Restore impl Future<Output = Type> to async blocks

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -912,12 +912,25 @@ pub trait PrettyPrinter<'tcx>:
                 }
 
                 for (assoc_item_def_id, term) in assoc_items {
-                    // Skip printing `<[generator@] as Generator<_>>::Return` from async blocks
-                    if let Some(ty) = term.skip_binder().ty() &&
-                       let ty::Projection(ty::ProjectionTy { item_def_id, .. }) = ty.kind() &&
-                       Some(*item_def_id) == self.tcx().lang_items().generator_return() {
-                        continue;
-                    }
+                    // Skip printing `<[generator@] as Generator<_>>::Return` from async blocks,
+                    // unless we can find out what generator return type it comes from.
+                    let term = if let Some(ty) = term.skip_binder().ty()
+                        && let ty::Projection(ty::ProjectionTy { item_def_id, substs }) = ty.kind()
+                        && Some(*item_def_id) == self.tcx().lang_items().generator_return()
+                    {
+                        if let ty::Generator(_, substs, _) = substs.type_at(0).kind() {
+                            let return_ty = substs.as_generator().return_ty();
+                            if !return_ty.is_ty_infer() {
+                                return_ty.into()
+                            } else {
+                                continue;
+                            }
+                        } else {
+                            continue;
+                        }
+                    } else {
+                        term.skip_binder()
+                    };
 
                     if first {
                         p!("<");
@@ -928,7 +941,7 @@ pub trait PrettyPrinter<'tcx>:
 
                     p!(write("{} = ", self.tcx().associated_item(assoc_item_def_id).name));
 
-                    match term.skip_binder() {
+                    match term {
                         Term::Ty(ty) => {
                             p!(print(ty))
                         }

src/test/ui/async-await/async-block-control-flow-static-semantics.rs

@@ -24,7 +24,7 @@ async fn return_targets_async_block_not_async_fn() -> u8 {
         return 0u8;
     };
     let _: &dyn Future<Output = ()> = &block;
-    //~^ ERROR type mismatch resolving `<impl Future as Future>::Output == ()`
+    //~^ ERROR type mismatch resolving `<impl Future<Output = u8> as Future>::Output == ()`
 }
 
 fn no_break_in_async_block() {

src/test/ui/async-await/async-block-control-flow-static-semantics.stderr

@@ -31,7 +31,7 @@ LL | |
 LL | | }
    | |_^ expected `u8`, found `()`
 
-error[E0271]: type mismatch resolving `<impl Future as Future>::Output == ()`
+error[E0271]: type mismatch resolving `<impl Future<Output = u8> as Future>::Output == ()`
   --> $DIR/async-block-control-flow-static-semantics.rs:26:39
    |
 LL |     let _: &dyn Future<Output = ()> = &block;
@@ -47,7 +47,7 @@ LL | fn return_targets_async_block_not_fn() -> u8 {
    |    |
    |    implicitly returns `()` as its body has no tail or `return` expression
 
-error[E0271]: type mismatch resolving `<impl Future as Future>::Output == ()`
+error[E0271]: type mismatch resolving `<impl Future<Output = u8> as Future>::Output == ()`
   --> $DIR/async-block-control-flow-static-semantics.rs:17:39
    |
 LL |     let _: &dyn Future<Output = ()> = &block;

src/test/ui/async-await/generator-desc.stderr

@@ -46,8 +46,8 @@ LL | pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
    |                                           the expected opaque type
    |                                           the found opaque type
    |
-   = note: expected opaque type `impl Future` (`async` closure body)
-              found opaque type `impl Future` (`async` closure body)
+   = note: expected opaque type `impl Future<Output = ()>` (`async` closure body)
+              found opaque type `impl Future<Output = ()>` (`async` closure body)
 
 error: aborting due to 3 previous errors
 

src/test/ui/async-await/issue-67252-unnamed-future.stderr

@@ -4,7 +4,7 @@ error: future cannot be sent between threads safely
 LL |     spawn(async {
    |     ^^^^^ future created by async block is not `Send`
    |
-   = help: within `impl Future`, the trait `Send` is not implemented for `*mut ()`
+   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `*mut ()`
 note: future is not `Send` as this value is used across an await
   --> $DIR/issue-67252-unnamed-future.rs:20:16
    |

