Auto merge of #70679 - tmandry:issue-68112, r=nikomatsakis

Improve async-await/generator obligation errors in some cases

Fixes #68112.

This change is best read one commit at a time (I add a test at the beginning and update it in each change after).

The `test2` function is a case I found while writing the test that we don't handle with this code yet. I don't attempt to fix it in this PR, but it's a good candidate for future work.

r? @davidtwco, @nikomatsakis

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

@@ -12,7 +12,7 @@ note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:24:5
    |
 LL |     let x = non_send();
-   |         - has type `impl std::fmt::Debug`
+   |         - has type `impl std::fmt::Debug` which is not `Send`
 LL |     drop(x);
 LL |     fut().await;
    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
@@ -33,7 +33,7 @@ note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:33:20
    |
 LL |     match Some(non_send()) {
-   |                ---------- has type `impl std::fmt::Debug`
+   |                ---------- has type `impl std::fmt::Debug` which is not `Send`
 LL |         Some(_) => fut().await,
    |                    ^^^^^^^^^^^ await occurs here, with `non_send()` maybe used later
 ...
@@ -54,7 +54,7 @@ note: future is not `Send` as this value is used across an await
   --> $DIR/async-fn-nonsend.rs:42:9
    |
 LL |     let f: &mut std::fmt::Formatter = panic!();
-   |         - has type `&mut std::fmt::Formatter<'_>`
+   |         - has type `&mut std::fmt::Formatter<'_>` which is not `Send`
 LL |     if non_sync().fmt(f).unwrap() == () {
 LL |         fut().await;
    |         ^^^^^^^^^^^ await occurs here, with `f` maybe used later

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

@@ -12,7 +12,7 @@ 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`
+   |         - has type `Foo` which is not `Sync`
 LL |     baz().await;
    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
 LL | }

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

@@ -12,7 +12,7 @@ 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`
+   |         - has type `Foo` which is not `Send`
 LL |     baz().await;
    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
 LL | }

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

@@ -16,7 +16,7 @@ note: future does not implement `Qux` as this value is used across an await
   --> $DIR/issue-64130-3-other.rs:18:5
    |
 LL |     let x = Foo;
-   |         - has type `Foo`
+   |         - has type `Foo` which does not implement `Qux`
 LL |     baz().await;
    |     ^^^^^^^^^^^ await occurs here, with `x` maybe used later
 LL | }

src/test/ui/async-await/issue-64130-4-async-move.stderr

@@ -2,7 +2,7 @@ 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`
+   |                   ^^^^^^^^^^^^^^^^^^ future created by async block is not `Send`
 ...
 LL | /     async move {
 LL | |         match client.status() {
@@ -18,7 +18,7 @@ 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`
+   |               ------ has type `&Client` which is not `Send`
 LL |             200 => {
 LL |                 let _x = get().await;
    |                          ^^^^^^^^^^^ await occurs here, with `client` maybe used later

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

@@ -12,7 +12,7 @@ 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();
-   |         - has type `std::sync::MutexGuard<'_, u32>`
+   |         - has type `std::sync::MutexGuard<'_, u32>` which is not `Send`
 LL |     baz().await;
    |     ^^^^^^^^^^^ await occurs here, with `g` maybe used later
 LL | }

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

@@ -5,14 +5,14 @@ LL | fn spawn<T: Send>(_: T) {}
    |             ---- required by this bound in `spawn`
 ...
 LL |     spawn(async {
-   |     ^^^^^ future is not `Send`
+   |     ^^^^^ future created by async block is not `Send`
    |
    = help: within `impl std::future::Future`, the trait `std::marker::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:20:9
    |
 LL |         let _a = std::ptr::null_mut::<()>(); // `*mut ()` is not `Send`
-   |             -- has type `*mut ()`
+   |             -- has type `*mut ()` which is not `Send`
 LL |         AFuture.await;
    |         ^^^^^^^^^^^^^ await occurs here, with `_a` maybe used later
 LL |     });

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

@@ -0,0 +1,64 @@
+// edition:2018
+
+use std::{
+    future::Future,
+    cell::RefCell,
+    sync::Arc,
+    pin::Pin,
+    task::{Context, Poll},
+};
+
+fn require_send(_: impl Send) {}
+
+struct Ready<T>(Option<T>);
+impl<T> Future for Ready<T> {
+    type Output = T;
+    fn poll(mut self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<T> {
+        Poll::Ready(self.0.take().unwrap())
+    }
+}
+fn ready<T>(t: T) -> Ready<T> {
+    Ready(Some(t))
+}
+
+fn make_non_send_future1() -> impl Future<Output = Arc<RefCell<i32>>> {
+    ready(Arc::new(RefCell::new(0)))
+}
+
+fn test1() {
+    let send_fut = async {
+        let non_send_fut = make_non_send_future1();
+        let _ = non_send_fut.await;
+        ready(0).await;
+    };
+    require_send(send_fut);
+    //~^ ERROR future cannot be sent between threads
+}
+
+fn test1_no_let() {
+    let send_fut = async {
+        let _ = make_non_send_future1().await;
+        ready(0).await;
+    };
+    require_send(send_fut);
+    //~^ ERROR future cannot be sent between threads
+}
+
+async fn ready2<T>(t: T) -> T { t }
+fn make_non_send_future2() -> impl Future<Output = Arc<RefCell<i32>>> {
+    ready2(Arc::new(RefCell::new(0)))
+}
+
+// Ideally this test would have diagnostics similar to the test above, but right
+// now it doesn't.
+fn test2() {
+    let send_fut = async {
+        let non_send_fut = make_non_send_future2();
+        let _ = non_send_fut.await;
+        ready(0).await;
+    };
+    require_send(send_fut);
+    //~^ ERROR `std::cell::RefCell<i32>` cannot be shared between threads safely
+}
+
+fn main() {}

