Auto merge of #65345 - davidtwco:issue-64130-async-send-sync-error-improvements, r=nikomatsakis

async/await: improve not-send errors, part 2

Part of #64130. Fixes #65667.

This PR improves the errors introduced in #64895 so that they have specialized messages for `Send` and `Sync`.

r? @nikomatsakis

14 files changed, 265 insertions, 63 deletions

diff --git a/src/test/ui/async-await/async-fn-nonsend.rs b/src/test/ui/async-await/async-fn-nonsend.rs
index 1f1bf4250ea..645c903c6ba 100644
--- a/src/test/ui/async-await/async-fn-nonsend.rs
+++ b/src/test/ui/async-await/async-fn-nonsend.rs
@@ -48,10 +48,10 @@ fn assert_send(_: impl Send) {}
 
 pub fn pass_assert() {
     assert_send(local_dropped_before_await());
-    //~^ ERROR `std::rc::Rc<()>` cannot be sent between threads safely
+    //~^ ERROR future cannot be sent between threads safely
     assert_send(non_send_temporary_in_match());
-    //~^ ERROR `std::rc::Rc<()>` cannot be sent between threads safely
+    //~^ ERROR future cannot be sent between threads safely
     assert_send(non_sync_with_method_call());
-    //~^ ERROR `dyn std::fmt::Write` cannot be sent between threads safely
-    //~^^ ERROR `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
+    //~^ ERROR future cannot be sent between threads safely
+    //~^^ ERROR future cannot be sent between threads safely
 }
diff --git a/src/test/ui/async-await/async-fn-nonsend.stderr b/src/test/ui/async-await/async-fn-nonsend.stderr
index 6e89deb407e..5c870ca2d02 100644
--- a/src/test/ui/async-await/async-fn-nonsend.stderr
+++ b/src/test/ui/async-await/async-fn-nonsend.stderr
@@ -1,79 +1,88 @@
-error[E0277]: `std::rc::Rc<()>` cannot be sent between threads safely
+error: future cannot be sent between threads safely
   --> $DIR/async-fn-nonsend.rs:50:5
    |
 LL | fn assert_send(_: impl Send) {}
    |    -----------         ---- required by this bound in `assert_send`
 ...
 LL |     assert_send(local_dropped_before_await());
-   |     ^^^^^^^^^^^ `std::rc::Rc<()>` cannot be sent between threads safely
+   |     ^^^^^^^^^^^ future returned by `local_dropped_before_await` is not `Send`
    |
    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<()>`
-   = note: required because it appears within the type `impl std::fmt::Debug`
-   = note: required because it appears within the type `{impl std::fmt::Debug, impl std::future::Future, impl std::future::Future, ()}`
-   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:21:39: 26:2 {impl std::fmt::Debug, impl std::future::Future, impl std::future::Future, ()}]`
-   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:21:39: 26:2 {impl std::fmt::Debug, impl std::future::Future, impl std::future::Future, ()}]>`
-   = note: required because it appears within the type `impl std::future::Future`
-   = note: required because it appears within the type `impl std::future::Future`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/async-fn-nonsend.rs:25:5
+   |
+LL |     let x = non_send();
+   |         - has type `impl std::fmt::Debug`
+LL |     drop(x);
+LL |     fut().await;
+   |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+LL | }
+   | - `x` is later dropped here
 
-error[E0277]: `std::rc::Rc<()>` cannot be sent between threads safely
+error: future cannot be sent between threads safely
   --> $DIR/async-fn-nonsend.rs:52:5
    |
 LL | fn assert_send(_: impl Send) {}
    |    -----------         ---- required by this bound in `assert_send`
 ...
 LL |     assert_send(non_send_temporary_in_match());
-   |     ^^^^^^^^^^^ `std::rc::Rc<()>` cannot be sent between threads safely
+   |     ^^^^^^^^^^^ future returned by `non_send_temporary_in_match` is not `Send`
    |
    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<()>`
-   = note: required because it appears within the type `impl std::fmt::Debug`
-   = note: required because it appears within the type `{impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, impl std::future::Future, impl std::future::Future, ()}`
-   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:28:40: 37:2 {impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, impl std::future::Future, impl std::future::Future, ()}]`
-   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:28:40: 37:2 {impl std::fmt::Debug, std::option::Option<impl std::fmt::Debug>, impl std::future::Future, impl std::future::Future, ()}]>`
-   = note: required because it appears within the type `impl std::future::Future`
-   = note: required because it appears within the type `impl std::future::Future`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/async-fn-nonsend.rs:34:20
+   |
+LL |     match Some(non_send()) {
+   |                ---------- has type `impl std::fmt::Debug`
+LL |         Some(_) => fut().await,
+   |                    ^^^^^^^^^^^ await occurs here, with `non_send()` maybe used later
+...
+LL | }
+   | - `non_send()` is later dropped here
 
