Suggest using Arc on !Send/!Sync types

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2743,6 +2743,12 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
             ObligationCauseCode::BindingObligation(item_def_id, span)
             | ObligationCauseCode::ExprBindingObligation(item_def_id, span, ..) => {
+                if self.tcx.is_diagnostic_item(sym::Send, item_def_id)
+                    || self.tcx.lang_items().sync_trait() == Some(item_def_id)
+                {
+                    return;
+                }
+
                 let item_name = tcx.def_path_str(item_def_id);
                 let short_item_name = with_forced_trimmed_paths!(tcx.def_path_str(item_def_id));
                 let mut multispan = MultiSpan::from(span);

library/core/src/marker.rs

@@ -76,8 +76,11 @@ macro marker_impls {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Send")]
 #[rustc_on_unimplemented(
+    on(_Self = "std::rc::Rc<T, A>", note = "use `std::sync::Arc` instead of `std::rc::Rc`"),
     message = "`{Self}` cannot be sent between threads safely",
-    label = "`{Self}` cannot be sent between threads safely"
+    label = "`{Self}` cannot be sent between threads safely",
+    note = "consider using `std::sync::Arc<{Self}>`; for more information visit \
+            <https://doc.rust-lang.org/book/ch16-03-shared-state.html>"
 )]
 pub unsafe auto trait Send {
     // empty.
@@ -628,8 +631,11 @@ impl<T: ?Sized> Copy for &T {}
         any(_Self = "core::cell::RefCell<T>", _Self = "std::cell::RefCell<T>"),
         note = "if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead",
     ),
+    on(_Self = "std::rc::Rc<T, A>", note = "use `std::sync::Arc` instead of `std::rc::Rc`"),
     message = "`{Self}` cannot be shared between threads safely",
-    label = "`{Self}` cannot be shared between threads safely"
+    label = "`{Self}` cannot be shared between threads safely",
+    note = "consider using `std::sync::Arc<{Self}>`; for more information visit \
+            <https://doc.rust-lang.org/book/ch16-03-shared-state.html>"
 )]
 pub unsafe auto trait Sync {
     // FIXME(estebank): once support to add notes in `rustc_on_unimplemented`

tests/ui/associated-type-bounds/bad-bounds-on-assoc-in-trait.stderr

@@ -5,6 +5,7 @@ LL |     type C: Clone + Iterator<Item: Send + Iterator<Item: for<'a> Lam<&'a u8
    |                                    ^^^^ `<<Self as Case1>::C as Iterator>::Item` cannot be sent between threads safely
    |
    = help: the trait `Send` is not implemented for `<<Self as Case1>::C as Iterator>::Item`
+   = note: consider using `std::sync::Arc<<<Self as Case1>::C as Iterator>::Item>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 help: consider further restricting the associated type
    |
 LL | trait Case1 where <<Self as Case1>::C as Iterator>::Item: Send {
@@ -29,6 +30,7 @@ LL |     type C: Clone + Iterator<Item: Send + Iterator<Item: for<'a> Lam<&'a u8
    |                                                                                             ^^^^ `<<Self as Case1>::C as Iterator>::Item` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `<<Self as Case1>::C as Iterator>::Item`
+   = note: consider using `std::sync::Arc<<<Self as Case1>::C as Iterator>::Item>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 help: consider further restricting the associated type
    |
 LL | trait Case1 where <<Self as Case1>::C as Iterator>::Item: Sync {

tests/ui/associated-type-bounds/return-type-notation/basic.without.stderr

@@ -14,6 +14,7 @@ LL |     is_send(foo::<T>());
    |             ^^^^^^^^^^ future returned by `foo` is not `Send`
    |
    = help: within `impl Future<Output = Result<(), ()>>`, the trait `Send` is not implemented for `impl Future<Output = Result<(), ()>>`
+   = note: consider using `std::sync::Arc<impl Future<Output = Result<(), ()>>>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as it awaits another future which is not `Send`
   --> $DIR/basic.rs:13:5
    |

tests/ui/async-await/async-await-let-else.drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     is_send(foo(Some(true)));
    |             ^^^^^^^^^^^^^^^ future returned by `foo` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:11:15
    |
@@ -32,6 +33,7 @@ LL |     is_send(foo2(Some(true)));
    |     required by a bound introduced by this call
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: required because it's used within this `async fn` body
   --> $DIR/async-await-let-else.rs:27:29
    |
@@ -64,6 +66,7 @@ LL |     is_send(foo3(Some(true)));
    |             ^^^^^^^^^^^^^^^^ future returned by `foo3` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:33:29
    |
@@ -85,6 +88,7 @@ LL |     is_send(foo4(Some(true)));
    |             ^^^^^^^^^^^^^^^^ future returned by `foo4` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:41:15
    |