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

@@ -0,0 +1,56 @@
+error: future cannot be sent between threads safely
+  --> $DIR/issue-68112.rs:34:5
+   |
+LL | fn require_send(_: impl Send) {}
+   |                         ---- required by this bound in `require_send`
+...
+LL |     require_send(send_fut);
+   |     ^^^^^^^^^^^^ future created by async block is not `Send`
+   |
+   = help: the trait `std::marker::Sync` is not implemented for `std::cell::RefCell<i32>`
+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 std::future::Future`, which is not `Send`
+
+error: future cannot be sent between threads safely
+  --> $DIR/issue-68112.rs:43:5
+   |
+LL | fn require_send(_: impl Send) {}
+   |                         ---- required by this bound in `require_send`
+...
+LL |     require_send(send_fut);
+   |     ^^^^^^^^^^^^ future created by async block is not `Send`
+   |
+   = help: the trait `std::marker::Sync` is not implemented for `std::cell::RefCell<i32>`
+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 std::future::Future`, which is not `Send`
+
+error[E0277]: `std::cell::RefCell<i32>` cannot be shared between threads safely
+  --> $DIR/issue-68112.rs:60:5
+   |
+LL | fn require_send(_: impl Send) {}
+   |                         ---- required by this bound in `require_send`
+...
+LL |     require_send(send_fut);
+   |     ^^^^^^^^^^^^ `std::cell::RefCell<i32>` cannot be shared between threads safely
+   |
+   = help: the trait `std::marker::Sync` is not implemented for `std::cell::RefCell<i32>`
+   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<std::cell::RefCell<i32>>`
+   = note: required because it appears within the type `[static generator@$DIR/issue-68112.rs:47:31: 47:36 t:std::sync::Arc<std::cell::RefCell<i32>> {}]`
+   = note: required because it appears within the type `std::future::from_generator::GenFuture<[static generator@$DIR/issue-68112.rs:47:31: 47:36 t:std::sync::Arc<std::cell::RefCell<i32>> {}]>`
+   = note: required because it appears within the type `impl std::future::Future`
+   = note: required because it appears within the type `impl std::future::Future`
+   = note: required because it appears within the type `impl std::future::Future`
+   = note: required because it appears within the type `{std::future::ResumeTy, impl std::future::Future, (), i32, Ready<i32>}`
+   = note: required because it appears within the type `[static generator@$DIR/issue-68112.rs:55:26: 59:6 {std::future::ResumeTy, impl std::future::Future, (), i32, Ready<i32>}]`
+   = note: required because it appears within the type `std::future::from_generator::GenFuture<[static generator@$DIR/issue-68112.rs:55:26: 59:6 {std::future::ResumeTy, impl std::future::Future, (), i32, Ready<i32>}]>`
+   = note: required because it appears within the type `impl std::future::Future`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

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

@@ -5,7 +5,7 @@ LL | fn assert_send<T: Send>(_: T) {}
    |                   ---- required by this bound in `assert_send`
 ...
 LL |     assert_send(async {
-   |     ^^^^^^^^^^^ future returned by `main` is not `Send`
+   |     ^^^^^^^^^^^ future created by async block is not `Send`
    |
    = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `*const u8`
 note: future is not `Send` as this value is used across an await
@@ -14,7 +14,7 @@ note: future is not `Send` as this value is used across an await
 LL |         bar(Foo(std::ptr::null())).await;
    |         ^^^^^^^^----------------^^^^^^^^- `std::ptr::null()` is later dropped here
    |         |       |
-   |         |       has type `*const u8`
+   |         |       has type `*const u8` which is not `Send`
    |         await occurs here, with `std::ptr::null()` maybe used later
 help: consider moving this into a `let` binding to create a shorter lived borrow
   --> $DIR/issue-65436-raw-ptr-not-send.rs:14:13

