From 89086f7d36e3a692b48cae8a408057734f044567 Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 4 Jan 2023 00:26:53 +0000
Subject: [PATCH] Restore Fn trait note

---
 compiler/rustc_hir_analysis/src/collect.rs   |  6 +++---
 src/test/ui/fn/issue-80179.rs                |  1 +
 src/test/ui/fn/issue-80179.stderr            |  2 ++
 src/test/ui/fn/suggest-return-closure.rs     |  3 +++
 src/test/ui/fn/suggest-return-closure.stderr | 10 ++++++++--
 5 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
index 53a58df0916f..cf847047c906 100644
--- a/compiler/rustc_hir_analysis/src/collect.rs
+++ b/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",
diff --git a/src/test/ui/fn/issue-80179.rs b/src/test/ui/fn/issue-80179.rs
index bea8deadc898..35e39bebb29a 100644
--- a/src/test/ui/fn/issue-80179.rs
+++ b/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
 }
 
diff --git a/src/test/ui/fn/issue-80179.stderr b/src/test/ui/fn/issue-80179.stderr
index dff8252b3715..f5d6c44db751 100644
--- a/src/test/ui/fn/issue-80179.stderr
+++ b/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
 
diff --git a/src/test/ui/fn/suggest-return-closure.rs b/src/test/ui/fn/suggest-return-closure.rs
index 1b9094cea38e..33daa1ea0b40 100644
--- a/src/test/ui/fn/suggest-return-closure.rs
+++ b/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
 }
 
diff --git a/src/test/ui/fn/suggest-return-closure.stderr b/src/test/ui/fn/suggest-return-closure.stderr
index f4b2c7f52740..341044469ea3 100644
--- a/src/test/ui/fn/suggest-return-closure.stderr
+++ b/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