Elaborate Future::Output when printing opaque impl Future type

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

@@ -649,30 +649,74 @@ pub trait PrettyPrinter<'tcx>:
 
  let mut first = true;
  let mut is_sized = false;
+ let mut is_future = false;
+ let mut future_output_ty = None;
+
  p!("impl");
  for (predicate, _) in bounds {
  let predicate = predicate.subst(self.tcx(), substs);
  let bound_predicate = predicate.kind();
- if let ty::PredicateKind::Trait(pred) = bound_predicate.skip_binder() {
- let trait_ref = bound_predicate.rebind(pred.trait_ref);
- // Don't print +Sized, but rather +?Sized if absent.
- if Some(trait_ref.def_id()) == self.tcx().lang_items().sized_trait() {
- is_sized = true;
- continue;
+
+ match bound_predicate.skip_binder() {
+ ty::PredicateKind::Projection(projection_predicate) => {
+ let Some(future_trait) = self.tcx().lang_items().future_trait() else { continue };
+ let future_output_def_id =
+ self.tcx().associated_item_def_ids(future_trait)[0];
+
+ if projection_predicate.projection_ty.item_def_id
+ == future_output_def_id
+ {
+ // We don't account for multiple `Future::Output = Ty` contraints.
+ is_future = true;
+ future_output_ty = Some(projection_predicate.ty);
+ }
  }
+ ty::PredicateKind::Trait(pred) => {
+ let trait_ref = bound_predicate.rebind(pred.trait_ref);
+ // Don't print +Sized, but rather +?Sized if absent.
+ if Some(trait_ref.def_id()) == self.tcx().lang_items().sized_trait()
+ {
+ is_sized = true;
+ continue;
+ }
 
- p!(
- write("{}", if first { " " } else { "+" }),
- print(trait_ref.print_only_trait_path())
- );
- first = false;
+ if Some(trait_ref.def_id())
+ == self.tcx().lang_items().future_trait()
+ {
+ is_future = true;
+ continue;
+ }
+
+ p!(
+ write("{}", if first { " " } else { "+" }),
+ print(trait_ref.print_only_trait_path())
+ );
+
+ first = false;
+ }
+ _ => {}
  }
  }
+
+ if is_future {
+ p!(write("{}Future", if first { " " } else { "+" }));
+ first = false;
+
+ if let Some(future_output_ty) = future_output_ty {
+ // Don't print projection types, which we (unfortunately) see often
+ // in the error outputs involving async blocks.
+ if !matches!(future_output_ty.kind(), ty::Projection(_)) {
+ p!("<Output = ", print(future_output_ty), ">");
+ }
+ }
+ }
+
  if !is_sized {
  p!(write("{}?Sized", if first { " " } else { "+" }));
  } else if first {
  p!(" Sized");
  }
+
  Ok(self)
  });
  }

src/test/ui/async-await/async-fn-nonsend.stderr

@@ -4,7 +4,7 @@ error: future cannot be sent between threads safely
 LL | assert_send(local_dropped_before_await());
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `local_dropped_before_await` is not `Send`
  |
- = help: within `impl Future`, the trait `Send` is not implemented for `Rc<()>`
+ = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
 note: future is not `Send` as this value is used across an await
  --> $DIR/async-fn-nonsend.rs:24:5
  |
@@ -27,7 +27,7 @@ error: future cannot be sent between threads safely
 LL | assert_send(non_send_temporary_in_match());
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_send_temporary_in_match` is not `Send`
  |
- = help: within `impl Future`, the trait `Send` is not implemented for `Rc<()>`
+ = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
 note: future is not `Send` as this value is used across an await
  --> $DIR/async-fn-nonsend.rs:33:20
  |

src/test/ui/async-await/dont-suggest-missing-await.stderr

@@ -10,7 +10,7 @@ note: while checking the return type of the `async fn`
 LL | async fn make_u32() -> u32 {
  | ^^^ checked the `Output` of this `async fn`, found opaque type
  = note: expected type `u32`
- found opaque type `impl Future`
+ found opaque type `impl Future<Output = u32>`
 help: consider `await`ing on the `Future`
  |
 LL | take_u32(x.await)