src/test/ui/generator/issue-68112.rs

@@ -0,0 +1,56 @@
+#![feature(generators, generator_trait)]
+
+use std::{
+    cell::RefCell,
+    sync::Arc,
+    pin::Pin,
+    ops::{Generator, GeneratorState},
+};
+
+pub struct Ready<T>(Option<T>);
+impl<T> Generator<()> for Ready<T> {
+    type Return = T;
+    type Yield = ();
+    fn resume(mut self: Pin<&mut Self>, _args: ()) -> GeneratorState<(), T> {
+        GeneratorState::Complete(self.0.take().unwrap())
+    }
+}
+pub fn make_gen1<T>(t: T) -> Ready<T> {
+    Ready(Some(t))
+}
+
+fn require_send(_: impl Send) {}
+
+fn make_non_send_generator() -> impl Generator<Return = Arc<RefCell<i32>>> {
+    make_gen1(Arc::new(RefCell::new(0)))
+}
+
+fn test1() {
+    let send_gen = || {
+        let _non_send_gen = make_non_send_generator();
+        yield;
+    };
+    require_send(send_gen);
+    //~^ ERROR generator cannot be sent between threads
+}
+
+pub fn make_gen2<T>(t: T) -> impl Generator<Return = T> {
+    || {
+        yield;
+        t
+    }
+}
+fn make_non_send_generator2() -> impl Generator<Return = Arc<RefCell<i32>>> {
+    make_gen2(Arc::new(RefCell::new(0)))
+}
+
+fn test2() {
+    let send_gen = || {
+        let _non_send_gen = make_non_send_generator2();
+        yield;
+    };
+    require_send(send_gen);
+    //~^ ERROR `std::cell::RefCell<i32>` cannot be shared between threads safely
+}
+
+fn main() {}

src/test/ui/generator/issue-68112.stderr

@@ -0,0 +1,40 @@
+error: generator cannot be sent between threads safely
+  --> $DIR/issue-68112.rs:33:5
+   |
+LL | fn require_send(_: impl Send) {}
+   |                         ---- required by this bound in `require_send`
+...
+LL |     require_send(send_gen);
+   |     ^^^^^^^^^^^^ generator is not `Send`
+   |
+   = help: the trait `std::marker::Sync` is not implemented for `std::cell::RefCell<i32>`
+note: generator is not `Send` as this value is used across a yield
+  --> $DIR/issue-68112.rs:31:9
+   |
+LL |         let _non_send_gen = make_non_send_generator();
+   |             ------------- has type `impl std::ops::Generator` which is not `Send`
+LL |         yield;
+   |         ^^^^^ yield occurs here, with `_non_send_gen` maybe used later
+LL |     };
+   |     - `_non_send_gen` is later dropped here
+
+error[E0277]: `std::cell::RefCell<i32>` cannot be shared between threads safely
+  --> $DIR/issue-68112.rs:52:5
+   |
+LL | fn require_send(_: impl Send) {}
+   |                         ---- required by this bound in `require_send`
+...
+LL |     require_send(send_gen);
+   |     ^^^^^^^^^^^^ `std::cell::RefCell<i32>` cannot be shared between threads safely
+   |
+   = help: the trait `std::marker::Sync` is not implemented for `std::cell::RefCell<i32>`
+   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<std::cell::RefCell<i32>>`
+   = note: required because it appears within the type `[generator@$DIR/issue-68112.rs:38:5: 41:6 t:std::sync::Arc<std::cell::RefCell<i32>> {()}]`
+   = note: required because it appears within the type `impl std::ops::Generator`
+   = note: required because it appears within the type `impl std::ops::Generator`
+   = note: required because it appears within the type `{impl std::ops::Generator, ()}`
+   = note: required because it appears within the type `[generator@$DIR/issue-68112.rs:48:20: 51:6 {impl std::ops::Generator, ()}]`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/generator/not-send-sync.rs

@@ -7,7 +7,7 @@ fn main() {
     fn assert_send<T: Send>(_: T) {}
 
     assert_sync(|| {
-        //~^ ERROR: future cannot be shared between threads safely
+        //~^ ERROR: generator cannot be shared between threads safely
         let a = Cell::new(2);
         yield;
     });

src/test/ui/generator/not-send-sync.stderr

@@ -11,21 +11,21 @@ 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: future cannot be shared between threads safely
+error: generator cannot be shared between threads safely
   --> $DIR/not-send-sync.rs:9:5
    |
 LL |     fn assert_sync<T: Sync>(_: T) {}
    |                       ---- required by this bound in `main::assert_sync`
 ...
 LL |     assert_sync(|| {
-   |     ^^^^^^^^^^^ future returned by `main` is not `Sync`
+   |     ^^^^^^^^^^^ generator is not `Sync`
    |
    = help: within `[generator@$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: future is not `Sync` as this value is used across an yield
+note: generator is not `Sync` as this value is used across a yield
   --> $DIR/not-send-sync.rs:12:9
    |
 LL |         let a = Cell::new(2);
-   |             - has type `std::cell::Cell<i32>`
+   |             - has type `std::cell::Cell<i32>` which is not `Sync`
 LL |         yield;
    |         ^^^^^ yield occurs here, with `a` maybe used later
 LL |     });