tests/ui/async-await/async-await-let-else.drop_tracking_mir.stderr

@@ -5,6 +5,7 @@ LL |     is_send(foo(Some(true)));
    |             ^^^^^^^^^^^^^^^ future returned by `foo` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:11:15
    |
@@ -30,6 +31,7 @@ LL |     is_send(foo2(Some(true)));
    |     required by a bound introduced by this call
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: required because it's used within this `async fn` body
   --> $DIR/async-await-let-else.rs:27:29
    |
@@ -62,6 +64,7 @@ LL |     is_send(foo3(Some(true)));
    |             ^^^^^^^^^^^^^^^^ future returned by `foo3` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:33:29
    |
@@ -82,6 +85,7 @@ LL |     is_send(foo4(Some(true)));
    |             ^^^^^^^^^^^^^^^^ future returned by `foo4` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:41:15
    |

tests/ui/async-await/async-await-let-else.no_drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     is_send(foo(Some(true)));
    |             ^^^^^^^^^^^^^^^ future returned by `foo` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:11:15
    |
@@ -27,6 +28,7 @@ LL |     is_send(foo2(Some(true)));
    |             ^^^^^^^^^^^^^^^^ future returned by `foo2` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:23:27
    |
@@ -49,6 +51,7 @@ LL |     is_send(foo3(Some(true)));
    |             ^^^^^^^^^^^^^^^^ future returned by `foo3` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:33:29
    |
@@ -70,6 +73,7 @@ LL |     is_send(foo4(Some(true)));
    |             ^^^^^^^^^^^^^^^^ future returned by `foo4` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-await-let-else.rs:41:15
    |

tests/ui/async-await/async-fn-nonsend.drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(non_send_temporary_in_match());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_send_temporary_in_match` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:36:26
    |
@@ -28,6 +29,7 @@ LL |     assert_send(non_sync_with_method_call());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_sync_with_method_call` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `dyn std::fmt::Write`
+   = note: consider using `std::sync::Arc<dyn std::fmt::Write>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:49:15
    |

tests/ui/async-await/async-fn-nonsend.drop_tracking_mir.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(non_send_temporary_in_match());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_send_temporary_in_match` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:36:26
    |
@@ -25,6 +26,7 @@ LL |     assert_send(non_sync_with_method_call());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_sync_with_method_call` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `dyn std::fmt::Write`
+   = note: consider using `std::sync::Arc<dyn std::fmt::Write>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:49:15
    |

tests/ui/async-await/async-fn-nonsend.no_drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(local_dropped_before_await());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `local_dropped_before_await` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:27:11
    |
@@ -28,6 +29,7 @@ LL |     assert_send(non_send_temporary_in_match());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_send_temporary_in_match` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:36:26
    |
@@ -51,6 +53,7 @@ LL |     assert_send(non_sync_with_method_call());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_sync_with_method_call` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `dyn std::fmt::Write`
+   = note: consider using `std::sync::Arc<dyn std::fmt::Write>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:49:15
    |
@@ -75,6 +78,7 @@ LL |     assert_send(non_sync_with_method_call_panic());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_sync_with_method_call_panic` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `dyn std::fmt::Write`
+   = note: consider using `std::sync::Arc<dyn std::fmt::Write>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:56:15
    |
@@ -99,6 +103,7 @@ LL |     assert_send(non_sync_with_method_call_infinite_loop());
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ future returned by `non_sync_with_method_call_infinite_loop` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `dyn std::fmt::Write`
+   = note: consider using `std::sync::Arc<dyn std::fmt::Write>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:63:15
    |

tests/ui/async-await/drop-track-field-assign-nonsend.drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(agent.handle());
    |                 ^^^^^^^^^^^^^^ future returned by `handle` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<String>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/drop-track-field-assign-nonsend.rs:23:39
    |

tests/ui/async-await/drop-track-field-assign-nonsend.drop_tracking_mir.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(agent.handle());
    |                 ^^^^^^^^^^^^^^ future returned by `handle` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<String>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/drop-track-field-assign-nonsend.rs:23:39
    |

tests/ui/async-await/drop-track-field-assign-nonsend.no_drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(agent.handle());
    |                 ^^^^^^^^^^^^^^ future returned by `handle` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<String>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/drop-track-field-assign-nonsend.rs:23:39
    |

tests/ui/async-await/field-assign-nonsend.drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(agent.handle());
    |                 ^^^^^^^^^^^^^^ future returned by `handle` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<String>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/field-assign-nonsend.rs:23:39
    |