-error[E0277]: `dyn std::fmt::Write` cannot be sent between threads safely
+error: future cannot be sent between threads safely
   --> $DIR/async-fn-nonsend.rs:54:5
    |
 LL | fn assert_send(_: impl Send) {}
    |    -----------         ---- required by this bound in `assert_send`
 ...
 LL |     assert_send(non_sync_with_method_call());
-   |     ^^^^^^^^^^^ `dyn std::fmt::Write` cannot be sent between threads safely
+   |     ^^^^^^^^^^^ future returned by `non_sync_with_method_call` is not `Send`
    |
    = help: the trait `std::marker::Send` is not implemented for `dyn std::fmt::Write`
-   = note: required because of the requirements on the impl of `std::marker::Send` for `&mut dyn std::fmt::Write`
-   = note: required because it appears within the type `std::fmt::Formatter<'_>`
-   = note: required because of the requirements on the impl of `std::marker::Send` for `&mut std::fmt::Formatter<'_>`
-   = note: required because it appears within the type `for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, bool, impl std::future::Future, impl std::future::Future, ()}`
-   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, bool, impl std::future::Future, impl std::future::Future, ()}]`
-   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, bool, impl std::future::Future, impl std::future::Future, ()}]>`
-   = note: required because it appears within the type `impl std::future::Future`
-   = note: required because it appears within the type `impl std::future::Future`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/async-fn-nonsend.rs:43:9
+   |
+LL |     let f: &mut std::fmt::Formatter = panic!();
+   |         - has type `&mut std::fmt::Formatter<'_>`
+LL |     if non_sync().fmt(f).unwrap() == () {
+LL |         fut().await;
+   |         ^^^^^^^^^^^ await occurs here, with `f` maybe used later
+LL |     }
+LL | }
+   | - `f` is later dropped here
 
-error[E0277]: `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
+error: future cannot be sent between threads safely
   --> $DIR/async-fn-nonsend.rs:54:5
    |
 LL | fn assert_send(_: impl Send) {}
    |    -----------         ---- required by this bound in `assert_send`
 ...
 LL |     assert_send(non_sync_with_method_call());
-   |     ^^^^^^^^^^^ `*mut (dyn std::ops::Fn() + 'static)` cannot be shared between threads safely
+   |     ^^^^^^^^^^^ future returned by `non_sync_with_method_call` is not `Send`
    |
    = help: within `std::fmt::ArgumentV1<'_>`, the trait `std::marker::Sync` is not implemented for `*mut (dyn std::ops::Fn() + 'static)`
-   = note: required because it appears within the type `std::marker::PhantomData<*mut (dyn std::ops::Fn() + 'static)>`
-   = note: required because it appears within the type `core::fmt::Void`
-   = note: required because it appears within the type `&core::fmt::Void`
-   = note: required because it appears within the type `std::fmt::ArgumentV1<'_>`
-   = note: required because of the requirements on the impl of `std::marker::Send` for `std::slice::Iter<'_, std::fmt::ArgumentV1<'_>>`
-   = note: required because it appears within the type `std::fmt::Formatter<'_>`
-   = note: required because of the requirements on the impl of `std::marker::Send` for `&mut std::fmt::Formatter<'_>`
-   = note: required because it appears within the type `for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, bool, impl std::future::Future, impl std::future::Future, ()}`
-   = note: required because it appears within the type `[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, bool, impl std::future::Future, impl std::future::Future, ()}]`
-   = note: required because it appears within the type `std::future::GenFuture<[static generator@$DIR/async-fn-nonsend.rs:39:38: 45:2 for<'r, 's> {&'r mut std::fmt::Formatter<'s>, bool, bool, impl std::future::Future, impl std::future::Future, ()}]>`
-   = note: required because it appears within the type `impl std::future::Future`
-   = note: required because it appears within the type `impl std::future::Future`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/async-fn-nonsend.rs:43:9
+   |
+LL |     let f: &mut std::fmt::Formatter = panic!();
+   |         - has type `&mut std::fmt::Formatter<'_>`
+LL |     if non_sync().fmt(f).unwrap() == () {
+LL |         fut().await;
+   |         ^^^^^^^^^^^ await occurs here, with `f` maybe used later
+LL |     }
+LL | }
+   | - `f` is later dropped here
 
