From 4f6be466fdec69dbf6ce768d41a43acf0fc3b47b Mon Sep 17 00:00:00 2001
From: ThePuzzlemaker <tpzker@thepuzzlemaker.info>
Date: Mon, 21 Dec 2020 20:23:21 -0600
Subject: [PATCH 1/2] Suggest fn ptr rather than fn item and suggest to use
 `Fn` trait bounds rather than the unique closure type in E0121

This is a squash of the titular commit along with these minor commits:
- Improve note
- Improve note pt2
---
 compiler/rustc_typeck/src/collect.rs | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index bc6b2037c184e..ebbcf6304b65a 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -1544,12 +1544,27 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
                     let mut diag = bad_placeholder_type(tcx, visitor.0);
                     let ret_ty = fn_sig.output();
                     if ret_ty != tcx.ty_error() {
-                        diag.span_suggestion(
-                            ty.span,
-                            "replace with the correct return type",
-                            ret_ty.to_string(),
-                            Applicability::MaybeIncorrect,
-                        );
+                        if !ret_ty.is_closure() {
+                            let ret_ty_str = match ret_ty.kind() {
+                                // Suggest a function pointer return type instead of a unique function definition
+                                // (e.g. `fn() -> i32` instead of `fn() -> i32 { f }`, the latter of which is invalid
+                                // syntax)
+                                ty::FnDef(..) => ret_ty.fn_sig(tcx).to_string(),
+                                _ => ret_ty.to_string(),
+                            };
+                            diag.span_suggestion(
+                                ty.span,
+                                "replace with the correct return type",
+                                ret_ty_str,
+                                Applicability::MaybeIncorrect,
+                            );
+                        } else {
+                            // 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");
+                            diag.note("for more information on `Fn` traits and closure types, see https://doc.rust-lang.org/book/ch13-01-closures.html");
+                        }
                     }
                     diag.emit();
                     ty::Binder::bind(fn_sig)

From 5e6dc927f7a935874a08c2c3913f24295711d8f2 Mon Sep 17 00:00:00 2001
From: ThePuzzlemaker <tpzker@thepuzzlemaker.info>
Date: Tue, 22 Dec 2020 13:25:37 -0600
Subject: [PATCH 2/2] Add regression test for #80179

---
 src/test/ui/fn/issue-80179.rs     | 27 +++++++++++++++++++++++++++
 src/test/ui/fn/issue-80179.stderr | 21 +++++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100644 src/test/ui/fn/issue-80179.rs
 create mode 100644 src/test/ui/fn/issue-80179.stderr

diff --git a/src/test/ui/fn/issue-80179.rs b/src/test/ui/fn/issue-80179.rs
new file mode 100644
index 0000000000000..7609b1525cc90
--- /dev/null
+++ b/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() {}
diff --git a/src/test/ui/fn/issue-80179.stderr b/src/test/ui/fn/issue-80179.stderr
new file mode 100644
index 0000000000000..63571e71b34f4
--- /dev/null
+++ b/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`.