tests/ui/async-await/field-assign-nonsend.drop_tracking_mir.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(agent.handle());
    |                 ^^^^^^^^^^^^^^ future returned by `handle` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<String>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/field-assign-nonsend.rs:23:39
    |

tests/ui/async-await/field-assign-nonsend.no_drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     assert_send(agent.handle());
    |                 ^^^^^^^^^^^^^^ future returned by `handle` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<String>`
+   = note: use `std::sync::Arc` instead of `std::rc::Rc`
 note: future is not `Send` as this value is used across an await
   --> $DIR/field-assign-nonsend.rs:23:39
    |

tests/ui/async-await/in-trait/missing-send-bound.stderr

@@ -5,6 +5,7 @@ LL |     assert_is_send(test::<T>());
    |                    ^^^^^^^^^^^ future returned by `test` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `impl Future<Output = ()>`
+   = note: consider using `std::sync::Arc<impl Future<Output = ()>>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as it awaits another future which is not `Send`
   --> $DIR/missing-send-bound.rs:10:5
    |

tests/ui/async-await/issue-64130-1-sync.drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     is_sync(bar());
    |             ^^^^^ future returned by `bar` is not `Sync`
    |
    = help: within `impl Future<Output = ()>`, the trait `Sync` is not implemented for `Foo`
+   = note: consider using `std::sync::Arc<Foo>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Sync` as this value is used across an await
   --> $DIR/issue-64130-1-sync.rs:18:11
    |

tests/ui/async-await/issue-64130-1-sync.drop_tracking_mir.stderr

@@ -5,6 +5,7 @@ LL |     is_sync(bar());
    |             ^^^^^ future returned by `bar` is not `Sync`
    |
    = help: within `impl Future<Output = ()>`, the trait `Sync` is not implemented for `Foo`
+   = note: consider using `std::sync::Arc<Foo>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Sync` as this value is used across an await
   --> $DIR/issue-64130-1-sync.rs:18:11
    |

tests/ui/async-await/issue-64130-1-sync.no_drop_tracking.stderr

@@ -5,6 +5,7 @@ LL |     is_sync(bar());
    |             ^^^^^ future returned by `bar` is not `Sync`
    |
    = help: within `impl Future<Output = ()>`, the trait `Sync` is not implemented for `Foo`
+   = note: consider using `std::sync::Arc<Foo>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Sync` as this value is used across an await
   --> $DIR/issue-64130-1-sync.rs:18:11
    |

tests/ui/async-await/issue-64130-4-async-move.no_drop_tracking.stderr

@@ -5,6 +5,7 @@ LL | pub fn foo() -> impl Future + Send {
    |                 ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
    |
    = help: the trait `Sync` is not implemented for `(dyn Any + Send + 'static)`
+   = note: consider using `std::sync::Arc<(dyn Any + Send + 'static)>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/issue-64130-4-async-move.rs:27:23
    |

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

@@ -5,6 +5,7 @@ LL |     is_send(foo());
    |             ^^^^^ future returned by `foo` is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `MutexGuard<'_, u32>`
+   = note: consider using `std::sync::Arc<MutexGuard<'_, u32>>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/issue-64130-non-send-future-diags.rs:17:11
    |

tests/ui/async-await/issue-67252-unnamed-future.drop_tracking.stderr

@@ -10,6 +10,7 @@ LL | |     });
    | |_____^ future created by async block is not `Send`
    |
    = help: within `[async block@$DIR/issue-67252-unnamed-future.rs:21:11: 25:6]`, the trait `Send` is not implemented for `*mut ()`
+   = note: consider using `std::sync::Arc<*mut ()>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/issue-67252-unnamed-future.rs:23:17
    |

tests/ui/async-await/issue-67252-unnamed-future.drop_tracking_mir.stderr

@@ -5,6 +5,7 @@ LL |     spawn(async {
    |     ^^^^^ future created by async block is not `Send`
    |
    = help: within `[async block@$DIR/issue-67252-unnamed-future.rs:21:11: 25:6]`, the trait `Send` is not implemented for `*mut ()`
+   = note: consider using `std::sync::Arc<*mut ()>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/issue-67252-unnamed-future.rs:23:17
    |

tests/ui/async-await/issue-67252-unnamed-future.no_drop_tracking.stderr

@@ -10,6 +10,7 @@ LL | |     });
    | |_____^ future created by async block is not `Send`
    |
    = help: within `[async block@$DIR/issue-67252-unnamed-future.rs:21:11: 25:6]`, the trait `Send` is not implemented for `*mut ()`
+   = note: consider using `std::sync::Arc<*mut ()>`; for more information visit <https://doc.rust-lang.org/book/ch16-03-shared-state.html>
 note: future is not `Send` as this value is used across an await
   --> $DIR/issue-67252-unnamed-future.rs:23:17
    |