Restore Fn trait note

Author: compiler-errors

compiler/rustc_hir_analysis/src/collect.rs

@@ -1231,10 +1231,10 @@ fn infer_return_ty_for_fn_sig<'tcx>(
                     Applicability::MachineApplicable,
                 );
             } else if ret_ty.is_closure() {
-                // We're dealing with a closure, so we should suggest using `impl Fn` or trait bounds
-                // to prevent the user from getting a papercut while trying to use the unique closure
-                // syntax (e.g. `[closure@src/lib.rs:2:5: 2:9]`).
                 diag.help("consider using an `Fn`, `FnMut`, or `FnOnce` trait bound");
+            }
+            // Also note how `Fn` traits work just in case!
+            if ret_ty.is_closure() {
                 diag.note(
                     "for more information on `Fn` traits and closure types, see \
                      https://doc.rust-lang.org/book/ch13-01-closures.html",

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

@@ -20,6 +20,7 @@ fn returns_closure() -> _ {
 //~| NOTE not allowed in type signatures
 //~| HELP replace with an appropriate return type
 //~| SUGGESTION impl Fn() -> i32
+//~| NOTE for more information on `Fn` traits and closure types
     || 0
 }
 

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

@@ -15,6 +15,8 @@ LL | fn returns_closure() -> _ {
    |                         |
    |                         not allowed in type signatures
    |                         help: replace with an appropriate return type: `impl Fn() -> i32`
+   |
+   = 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
 

src/test/ui/fn/suggest-return-closure.rs

@@ -3,6 +3,7 @@ fn fn_once() -> _ {
     //~| NOTE not allowed in type signatures
     //~| HELP replace with an appropriate return type
     //~| SUGGESTION impl FnOnce()
+    //~| NOTE for more information on `Fn` traits and closure types
     let x = String::new();
     || {
         drop(x);
@@ -14,6 +15,7 @@ fn fn_mut() -> _ {
     //~| NOTE not allowed in type signatures
     //~| HELP replace with an appropriate return type
     //~| SUGGESTION impl FnMut(char)
+    //~| NOTE for more information on `Fn` traits and closure types
     let x = String::new();
     |c| {
         x.push(c);
@@ -25,6 +27,7 @@ fn fun() -> _ {
     //~| NOTE not allowed in type signatures
     //~| HELP replace with an appropriate return type
     //~| SUGGESTION impl Fn() -> i32
+    //~| NOTE for more information on `Fn` traits and closure types
     || 1i32
 }
 

src/test/ui/fn/suggest-return-closure.stderr

@@ -6,24 +6,30 @@ LL | fn fn_once() -> _ {
    |                 |
    |                 not allowed in type signatures
    |                 help: replace with an appropriate return type: `impl FnOnce()`
+   |
+   = note: for more information on `Fn` traits and closure types, see https://doc.rust-lang.org/book/ch13-01-closures.html
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
-  --> $DIR/suggest-return-closure.rs:12:16
+  --> $DIR/suggest-return-closure.rs:13:16
    |
 LL | fn fn_mut() -> _ {
    |                ^
    |                |
    |                not allowed in type signatures
    |                help: replace with an appropriate return type: `impl FnMut(char)`
+   |
+   = note: for more information on `Fn` traits and closure types, see https://doc.rust-lang.org/book/ch13-01-closures.html
 
 error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
-  --> $DIR/suggest-return-closure.rs:23:13
+  --> $DIR/suggest-return-closure.rs:25:13
    |
 LL | fn fun() -> _ {
    |             ^
    |             |
    |             not allowed in type signatures
    |             help: replace with an appropriate return type: `impl Fn() -> i32`
+   |
+   = 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 3 previous errors