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

@@ -25,8 +25,8 @@ note: while checking the return type of the `async fn`
  |
 LL | async fn two() {}
  | ^ checked the `Output` of this `async fn`, found opaque type
- = note: expected opaque type `impl Future` (opaque type at <$DIR/generator-desc.rs:5:16>)
- found opaque type `impl Future` (opaque type at <$DIR/generator-desc.rs:6:16>)
+ = note: expected opaque type `impl Future<Output = ()>` (opaque type at <$DIR/generator-desc.rs:5:16>)
+ found opaque type `impl Future<Output = ()>` (opaque type at <$DIR/generator-desc.rs:6:16>)
  = help: consider `await`ing on both `Future`s
  = note: distinct uses of `impl Trait` result in different opaque types
 

src/test/ui/async-await/issue-61076.rs

@@ -40,8 +40,8 @@ async fn foo() -> Result<(), ()> {
 
 async fn bar() -> Result<(), ()> {
  foo()?; //~ ERROR the `?` operator can only be applied to values that implement `Try`
- //~^ NOTE the `?` operator cannot be applied to type `impl Future`
- //~| HELP the trait `Try` is not implemented for `impl Future`
+ //~^ NOTE the `?` operator cannot be applied to type `impl Future<Output = Result<(), ()>>`
+ //~| HELP the trait `Try` is not implemented for `impl Future<Output = Result<(), ()>>`
  //~| NOTE required by `branch`
  //~| HELP consider `await`ing on the `Future`
  //~| NOTE in this expansion of desugaring of operator `?`
@@ -84,7 +84,7 @@ async fn baz() -> Result<(), ()> {
  //~| NOTE field not available in `impl Future`
 
  struct_().method(); //~ ERROR no method named
- //~^ NOTE method not found in `impl Future`
+ //~^ NOTE method not found in `impl Future<Output = Struct>`
  //~| HELP consider `await`ing on the `Future`
  Ok(())
 }
@@ -93,7 +93,7 @@ async fn match_() {
  match tuple() { //~ HELP consider `await`ing on the `Future`
  Tuple(_) => {} //~ ERROR mismatched types
  //~^ NOTE expected opaque type, found struct `Tuple`
- //~| NOTE expected opaque type `impl Future`
+ //~| NOTE expected opaque type `impl Future<Output = Tuple>`
  }
 }
 

src/test/ui/async-await/issue-61076.stderr

@@ -2,9 +2,9 @@ error[E0277]: the `?` operator can only be applied to values that implement `Try
  --> $DIR/issue-61076.rs:42:5
  |
 LL | foo()?;
- | ^^^^^^ the `?` operator cannot be applied to type `impl Future`
+ | ^^^^^^ the `?` operator cannot be applied to type `impl Future<Output = Result<(), ()>>`
  |
- = help: the trait `Try` is not implemented for `impl Future`
+ = help: the trait `Try` is not implemented for `impl Future<Output = Result<(), ()>>`
 note: required by `branch`
  --> $SRC_DIR/core/src/ops/try_trait.rs:LL:COL
  |
@@ -32,7 +32,7 @@ help: consider `await`ing on the `Future`
 LL | t.await?;
  | ++++++
 
-error[E0609]: no field `0` on type `impl Future`
+error[E0609]: no field `0` on type `impl Future<Output = Tuple>`
  --> $DIR/issue-61076.rs:78:26
  |
 LL | let _: i32 = tuple().0;
@@ -43,7 +43,7 @@ help: consider `await`ing on the `Future` and access the field of its `Output`
 LL | let _: i32 = tuple().await.0;
  | ++++++
 
-error[E0609]: no field `a` on type `impl Future`
+error[E0609]: no field `a` on type `impl Future<Output = Struct>`
  --> $DIR/issue-61076.rs:82:28
  |
 LL | let _: i32 = struct_().a;
@@ -54,11 +54,11 @@ help: consider `await`ing on the `Future` and access the field of its `Output`
 LL | let _: i32 = struct_().await.a;
  | ++++++
 
