Rollup merge of rust-lang#73672 - nellshamrell:async-fix, r=estebank

Adds a clearer message for when the async keyword is missing from a f…

…unction

This is a somewhat simple fix for rust-lang#66731.

Under the current version of Rust, if a user has a rust file that looks like this:

```rust
fn boo (){}

async fn foo() {
    boo().await;
}

fn main() {

}
```

And they attempt to run it, they will receive an error message that looks like this:

```bash
error: incorrect use of `await`                                                                                                        --> test.rs:4:14                                                                                                                       |                                                                                                                                   4 |     boo.await();                                                                                                                    |              ^^ help: `await` is not a method call, remove the parentheses                                                                                                                                                                                              error[E0277]: the trait bound `fn() {boo}: std::future::Future` is not satisfied                                                        --> test.rs:4:5                                                                                                                        |                                                                                                                                  4  |     boo.await();                                                                                                                    |     ^^^^^^^^^ the trait `std::future::Future` is not implemented for `fn() {boo}`                                                                                                                                                                                      error: aborting due to 2 previous errors                                                                                                                                                                                                                                    For more information about this error, try `rustc --explain E0277`.
```

This is not very clear.

With the changes made in this PR, when a user compiles and runs that same rust code, they will receive an error message that looks like this:

```bash
error[E0277]: `()` is not a future.
 --> test.rs:4:5
  |
4 |     boo().await;
  |     ^^^^^^^^^^^ `()` is not a future
  |
  = help: the trait `std::future::Future` is not implemented for `()`
  = note: required by `std::future::Future::poll`
```

In the future, I think we should make this error message even clearer, perhaps through a solution like the one described in [this comment](rust-lang#66731 (comment)). However, as that potentially involves a major change proposal, I would rather get this change in now and make the error message a little clearer while an MCP is drafted and discussed.

Signed-off-by: Nell Shamrell <nellshamrell@gmail.com>

src/libcore/future/future.rs

@@ -27,6 +27,7 @@ use crate::task::{Context, Poll};
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 #[stable(feature = "futures_api", since = "1.36.0")]
 #[lang = "future_trait"]
+#[rustc_on_unimplemented(label = "`{Self}` is not a future", message = "`{Self}` is not a future")]
 pub trait Future {
     /// The type of value produced on completion.
     #[stable(feature = "futures_api", since = "1.36.0")]

src/test/ui/async-await/async-error-span.rs

@@ -5,7 +5,7 @@
 use std::future::Future;
 
 fn get_future() -> impl Future<Output = ()> {
-//~^ ERROR the trait bound `(): std::future::Future` is not satisfied
+//~^ ERROR `()` is not a future
     panic!()
 }
 

src/test/ui/async-await/async-error-span.stderr

@@ -1,12 +1,13 @@
-error[E0277]: the trait bound `(): std::future::Future` is not satisfied
+error[E0277]: `()` is not a future
   --> $DIR/async-error-span.rs:7:20
    |
 LL | fn get_future() -> impl Future<Output = ()> {
-   |                    ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `()`
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^ `()` is not a future
 LL |
 LL |     panic!()
    |     -------- this returned value is of type `!`
    |
+   = help: the trait `std::future::Future` is not implemented for `()`
    = note: the return type of a function must have a statically known size
 
 error[E0698]: type inside `async fn` body must be known in this context

src/test/ui/async-await/issue-70594.rs

@@ -6,7 +6,7 @@ async fn fun() {
     //~| error: `.await` is not allowed in a `const`
     //~| error: `loop` is not allowed in a `const`
     //~| error: `.await` is not allowed in a `const`
-    //~| error: the trait bound `(): std::future::Future` is not satisfied
+    //~| error: `()` is not a future
 }
 
 fn main() {}

src/test/ui/async-await/issue-70594.stderr

@@ -27,12 +27,13 @@ error[E0744]: `.await` is not allowed in a `const`
 LL |     [1; ().await];
    |         ^^^^^^^^
 
