Add regression test for rust-lang#80179

Author: ThePuzzlemaker

src/test/ui/fn/issue-80179.rs

@@ -0,0 +1,27 @@
+// Functions with a type placeholder `_` as the return type should
+// show a function pointer suggestion when given a function item
+// and suggest how to return closures correctly from a function.
+// This is a regression test of #80179
+
+fn returns_i32() -> i32 {
+    0
+}
+
+fn returns_fn_ptr() -> _ {
+//~^ ERROR the type placeholder `_` is not allowed within types on item signatures [E0121]
+//~| NOTE not allowed in type signatures
+//~| HELP replace with the correct return type
+//~| SUGGESTION fn() -> i32
+    returns_i32
+}
+
+fn returns_closure() -> _ {
+//~^ ERROR the type placeholder `_` is not allowed within types on item signatures [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
+    || 0
+}
+
+fn main() {}

src/test/ui/fn/issue-80179.stderr

@@ -0,0 +1,21 @@
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures
+  --> $DIR/issue-80179.rs:10:24
+   |
+LL | fn returns_fn_ptr() -> _ {
+   |                        ^
+   |                        |
+   |                        not allowed in type signatures
+   |                        help: replace with the correct return type: `fn() -> i32`
+
+error[E0121]: the type 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
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0121`.