-future.rs:41:18: 41:23]`
+   = note: [generator@$DIR/generator-not-future.rs:41:18: 41:23] must be a future or must implement `IntoFuture` to be awaited
+note: required by a bound in `takes_future`
+  --> $DIR/generator-not-future.rs:17:26
+   |
+LL | fn takes_future(_f: impl Future<Output = ()>) {}
+   |                          ^^^^^^^^^^^^^^^^^^^ required by this bound in `takes_future`
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0277`.