don't use generator witness for implied bounds

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -608,8 +608,6 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                 | ty::Float(..)
                 | ty::Error(_)
                 | ty::Str
-                | ty::GeneratorWitness(..)
-                | ty::GeneratorWitnessMIR(..)
                 | ty::Never
                 | ty::Param(_)
                 | ty::Bound(..)
@@ -681,6 +679,12 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
                     }
                 }
 
+                ty::GeneratorWitness(..) | ty::GeneratorWitnessMIR(..) => {
+                    // Unspecified in RFC 1214.
+                    // They are always WF when the gnereator passes typeck/borrowck.
+                    walker.skip_current_subtree();
+                }
+
                 ty::Generator(did, args, ..) => {
                     // Walk ALL the types in the generator: this will
                     // include the upvar types as well as the yield

tests/ui/async-await/generator-wf-check.output_wf.stderr

@@ -0,0 +1,17 @@
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/generator-wf-check.rs:24:5
+   |
+LL | /     wf(async {
+LL | |
+LL | |         None::<Static<T>>
+LL | |     });
+   | |______^ ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL | fn test_output_wf<T: 'static>() {
+   |                    +++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0310`.

tests/ui/async-await/generator-wf-check.rs

@@ -0,0 +1,52 @@
+//! Tests if we draw implied bounds from the generator output or witness types,
+//! and if so, whether we check these types are WF when checking the generator.
+
+// edition: 2021
+// revisions: output output_wf witness witness_wf
+//[output] check-pass
+//[output_wf] check-fail
+//[witness] check-fail
+//[witness_wf] check-fail
+
+struct Static<T: 'static>(T);
+
+fn wf<T>(_: T) {}
+
+#[cfg(output)]
+fn test_output<T>() {
+    async {
+        None::<Static<T>>
+    };
+}
+
+#[cfg(output_wf)]
+fn test_output_wf<T>() {
+    wf(async {
+    //[output_wf]~^ ERROR `T` may not live long enough
+        None::<Static<T>>
+    });
+}
+
+#[cfg(witness)]
+fn test_witness<T>() {
+    async {
+        let witness: Option<Static<T>> = None;
+        //[witness]~^ ERROR `T` may not live long enough
+        async {}.await;
+        drop(witness);
+        //[witness]~^ ERROR `T` may not live long enough
+    };
+}
+
+#[cfg(witness_wf)]
+fn test_witness_wf<T>() {
+    wf(async {
+        let witness: Option<Static<T>> = None;
+        //[witness_wf]~^ ERROR `T` may not live long enough
+        async {}.await;
+        drop(witness);
+        //[witness_wf]~^ ERROR `T` may not live long enough
+    });
+}
+
+fn main() {}

tests/ui/async-await/generator-wf-check.witness.stderr

@@ -0,0 +1,25 @@
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/generator-wf-check.rs:33:22
+   |
+LL |         let witness: Option<Static<T>> = None;
+   |                      ^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL | fn test_witness<T: 'static>() {
+   |                  +++++++++
+
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/generator-wf-check.rs:36:9
+   |
+LL |         drop(witness);
+   |         ^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL | fn test_witness<T: 'static>() {
+   |                  +++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0310`.

tests/ui/async-await/generator-wf-check.witness_wf.stderr

@@ -0,0 +1,25 @@
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/generator-wf-check.rs:44:22
+   |
+LL |         let witness: Option<Static<T>> = None;
+   |                      ^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL | fn test_witness_wf<T: 'static>() {
+   |                     +++++++++
+
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/generator-wf-check.rs:47:9
+   |
+LL |         drop(witness);
+   |         ^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL | fn test_witness_wf<T: 'static>() {
+   |                     +++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0310`.