-error[E0599]: no method named `method` found for opaque type `impl Future` in the current scope
+error[E0599]: no method named `method` found for opaque type `impl Future<Output = Struct>` in the current scope
  --> $DIR/issue-61076.rs:86:15
  |
 LL | struct_().method();
- | ^^^^^^ method not found in `impl Future`
+ | ^^^^^^ method not found in `impl Future<Output = Struct>`
  |
 help: consider `await`ing on the `Future` and calling the method on its `Output`
  |
@@ -76,7 +76,7 @@ note: while checking the return type of the `async fn`
  |
 LL | async fn tuple() -> Tuple {
  | ^^^^^ checked the `Output` of this `async fn`, expected opaque type
- = note: expected opaque type `impl Future`
+ = note: expected opaque type `impl Future<Output = Tuple>`
  found struct `Tuple`
 help: consider `await`ing on the `Future`
  |

src/test/ui/async-await/issue-64130-1-sync.stderr

@@ -4,7 +4,7 @@ error: future cannot be shared between threads safely
 LL | is_sync(bar());
  | ^^^^^ future returned by `bar` is not `Sync`
  |
- = help: within `impl Future`, the trait `Sync` is not implemented for `Foo`
+ = help: within `impl Future<Output = ()>`, the trait `Sync` is not implemented for `Foo`
 note: future is not `Sync` as this value is used across an await
  --> $DIR/issue-64130-1-sync.rs:15:5
  |

src/test/ui/async-await/issue-64130-2-send.stderr

@@ -4,7 +4,7 @@ error: future cannot be sent between threads safely
 LL | is_send(bar());
  | ^^^^^ future returned by `bar` is not `Send`
  |
- = help: within `impl Future`, the trait `Send` is not implemented for `Foo`
+ = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Foo`
 note: future is not `Send` as this value is used across an await
  --> $DIR/issue-64130-2-send.rs:15:5
  |

src/test/ui/async-await/issue-64130-3-other.rs

@@ -5,22 +5,22 @@
 // This tests the the unspecialized async-await-specific error when futures don't implement an
 // auto trait (which is not Send or Sync) due to some type that was captured.
 
-auto trait Qux { }
+auto trait Qux {}
 
 struct Foo;
 
 impl !Qux for Foo {}
 
-fn is_qux<T: Qux>(t: T) { }
+fn is_qux<T: Qux>(t: T) {}
 
 async fn bar() {
  let x = Foo;
  baz().await;
 }
 
-async fn baz() { }
+async fn baz() {}
 
 fn main() {
  is_qux(bar());
- //~^ ERROR the trait bound `Foo: Qux` is not satisfied in `impl Future`
+ //~^ ERROR the trait bound `Foo: Qux` is not satisfied in `impl Future<Output = ()>`
 }

src/test/ui/async-await/issue-64130-3-other.stderr

@@ -1,11 +1,11 @@
-error[E0277]: the trait bound `Foo: Qux` is not satisfied in `impl Future`
+error[E0277]: the trait bound `Foo: Qux` is not satisfied in `impl Future<Output = ()>`
  --> $DIR/issue-64130-3-other.rs:24:12
  |
 LL | async fn bar() {
- | - within this `impl Future`
+ | - within this `impl Future<Output = ()>`
 ...
 LL | is_qux(bar());
- | ^^^^^ within `impl Future`, the trait `Qux` is not implemented for `Foo`
+ | ^^^^^ within `impl Future<Output = ()>`, the trait `Qux` is not implemented for `Foo`
  |
 note: future does not implement `Qux` as this value is used across an await
  --> $DIR/issue-64130-3-other.rs:18:5
@@ -19,7 +19,7 @@ LL | }
 note: required by a bound in `is_qux`
  --> $DIR/issue-64130-3-other.rs:14:14
  |
-LL | fn is_qux<T: Qux>(t: T) { }
+LL | fn is_qux<T: Qux>(t: T) {}
  | ^^^ required by this bound in `is_qux`
 
 error: aborting due to previous error

src/test/ui/async-await/issue-64130-non-send-future-diags.stderr

@@ -4,7 +4,7 @@ error: future cannot be sent between threads safely
 LL | is_send(foo());
  | ^^^^^ future returned by `foo` is not `Send`
  |
- = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, u32>`
+ = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `MutexGuard<'_, u32>`
 note: future is not `Send` as this value is used across an await
  --> $DIR/issue-64130-non-send-future-diags.rs:17:5
  |

