Suggest more impl Trait on `-> _`

6 files changed, 111 insertions, 7 deletions

diff --git a/src/test/ui/fn/issue-80179.rs b/src/test/ui/fn/issue-80179.rs
index fcef6f1b60e..bea8deadc89 100644
--- a/src/test/ui/fn/issue-80179.rs
+++ b/src/test/ui/fn/issue-80179.rs
@@ -18,9 +18,8 @@ fn returns_fn_ptr() -> _ {
 fn returns_closure() -> _ {
 //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types [E0121]
 //~| NOTE not allowed in type signatures
-//~| HELP consider using an `Fn`, `FnMut`, or `FnOnce` trait bound
-//~| NOTE for more information on `Fn` traits and closure types, see
-//        https://doc.rust-lang.org/book/ch13-01-closures.html
+//~| HELP replace with an appropriate return type
+//~| SUGGESTION impl Fn() -> i32
     || 0
 }
 
diff --git a/src/test/ui/fn/issue-80179.stderr b/src/test/ui/fn/issue-80179.stderr
index 2ca4ae982d9..dff8252b371 100644
--- a/src/test/ui/fn/issue-80179.stderr
+++ b/src/test/ui/fn/issue-80179.stderr
@@ -11,10 +11,10 @@ error[E0121]: the placeholder `_` is not allowed within types on item signatures
   --> $DIR/issue-80179.rs:18:25
    |
 LL | fn returns_closure() -> _ {
-   |                         ^ not allowed in type signatures
-   |
-   = help: consider using an `Fn`, `FnMut`, or `FnOnce` trait bound
-   = note: for more information on `Fn` traits and closure types, see https://doc.rust-lang.org/book/ch13-01-closures.html
+   |                         ^
+   |                         |
+   |                         not allowed in type signatures
+   |                         help: replace with an appropriate return type: `impl Fn() -> i32`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/fn/suggest-return-closure.rs b/src/test/ui/fn/suggest-return-closure.rs
new file mode 100644
index 00000000000..1b9094cea38
--- /dev/null
+++ b/src/test/ui/fn/suggest-return-closure.rs
@@ -0,0 +1,31 @@
+fn fn_once() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types [E0121]
+    //~| NOTE not allowed in type signatures
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl FnOnce()
+    let x = String::new();
+    || {
+        drop(x);
+    }
+}
+
+fn fn_mut() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types [E0121]
+    //~| NOTE not allowed in type signatures
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl FnMut(char)
+    let x = String::new();
+    |c| {
+        x.push(c);
+    }
+}
+
+fn fun() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types [E0121]
+    //~| NOTE not allowed in type signatures
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl Fn() -> i32
+    || 1i32
+}
+
+fn main() {}
diff --git a/src/test/ui/fn/suggest-return-closure.stderr b/src/test/ui/fn/suggest-return-closure.stderr
new file mode 100644
index 00000000000..f4b2c7f5274
--- /dev/null
+++ b/src/test/ui/fn/suggest-return-closure.stderr
@@ -0,0 +1,30 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-return-closure.rs:1:17
+   |
+LL | fn fn_once() -> _ {
+   |                 ^
+   |                 |
+   |                 not allowed in type signatures
+   |                 help: replace with an appropriate return type: `impl FnOnce()`
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-return-closure.rs:12:16
+   |
+LL | fn fn_mut() -> _ {
+   |                ^
+   |                |
+   |                not allowed in type signatures
+   |                help: replace with an appropriate return type: `impl FnMut(char)`
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-return-closure.rs:23:13
+   |
+LL | fn fun() -> _ {
+   |             ^
+   |             |
+   |             not allowed in type signatures
+   |             help: replace with an appropriate return type: `impl Fn() -> i32`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0121`.
diff --git a/src/test/ui/fn/suggest-return-future.rs b/src/test/ui/fn/suggest-return-future.rs
new file mode 100644
index 00000000000..750740d9426
--- /dev/null
+++ b/src/test/ui/fn/suggest-return-future.rs
@@ -0,0 +1,23 @@
+// edition: 2021
+
+async fn a() -> i32 {
+    0
+}
+
+fn foo() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types [E0121]
+    //~| NOTE not allowed in type signatures
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl Future<Output = i32>
+    a()
+}
+
+fn bar() -> _ {
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types [E0121]
+    //~| NOTE not allowed in type signatures
+    //~| HELP replace with an appropriate return type
+    //~| SUGGESTION impl Future<Output = i32>
+    async { a().await }
+}
+
+fn main() {}
diff --git a/src/test/ui/fn/suggest-return-future.stderr b/src/test/ui/fn/suggest-return-future.stderr
new file mode 100644
index 00000000000..a4c8b5d8c4b
--- /dev/null
+++ b/src/test/ui/fn/suggest-return-future.stderr
@@ -0,0 +1,21 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-return-future.rs:7:13
+   |
+LL | fn foo() -> _ {
+   |             ^
+   |             |
+   |             not allowed in type signatures
+   |             help: replace with an appropriate return type: `impl Future<Output = i32>`
+
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-return-future.rs:15:13
+   |
+LL | fn bar() -> _ {
+   |             ^
+   |             |
+   |             not allowed in type signatures
+   |             help: replace with an appropriate return type: `impl Future<Output = i32>`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0121`.