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

Manishearth · web-flow · commit d227a825bc61 · 2020-06-27T22:29:55.000-07:00
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>
diff --git a/src/libcore/future/future.rs b/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")]
diff --git a/src/test/ui/async-await/async-error-span.rs b/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!()
 }
 
diff --git a/src/test/ui/async-await/async-error-span.stderr b/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
diff --git a/src/test/ui/async-await/issue-70594.rs b/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() {}
diff --git a/src/test/ui/async-await/issue-70594.stderr b/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
diff --git a/src/test/ui/async-await/issues/issue-62009-1.stderr b/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
diff --git a/src/test/ui/issues-71798.rs b/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() {
diff --git a/src/test/ui/issues-71798.stderr b/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
diff --git a/src/test/ui/suggestions/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.stderr b/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());

Original file line number	Diff line number	Diff line change
`@@ -5,7 +5,7 @@`
`5`	`5`	`use std::future::Future;`
`6`	`6`
`7`	`7`	`fn get_future() -> impl Future<Output = ()> {`
`8`		-//~^ ERROR the trait bound `(): std::future::Future` is not satisfied
	`8`	+//~^ ERROR `()` is not a future
`9`	`9`	`panic!()`
`10`	`10`	`}`
`11`	`11`
Original file line number	Diff line number	Diff line change
`@@ -6,7 +6,7 @@ async fn fun() {`
`6`	`6`	//~\| error: `.await` is not allowed in a `const`
`7`	`7`	//~\| error: `loop` is not allowed in a `const`
`8`	`8`	//~\| error: `.await` is not allowed in a `const`
`9`		- //~\| error: the trait bound `(): std::future::Future` is not satisfied
	`9`	+ //~\| error: `()` is not a future
`10`	`10`	`}`
`11`	`11`
`12`	`12`	`fn main() {}`
Original file line number	Diff line number	Diff line change
@@ -27,12 +27,13 @@ error[E0744]: `.await` is not allowed in a `const`
`27`	`27`	`LL \| [1; ().await];`
`28`	`28`	`\| ^^^^^^^^`
`29`	`29`
`30`		-error[E0277]: the trait bound `(): std::future::Future` is not satisfied
	`30`	+error[E0277]: `()` is not a future
`31`	`31`	`--> $DIR/issue-70594.rs:4:9`
`32`	`32`	`\|`
`33`	`33`	`LL \| [1; ().await];`
`34`		- \| ^^^^^^^^ the trait `std::future::Future` is not implemented for `()`
	`34`	+ \| ^^^^^^^^ `()` is not a future
`35`	`35`	`\|`
	`36`	+ = help: the trait `std::future::Future` is not implemented for `()`
`36`	`37`	= note: required by `std::future::Future::poll`
`37`	`38`
`38`	`39`	`error: aborting due to 5 previous errors`
Original file line number	Diff line number	Diff line change
`@@ -27,12 +27,13 @@ LL \| fn main() {`
`27`	`27`	`LL \| (\|_\| 2333).await;`
`28`	`28`	\| ^^^^^^^^^^^^^^^^ only allowed inside `async` functions and blocks
`29`	`29`
`30`		-error[E0277]: the trait bound `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]: std::future::Future` is not satisfied
	`30`	+error[E0277]: `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]` is not a future
`31`	`31`	`--> $DIR/issue-62009-1.rs:12:5`
`32`	`32`	`\|`
`33`	`33`	`LL \| (\|_\| 2333).await;`
`34`		- \| ^^^^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]`
	`34`	+ \| ^^^^^^^^^^^^^^^^ `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]` is not a future
`35`	`35`	`\|`
	`36`	+ = help: the trait `std::future::Future` is not implemented for `[closure@$DIR/issue-62009-1.rs:12:5: 12:15]`
`36`	`37`	= note: required by `std::future::Future::poll`
`37`	`38`
`38`	`39`	`error: aborting due to 4 previous errors`
Original file line number	Diff line number	Diff line change
`@@ -1,5 +1,5 @@`
`1`	`1`	`fn test_ref(x: &u32) -> impl std::future::Future<Output = u32> + '_ {`
`2`		- *x //~^ ERROR the trait bound `u32: std::future::Future` is not satisfied
	`2`	+ *x //~^ ERROR `u32` is not a future
`3`	`3`	`}`
`4`	`4`
`5`	`5`	`fn main() {`
Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		-error[E0277]: the trait bound `fn() -> impl std::future::Future {foo}: std::future::Future` is not satisfied
	`1`	+error[E0277]: `fn() -> impl std::future::Future {foo}` is not a future
`2`	`2`	`--> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:10:9`
`3`	`3`	`\|`
`4`	`4`	`LL \| async fn foo() {}`
`@@ -8,14 +8,15 @@ LL \| fn bar(f: impl Future<Output=()>) {}`
`8`	`8`	\| ----------------- required by this bound in `bar`
`9`	`9`	`...`
`10`	`10`	`LL \| bar(foo);`
`11`		- \| ^^^ the trait `std::future::Future` is not implemented for `fn() -> impl std::future::Future {foo}`
	`11`	+ \| ^^^ `fn() -> impl std::future::Future {foo}` is not a future
`12`	`12`	`\|`
	`13`	+ = help: the trait `std::future::Future` is not implemented for `fn() -> impl std::future::Future {foo}`
`13`	`14`	`help: use parentheses to call the function`
`14`	`15`	`\|`
`15`	`16`	`LL \| bar(foo());`
`16`	`17`	`\| ^^`
`17`	`18`
`18`		-error[E0277]: the trait bound `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]: std::future::Future` is not satisfied
	`19`	+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
`19`	`20`	`--> $DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:12:9`
`20`	`21`	`\|`
`21`	`22`	`LL \| fn bar(f: impl Future<Output=()>) {}`
`@@ -24,8 +25,9 @@ LL \| fn bar(f: impl Future<Output=()>) {}`
`24`	`25`	`LL \| let async_closure = async \|\| ();`
`25`	`26`	`\| -------- consider calling this closure`
`26`	`27`	`LL \| bar(async_closure);`
`27`		- \| ^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]`
	`28`	+ \| ^^^^^^^^^^^^^ `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]` is not a future
`28`	`29`	`\|`
	`30`	+ = help: the trait `std::future::Future` is not implemented for `[closure@$DIR/async-fn-ctor-passed-as-arg-where-it-should-have-been-called.rs:11:25: 11:36]`
`29`	`31`	`help: use parentheses to call the closure`
`30`	`32`	`\|`
`31`	`33`	`LL \| bar(async_closure());`