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

@@ -9,7 +9,7 @@ note: future is not `Send` as it awaits another future which is not `Send`
  --> $DIR/issue-68112.rs:31:17
  |
 LL | let _ = non_send_fut.await;
- | ^^^^^^^^^^^^ await occurs here on type `impl Future`, which is not `Send`
+ | ^^^^^^^^^^^^ await occurs here on type `impl Future<Output = Arc<RefCell<i32>>>`, which is not `Send`
 note: required by a bound in `require_send`
  --> $DIR/issue-68112.rs:11:25
  |
@@ -27,7 +27,7 @@ note: future is not `Send` as it awaits another future which is not `Send`
  --> $DIR/issue-68112.rs:40:17
  |
 LL | let _ = make_non_send_future1().await;
- | ^^^^^^^^^^^^^^^^^^^^^^^ await occurs here on type `impl Future`, which is not `Send`
+ | ^^^^^^^^^^^^^^^^^^^^^^^ await occurs here on type `impl Future<Output = Arc<RefCell<i32>>>`, which is not `Send`
 note: required by a bound in `require_send`
  --> $DIR/issue-68112.rs:11:25
  |
@@ -45,9 +45,9 @@ LL | require_send(send_fut);
  = note: required because it appears within the type `[static generator@$DIR/issue-68112.rs:47:31: 47:36]`
  = note: required because it appears within the type `from_generator::GenFuture<[static generator@$DIR/issue-68112.rs:47:31: 47:36]>`
  = note: required because it appears within the type `impl Future`
- = note: required because it appears within the type `impl Future`
- = note: required because it appears within the type `impl Future`
- = note: required because it appears within the type `{ResumeTy, impl Future, (), i32, Ready<i32>}`
+ = note: required because it appears within the type `impl Future<Output = Arc<RefCell<i32>>>`
+ = note: required because it appears within the type `impl Future<Output = Arc<RefCell<i32>>>`
+ = note: required because it appears within the type `{ResumeTy, impl Future<Output = Arc<RefCell<i32>>>, (), i32, Ready<i32>}`
  = note: required because it appears within the type `[static generator@$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`

src/test/ui/async-await/issue-71137.stderr

@@ -4,7 +4,7 @@ error: future cannot be sent between threads safely
 LL | fake_spawn(wrong_mutex());
  | ^^^^^^^^^^^^^ future returned by `wrong_mutex` is not `Send`
  |
- = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, i32>`
+ = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `MutexGuard<'_, i32>`
 note: future is not `Send` as this value is used across an await
  --> $DIR/issue-71137.rs:14:5
  |

src/test/ui/async-await/issue-74497-lifetime-in-opaque.stderr

@@ -4,7 +4,7 @@ error: lifetime may not live long enough
 LL | let _ = foo(|x| bar(x));
  | -- ^^^^^^ returning this value requires that `'1` must outlive `'2`
  | ||
- | |return type of closure `impl Future` contains a lifetime `'2`
+ | |return type of closure `impl Future<Output = ()>` contains a lifetime `'2`
  | has type `&'1 u8`
 
 error: aborting due to previous error

src/test/ui/async-await/issue-84841.stderr

@@ -2,9 +2,9 @@ error[E0277]: the `?` operator can only be applied to values that implement `Try
  --> $DIR/issue-84841.rs:9:5
  |
 LL | test()?;
- | ^^^^^^^ the `?` operator cannot be applied to type `impl Future`
+ | ^^^^^^^ the `?` operator cannot be applied to type `impl Future<Output = ()>`
  |
- = help: the trait `Try` is not implemented for `impl Future`
+ = help: the trait `Try` is not implemented for `impl Future<Output = ()>`
 note: required by `branch`
  --> $SRC_DIR/core/src/ops/try_trait.rs:LL:COL
  |

src/test/ui/async-await/issues/issue-67893.stderr

@@ -4,7 +4,7 @@ error: generator cannot be sent between threads safely
 LL | g(issue_67893::run())
  | ^^^^^^^^^^^^^^^^^^ generator is not `Send`
  |
- = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
+ = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
 note: required by a bound in `g`
  --> $DIR/issue-67893.rs:6:14
  |

src/test/ui/async-await/suggest-missing-await-closure.stderr

@@ -10,7 +10,7 @@ note: while checking the return type of the `async fn`
 LL | async fn make_u32() -> u32 {
  | ^^^ checked the `Output` of this `async fn`, found opaque type
  = note: expected type `u32`
- found opaque type `impl Future`
+ found opaque type `impl Future<Output = u32>`
 help: consider `await`ing on the `Future`
  |
 LL | take_u32(x.await)