-error[E0277]: the trait bound `(): std::future::Future` is not satisfied
+error[E0277]: `()` is not a future
   --> $DIR/issue-70594.rs:4:9
    |
 LL |     [1; ().await];
-   |         ^^^^^^^^ the trait `std::future::Future` is not implemented for `()`
+   |         ^^^^^^^^ `()` is not a future
    |
+   = help: the trait `std::future::Future` is not implemented for `()`
    = note: required by `std::future::Future::poll`
 
 error: aborting due to 5 previous errors

src/test/ui/async-await/issues/issue-62009-1.stderr

@@ -27,12 +27,13 @@ LL | fn main() {
 LL |     (|_| 2333).await;
    |     ^^^^^^^^^^^^^^^^ only allowed inside `async` functions and blocks
 
-error[E0277]: the trait bound `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]: std::future::Future` is not satisfied
+error[E0277]: `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]` is not a future
   --> $DIR/issue-62009-1.rs:12:5
    |
 LL |     (|_| 2333).await;
-   |     ^^^^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]`
+   |     ^^^^^^^^^^^^^^^^ `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]` is not a future
    |
+   = help: the trait `std::future::Future` is not implemented for `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]`
    = note: required by `std::future::Future::poll`
 
 error: aborting due to 4 previous errors

src/test/ui/issues-71798.rs

@@ -1,5 +1,5 @@
 fn test_ref(x: &u32) -> impl std::future::Future<Output = u32> + '_ {
-    *x //~^ ERROR the trait bound `u32: std::future::Future` is not satisfied
+    *x //~^ ERROR `u32` is not a future
 }
 
 fn main() {

src/test/ui/issues-71798.stderr

@@ -4,14 +4,15 @@ error[E0425]: cannot find value `u` in this scope
 LL |     let _ = test_ref & u;
    |                        ^ not found in this scope
 
-error[E0277]: the trait bound `u32: std::future::Future` is not satisfied
+error[E0277]: `u32` is not a future
   --> $DIR/issues-71798.rs:1:25
    |
 LL | fn test_ref(x: &u32) -> impl std::future::Future<Output = u32> + '_ {
-   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `u32`
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `u32` is not a future
 LL |     *x
    |     -- this returned value is of type `u32`
    |
+   = help: the trait `std::future::Future` is not implemented for `u32`
    = note: the return type of a function must have a statically known size
 
 error: aborting due to 2 previous errors

src/test/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr

@@ -1,4 +1,4 @@
-error[E0277]: the trait bound `fn() -> impl std::future::Future {foo}: std::future::Future` is not satisfied
+error[E0277]: `fn() -> impl std::future::Future {foo}` is not a future
   --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:10:9
    |
 LL | async fn foo() {}
@@ -8,14 +8,15 @@ LL | fn bar(f: impl Future<Output=()>) {}
    |                ----------------- required by this bound in `bar`
 ...
 LL |     bar(foo);
-   |         ^^^ the trait `std::future::Future` is not implemented for `fn() -> impl std::future::Future {foo}`
+   |         ^^^ `fn() -> impl std::future::Future {foo}` is not a future
    |
+   = help: the trait `std::future::Future` is not implemented for `fn() -> impl std::future::Future {foo}`
 help: use parentheses to call the function
    |
 LL |     bar(foo());
    |            ^^
 
-error[E0277]: the trait bound `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]: std::future::Future` is not satisfied
+error[E0277]: `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]` is not a future
   --> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:12:9
    |
 LL | fn bar(f: impl Future<Output=()>) {}
@@ -24,8 +25,9 @@ LL | fn bar(f: impl Future<Output=()>) {}
 LL |     let async_closure = async || ();
    |                         -------- consider calling this closure
 LL |     bar(async_closure);
-   |         ^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]`
+   |         ^^^^^^^^^^^^^ `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]` is not a future
    |
+   = help: the trait `std::future::Future` is not implemented for `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]`
 help: use parentheses to call the closure
    |
 LL |     bar(async_closure());