 error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/async-await/issue-64130-1-sync.rs b/src/test/ui/async-await/issue-64130-1-sync.rs
new file mode 100644
index 00000000000..cc5ca89f03a
--- /dev/null
+++ b/src/test/ui/async-await/issue-64130-1-sync.rs
@@ -0,0 +1,23 @@
+#![feature(optin_builtin_traits)]
+// edition:2018
+
+// This tests the the specialized async-await-specific error when futures don't implement an
+// auto trait (which is specifically Sync) due to some type that was captured.
+
+struct Foo;
+
+impl !Sync for Foo {}
+
+fn is_sync<T: Sync>(t: T) { }
+
+async fn bar() {
+    let x = Foo;
+    baz().await;
+}
+
+async fn baz() { }
+
+fn main() {
+    is_sync(bar());
+    //~^ ERROR future cannot be shared between threads safely
+}
diff --git a/src/test/ui/async-await/issue-64130-1-sync.stderr b/src/test/ui/async-await/issue-64130-1-sync.stderr
new file mode 100644
index 00000000000..8beb31f152a
--- /dev/null
+++ b/src/test/ui/async-await/issue-64130-1-sync.stderr
@@ -0,0 +1,22 @@
+error: future cannot be shared between threads safely
+  --> $DIR/issue-64130-1-sync.rs:21:5
+   |
+LL | fn is_sync<T: Sync>(t: T) { }
+   |    -------    ---- required by this bound in `is_sync`
+...
+LL |     is_sync(bar());
+   |     ^^^^^^^ future returned by `bar` is not `Sync`
+   |
+   = help: within `impl std::future::Future`, the trait `std::marker::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
+   |
+LL |     let x = Foo;
+   |         - has type `Foo`
+LL |     baz().await;
+   |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+LL | }
+   | - `x` is later dropped here
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/async-await/issue-64130-2-send.rs b/src/test/ui/async-await/issue-64130-2-send.rs
new file mode 100644
index 00000000000..1efe2ab3f85
--- /dev/null
+++ b/src/test/ui/async-await/issue-64130-2-send.rs
@@ -0,0 +1,23 @@
+#![feature(optin_builtin_traits)]
+// edition:2018
+
+// This tests the the specialized async-await-specific error when futures don't implement an
+// auto trait (which is specifically Send) due to some type that was captured.
+
+struct Foo;
+
+impl !Send for Foo {}
+
+fn is_send<T: Send>(t: T) { }
+
+async fn bar() {
+    let x = Foo;
+    baz().await;
+}
+
+async fn baz() { }
+
+fn main() {
+    is_send(bar());
+    //~^ ERROR future cannot be sent between threads safely
+}
diff --git a/src/test/ui/async-await/issue-64130-2-send.stderr b/src/test/ui/async-await/issue-64130-2-send.stderr
new file mode 100644
index 00000000000..823b88e18c5
--- /dev/null
+++ b/src/test/ui/async-await/issue-64130-2-send.stderr
@@ -0,0 +1,22 @@
+error: future cannot be sent between threads safely
+  --> $DIR/issue-64130-2-send.rs:21:5
+   |
+LL | fn is_send<T: Send>(t: T) { }
+   |    -------    ---- required by this bound in `is_send`
+...
+LL |     is_send(bar());
+   |     ^^^^^^^ future returned by `bar` is not `Send`
+   |
+   = help: within `impl std::future::Future`, the trait `std::marker::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
+   |
+LL |     let x = Foo;
+   |         - has type `Foo`
+LL |     baz().await;
+   |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+LL | }
+   | - `x` is later dropped here
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/async-await/issue-64130-3-other.rs b/src/test/ui/async-await/issue-64130-3-other.rs
new file mode 100644
index 00000000000..901544edba1
--- /dev/null
+++ b/src/test/ui/async-await/issue-64130-3-other.rs
@@ -0,0 +1,25 @@
+#![feature(optin_builtin_traits)]
+// edition:2018
+
+// 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 { }