src/test/ui/async-await/issue-68112.stderr

@@ -44,13 +44,13 @@ LL |     require_send(send_fut);
    = note: required because of the requirements on the impl of `Send` for `Arc<RefCell<i32>>`
    = note: required because it appears within the type `[static generator@$DIR/issue-68112.rs:47:31: 47:36]`
    = note: required because it appears within the type `from_generator::GenFuture<[static generator@$DIR/issue-68112.rs:47:31: 47:36]>`
-   = note: required because it appears within the type `impl Future`
+   = note: required because it appears within the type `impl Future<Output = Arc<RefCell<i32>>>`
    = note: required because it appears within the type `impl Future<Output = Arc<RefCell<i32>>>`
    = note: required because it appears within the type `impl Future<Output = Arc<RefCell<i32>>>`
    = note: required because it appears within the type `{ResumeTy, impl Future<Output = Arc<RefCell<i32>>>, (), i32, Ready<i32>}`
    = note: required because it appears within the type `[static generator@$DIR/issue-68112.rs:55:26: 59:6]`
    = note: required because it appears within the type `from_generator::GenFuture<[static generator@$DIR/issue-68112.rs:55:26: 59:6]>`
-   = note: required because it appears within the type `impl Future`
+   = note: required because it appears within the type `impl Future<Output = ()>`
 note: required by a bound in `require_send`
   --> $DIR/issue-68112.rs:11:25
    |

src/test/ui/async-await/issues/issue-65436-raw-ptr-not-send.stderr

@@ -4,7 +4,7 @@ error: future cannot be sent between threads safely
 LL |     assert_send(async {
    |     ^^^^^^^^^^^ future created by async block is not `Send`
    |
-   = help: within `impl Future`, the trait `Send` is not implemented for `*const u8`
+   = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `*const u8`
 note: future is not `Send` as this value is used across an await
   --> $DIR/issue-65436-raw-ptr-not-send.rs:14:35
    |

src/test/ui/async-await/partial-drop-partial-reinit.stderr

@@ -14,7 +14,7 @@ LL | async fn foo() {
    = note: required because it appears within the type `{ResumeTy, (NotSend,), impl Future<Output = ()>, ()}`
    = note: required because it appears within the type `[static generator@$DIR/partial-drop-partial-reinit.rs:22:16: 27:2]`
    = note: required because it appears within the type `from_generator::GenFuture<[static generator@$DIR/partial-drop-partial-reinit.rs:22:16: 27:2]>`
-   = note: required because it appears within the type `impl Future`
+   = note: required because it appears within the type `impl Future<Output = ()>`
    = note: required because it appears within the type `impl Future<Output = ()>`
 note: required by a bound in `gimme_send`
   --> $DIR/partial-drop-partial-reinit.rs:10:18

src/test/ui/chalkify/bugs/async.stderr

@@ -28,7 +28,7 @@ note: required by a bound in `from_generator`
 LL |     T: Generator<ResumeTy, Yield = ()>,
    |                            ^^^^^^^^^^ required by this bound in `from_generator`
 
-error[E0280]: the requirement `<impl Future as Future>::Output == u32` is not satisfied
+error[E0280]: the requirement `<impl Future<Output = u32> as Future>::Output == u32` is not satisfied
   --> $DIR/async.rs:7:29
    |
 LL |   async fn foo(x: u32) -> u32 {

src/test/ui/impl-trait/issue-55872-3.rs

@@ -13,7 +13,7 @@ impl<S> Bar for S {
     type E = impl std::marker::Copy;
     fn foo<T>() -> Self::E {
         async {}
-        //~^ ERROR the trait bound `impl Future: Copy` is not satisfied [E0277]
+        //~^ ERROR the trait bound `impl Future<Output = ()>: Copy` is not satisfied [E0277]
     }
 }
 

src/test/ui/impl-trait/issue-55872-3.stderr

@@ -1,8 +1,8 @@
-error[E0277]: the trait bound `impl Future: Copy` is not satisfied
+error[E0277]: the trait bound `impl Future<Output = ()>: Copy` is not satisfied
   --> $DIR/issue-55872-3.rs:15:9
    |
 LL |         async {}
-   |         ^^^^^^^^ the trait `Copy` is not implemented for `impl Future`
+   |         ^^^^^^^^ the trait `Copy` is not implemented for `impl Future<Output = ()>`
 
 error: aborting due to previous error
 

src/test/ui/impl-trait/issues/issue-78722.stderr

@@ -16,7 +16,7 @@ LL |         let f: F = async { 1 };
 LL |     }],
    |     - value is dropped here
 
-error[E0271]: type mismatch resolving `<impl Future as Future>::Output == u8`
+error[E0271]: type mismatch resolving `<impl Future<Output = ()> as Future>::Output == u8`
   --> $DIR/issue-78722.rs:10:13
    |
 LL |             async {}

src/test/ui/pattern/non-structural-match-types.stderr

@@ -4,7 +4,7 @@ error: `[closure@$DIR/non-structural-match-types.rs:9:17: 9:22]` cannot be used
 LL |         const { || {} } => {},
    |         ^^^^^^^^^^^^^^^
 
-error: `impl Future` cannot be used in patterns
+error: `impl Future<Output = ()>` cannot be used in patterns
   --> $DIR/non-structural-match-types.rs:12:9
    |
 LL |         const { async {} } => {},