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SwatinemSwatinem
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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`.
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3 files changed

+131
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lines changed

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

+5-1
Original file line numberDiff line numberDiff line change
@@ -203,7 +203,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
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// type/region parameters.
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let self_ty = obligation.self_ty().skip_binder();
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match self_ty.kind() {
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ty::Generator(..) => {
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// async constructs get lowered to a special kind of generator that
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// should *not* `impl Generator`.
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ty::Generator(did, ..) if !self.tcx().generator_is_async(*did) => {
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debug!(?self_ty, ?obligation, "assemble_generator_candidates",);
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candidates.vec.push(GeneratorCandidate);
@@ -223,6 +225,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
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) {
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let self_ty = obligation.self_ty().skip_binder();
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if let ty::Generator(did, ..) = self_ty.kind() {
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// async constructs get lowered to a special kind of generator that
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// should directly `impl Future`.
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if self.tcx().generator_is_async(*did) {
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debug!(?self_ty, ?obligation, "assemble_future_candidates",);
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‎src/test/ui/async-await/generator-not-future.rs

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

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

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

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