Fix suggestion for returning async closures

Author: chenyukang

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(),

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -698,6 +698,7 @@ impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for MakeSuggestableFolder<'tcx> {
  }
 
  Closure(..)
+ | CoroutineClosure(..)
  | FnDef(..)
  | Infer(..)
  | Coroutine(..)

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

@@ -0,0 +1,67 @@
+#![allow(dead_code)]
+//@ run-rustfix
+//@ edition: 2021
+
+// Regression test for issue #148493
+// the suggestion is `impl AsyncFn()` instead of `{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
+}
+
+// Test AsyncFnMut - mutably borrows captured variable
+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
+ }
+}
+
+// Test AsyncFnOnce - consumes captured variable
+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 +1,67 @@
+#![allow(dead_code)]
+//@ run-rustfix
+//@ edition: 2021
+
+// Regression test for issue #148493
+// the suggestion is `impl AsyncFn()` instead of `{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
+}
+
+// Test AsyncFnMut - mutably borrows captured variable
+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
+ }
+}
+
+// Test AsyncFnOnce - consumes captured variable
+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:8:15
+ |
+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:15:15
+ |
+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:22:15
+ |
+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:29:15
+ |
+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:36:15
+ |
+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:45:15
+ |
+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:57:15
+ |
+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`.