+
+struct Foo;
+
+impl !Qux for Foo {}
+
+fn is_qux<T: Qux>(t: T) { }
+
+async fn bar() {
+    let x = Foo;
+    baz().await;
+}
+
+async fn baz() { }
+
+fn main() {
+    is_qux(bar());
+    //~^ ERROR the trait bound `Foo: Qux` is not satisfied in `impl std::future::Future`
+}
diff --git a/src/test/ui/async-await/issue-64130-3-other.stderr b/src/test/ui/async-await/issue-64130-3-other.stderr
new file mode 100644
index 00000000000..155c5cc8ea1
--- /dev/null
+++ b/src/test/ui/async-await/issue-64130-3-other.stderr
@@ -0,0 +1,24 @@
+error[E0277]: the trait bound `Foo: Qux` is not satisfied in `impl std::future::Future`
+  --> $DIR/issue-64130-3-other.rs:23:5
+   |
+LL | fn is_qux<T: Qux>(t: T) { }
+   |    ------    --- required by this bound in `is_qux`
+...
+LL |     is_qux(bar());
+   |     ^^^^^^ within `impl std::future::Future`, the trait `Qux` is not implemented for `Foo`
+   |
+   = help: the following implementations were found:
+             <Foo as Qux>
+note: future does not implement `Qux` as this value is used across an await
+  --> $DIR/issue-64130-3-other.rs:17:5
+   |
+LL |     let x = Foo;
+   |         - has type `Foo`
+LL |     baz().await;
+   |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
+LL | }
+   | - `x` is later dropped here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/async-await/issue-64130-4-async-move.rs b/src/test/ui/async-await/issue-64130-4-async-move.rs
new file mode 100644
index 00000000000..2538f34351e
--- /dev/null
+++ b/src/test/ui/async-await/issue-64130-4-async-move.rs
@@ -0,0 +1,28 @@
+// edition:2018
+use std::any::Any;
+use std::future::Future;
+
+struct Client(Box<dyn Any + Send>);
+
+impl Client {
+    fn status(&self) -> u16 {
+        200
+    }
+}
+
+async fn get() { }
+
+pub fn foo() -> impl Future + Send {
+    //~^ ERROR future cannot be sent between threads safely
+    let client = Client(Box::new(true));
+    async move {
+        match client.status() {
+            200 => {
+                let _x = get().await;
+            },
+            _ => (),
+        }
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/async-await/issue-64130-4-async-move.stderr b/src/test/ui/async-await/issue-64130-4-async-move.stderr
new file mode 100644
index 00000000000..ddbb469b99c
--- /dev/null
+++ b/src/test/ui/async-await/issue-64130-4-async-move.stderr
@@ -0,0 +1,22 @@
+error: future cannot be sent between threads safely
+  --> $DIR/issue-64130-4-async-move.rs:15:17
+   |
+LL | pub fn foo() -> impl Future + Send {
+   |                 ^^^^^^^^^^^^^^^^^^ future returned by `foo` is not `Send`
+   |
+   = help: the trait `std::marker::Sync` is not implemented for `(dyn std::any::Any + std::marker::Send + 'static)`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/issue-64130-4-async-move.rs:21:26
+   |
+LL |         match client.status() {
+   |               ------ has type `&Client`
+LL |             200 => {
+LL |                 let _x = get().await;
+   |                          ^^^^^^^^^^^ await occurs here, with `client` maybe used later
+...
+LL |     }
+   |     - `client` is later dropped here
+   = note: the return type of a function must have a statically known size
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/async-await/issue-64130-non-send-future-diags.rs b/src/test/ui/async-await/issue-64130-non-send-future-diags.rs
index 1936d1a2ed5..656ade67c71 100644
--- a/src/test/ui/async-await/issue-64130-non-send-future-diags.rs
+++ b/src/test/ui/async-await/issue-64130-non-send-future-diags.rs
@@ -1,10 +1,10 @@
 // edition:2018
 
