Fix suggestion for returning async closures

compiler/rustc_hir_analysis/src/collect.rs

@@ -1263,6 +1263,21 @@ pub fn suggest_impl_trait<'tcx>(
             infcx.tcx.lang_items().future_output(),
             format_as_assoc,
         ),
+        (
+            infcx.tcx.lang_items().async_fn_trait(),
+            infcx.tcx.lang_items().async_fn_once_output(),
+            format_as_parenthesized,
+        ),
+        (
+            infcx.tcx.lang_items().async_fn_mut_trait(),
+            infcx.tcx.lang_items().async_fn_once_output(),
+            format_as_parenthesized,
+        ),
+        (
+            infcx.tcx.lang_items().async_fn_once_trait(),
+            infcx.tcx.lang_items().async_fn_once_output(),
+            format_as_parenthesized,
+        ),
         (
             infcx.tcx.lang_items().fn_trait(),
             infcx.tcx.lang_items().fn_once_output(),

tests/ui/async-await/async-closures/ice-async-closure-variance-issue-148488.stderr

@@ -11,6 +11,12 @@ error[E0121]: the placeholder `_` is not allowed within types on item signatures
    |
 LL | fn ord<a>() -> _ {
    |                ^ not allowed in type signatures
+   |
+help: replace with an appropriate return type
+   |
+LL - fn ord<a>() -> _ {
+LL + fn ord<a>() -> impl AsyncFn() {
+   |
 
 error[E0392]: lifetime parameter `'g` is never used
   --> $DIR/ice-async-closure-variance-issue-148488.rs:3:10

tests/ui/async-await/async-closures/suggest-impl-async-fn-issue-148493.fixed

@@ -0,0 +1,64 @@
+#![allow(dead_code)]
+//@ run-rustfix
+//@ edition: 2021
+
+// The suggestion should be `impl AsyncFn()` instead of something like `{async closure@...}`
+
+fn test1() -> impl AsyncFn() {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn()
+    async || {}
+}
+
+fn test2() -> impl AsyncFn(i32) -> i32 {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn(i32) -> i32
+    async |x: i32| x + 1
+}
+
+fn test3() -> impl AsyncFn(i32, i32) -> i32 {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn(i32, i32) -> i32
+    async |x: i32, y: i32| x + y
+}
+
+fn test4() -> impl AsyncFn() {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn()
+    async || -> () { () }
+}
+
+fn test5() -> impl AsyncFn() -> i32 {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn() -> i32
+    let z = 42;
+    async move || z
+}
+
+fn test6() -> impl AsyncFnMut() -> i32 {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFnMut() -> i32
+    let mut x = 0;
+    async move || {
+        x += 1;
+        x
+    }
+}
+
+fn test7() -> impl AsyncFnOnce() {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFnOnce()
+    let s = String::from("hello");
+    async move || {
+        drop(s);
+    }
+}
+
+fn main() {}

tests/ui/async-await/async-closures/suggest-impl-async-fn-issue-148493.rs

@@ -0,0 +1,64 @@
+#![allow(dead_code)]
+//@ run-rustfix
+//@ edition: 2021
+
+// The suggestion should be `impl AsyncFn()` instead of something like `{async closure@...}`
+
+fn test1() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn()
+    async || {}
+}
+
+fn test2() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn(i32) -> i32
+    async |x: i32| x + 1
+}
+
+fn test3() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn(i32, i32) -> i32
+    async |x: i32, y: i32| x + y
+}
+
+fn test4() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn()
+    async || -> () { () }
+}
+
+fn test5() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFn() -> i32
+    let z = 42;
+    async move || z
+}
+
+fn test6() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFnMut() -> i32
+    let mut x = 0;
+    async move || {
+        x += 1;
+        x
+    }
+}
+
+fn test7() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl AsyncFnOnce()
+    let s = String::from("hello");
+    async move || {
+        drop(s);
+    }
+}
+
+fn main() {}

tests/ui/async-await/async-closures/suggest-impl-async-fn-issue-148493.stderr

@@ -0,0 +1,87 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-impl-async-fn-issue-148493.rs:7:15
+   |
+LL | fn test1() -> _ {
+   |               ^ not allowed in type signatures
+   |
+help: replace with an appropriate return type
+   |
+LL - fn test1() -> _ {
+LL + fn test1() -> impl AsyncFn() {
+   |
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-impl-async-fn-issue-148493.rs:14:15
+   |
+LL | fn test2() -> _ {
+   |               ^ not allowed in type signatures
+   |
+help: replace with an appropriate return type
+   |
+LL - fn test2() -> _ {
+LL + fn test2() -> impl AsyncFn(i32) -> i32 {
+   |
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-impl-async-fn-issue-148493.rs:21:15
+   |
+LL | fn test3() -> _ {
+   |               ^ not allowed in type signatures
+   |
+help: replace with an appropriate return type
+   |
+LL - fn test3() -> _ {
+LL + fn test3() -> impl AsyncFn(i32, i32) -> i32 {
+   |
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-impl-async-fn-issue-148493.rs:28:15
+   |
+LL | fn test4() -> _ {
+   |               ^ not allowed in type signatures
+   |
+help: replace with an appropriate return type
+   |
+LL - fn test4() -> _ {
+LL + fn test4() -> impl AsyncFn() {
+   |
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-impl-async-fn-issue-148493.rs:35:15
+   |
+LL | fn test5() -> _ {
+   |               ^ not allowed in type signatures
+   |
+help: replace with an appropriate return type
+   |
+LL - fn test5() -> _ {
+LL + fn test5() -> impl AsyncFn() -> i32 {
+   |
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-impl-async-fn-issue-148493.rs:43:15
+   |
+LL | fn test6() -> _ {
+   |               ^ not allowed in type signatures
+   |
+help: replace with an appropriate return type
+   |
+LL - fn test6() -> _ {
+LL + fn test6() -> impl AsyncFnMut() -> i32 {
+   |
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-impl-async-fn-issue-148493.rs:54:15
+   |
+LL | fn test7() -> _ {
+   |               ^ not allowed in type signatures
+   |
+help: replace with an appropriate return type
+   |
+LL - fn test7() -> _ {
+LL + fn test7() -> impl AsyncFnOnce() {
+   |
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0121`.