-use std::sync::Mutex;
+// This tests the basic example case for the async-await-specific error.
 
-fn is_send<T: Send>(t: T) {
+use std::sync::Mutex;
 
-}
+fn is_send<T: Send>(t: T) { }
 
 async fn foo() {
     bar(&Mutex::new(22)).await;
@@ -15,11 +15,9 @@ async fn bar(x: &Mutex<u32>) {
     baz().await;
 }
 
-async fn baz() {
-
-}
+async fn baz() { }
 
 fn main() {
     is_send(foo());
-    //~^ ERROR `std::sync::MutexGuard<'_, u32>` cannot be sent between threads safely [E0277]
+    //~^ ERROR future cannot be sent between threads safely
 }
diff --git a/src/test/ui/async-await/issue-64130-non-send-future-diags.stderr b/src/test/ui/async-await/issue-64130-non-send-future-diags.stderr
index 9e9fc52e30b..662407f7017 100644
--- a/src/test/ui/async-await/issue-64130-non-send-future-diags.stderr
+++ b/src/test/ui/async-await/issue-64130-non-send-future-diags.stderr
@@ -1,14 +1,14 @@
-error[E0277]: `std::sync::MutexGuard<'_, u32>` cannot be sent between threads safely
-  --> $DIR/issue-64130-non-send-future-diags.rs:23:5
+error: future cannot be sent between threads safely
+  --> $DIR/issue-64130-non-send-future-diags.rs:21:5
    |
-LL | fn is_send<T: Send>(t: T) {
+LL | fn is_send<T: Send>(t: T) { }
    |    -------    ---- required by this bound in `is_send`
 ...
 LL |     is_send(foo());
-   |     ^^^^^^^ `std::sync::MutexGuard<'_, u32>` cannot be sent between threads safely
+   |     ^^^^^^^ future returned by `foo` is not `Send`
    |
    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::sync::MutexGuard<'_, u32>`
-note: future does not implement `std::marker::Send` as this value is used across an await
+note: future is not `Send` as this value is used across an await
   --> $DIR/issue-64130-non-send-future-diags.rs:15:5
    |
 LL |     let g = x.lock().unwrap();
@@ -20,4 +20,3 @@ LL | }
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/generator/not-send-sync.rs b/src/test/ui/generator/not-send-sync.rs
index ae0a288bbb4..0db01c6f756 100644
--- a/src/test/ui/generator/not-send-sync.rs
+++ b/src/test/ui/generator/not-send-sync.rs
@@ -7,7 +7,7 @@ fn main() {
     fn assert_send<T: Send>(_: T) {}
 
     assert_sync(|| {
-        //~^ ERROR: E0277
+        //~^ ERROR: future cannot be shared between threads safely
         let a = Cell::new(2);
         yield;
     });
diff --git a/src/test/ui/generator/not-send-sync.stderr b/src/test/ui/generator/not-send-sync.stderr
index 620db245d3e..0ac1d189b79 100644
--- a/src/test/ui/generator/not-send-sync.stderr
+++ b/src/test/ui/generator/not-send-sync.stderr
@@ -11,18 +11,25 @@ LL |     assert_send(|| {
    = note: required because of the requirements on the impl of `std::marker::Send` for `&std::cell::Cell<i32>`
    = note: required because it appears within the type `[generator@$DIR/not-send-sync.rs:16:17: 20:6 a:&std::cell::Cell<i32> _]`
 
-error[E0277]: `std::cell::Cell<i32>` cannot be shared between threads safely
+error: future cannot be shared between threads safely
   --> $DIR/not-send-sync.rs:9:5
    |
 LL |     fn assert_sync<T: Sync>(_: T) {}
    |        -----------    ---- required by this bound in `main::assert_sync`
 ...
 LL |     assert_sync(|| {
-   |     ^^^^^^^^^^^ `std::cell::Cell<i32>` cannot be shared between threads safely
+   |     ^^^^^^^^^^^ future returned by `main` is not `Sync`
    |
    = help: within `[generator@$DIR/not-send-sync.rs:9:17: 13:6 {std::cell::Cell<i32>, ()}]`, the trait `std::marker::Sync` is not implemented for `std::cell::Cell<i32>`
-   = note: required because it appears within the type `{std::cell::Cell<i32>, ()}`
-   = note: required because it appears within the type `[generator@$DIR/not-send-sync.rs:9:17: 13:6 {std::cell::Cell<i32>, ()}]`
+note: future is not `Sync` as this value is used across an yield
+  --> $DIR/not-send-sync.rs:12:9
+   |
+LL |         let a = Cell::new(2);
+   |             - has type `std::cell::Cell<i32>`
+LL |         yield;
+   |         ^^^^^ yield occurs here, with `a` maybe used later
+LL |     });
+   |     - `a` is later dropped here
 
 error: aborting due to 2 previous errors