Improvement for comparision against fn

src/librustc_typeck/check/op.rs

@@ -333,8 +333,18 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                 lhs_ty);
 
                             if !lhs_expr.span.eq(&rhs_expr.span) {
-                                self.add_type_neq_err_label(&mut err, lhs_expr.span, lhs_ty);
-                                self.add_type_neq_err_label(&mut err, rhs_expr.span, rhs_ty);
+                                self.add_type_neq_err_label(&mut err,
+                                                            lhs_expr.span,
+                                                            lhs_ty,
+                                                            rhs_ty,
+                                                            op,
+                                                            is_assign);
+                                self.add_type_neq_err_label(&mut err,
+                                                            rhs_expr.span,
+                                                            rhs_ty,
+                                                            lhs_ty,
+                                                            op,
+                                                            is_assign);
                             }
 
                             let mut suggested_deref = false;
@@ -420,10 +430,40 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         err: &mut errors::DiagnosticBuilder<'_>,
         span: Span,
         ty: Ty<'tcx>,
+        other_ty: Ty<'tcx>,
+        op: hir::BinOp,
+        is_assign: IsAssign,
     ) {
         err.span_label(span, ty.to_string());
-        if let FnDef(..) = ty.sty {
-            err.span_label(span, "did you forget `()`?");
+        if let FnDef(def_id, _) = ty.sty {
+            let source_map = self.tcx.sess.source_map();
+            let hir_id = &self.tcx.hir().as_local_hir_id(def_id).unwrap();
+            let fn_sig = {
+                match self.tcx.typeck_tables_of(def_id).liberated_fn_sigs().get(*hir_id) {
+                    Some(f) => f.clone(),
+                    None => {
+                        bug!("No fn-sig entry for def_id={:?}", def_id);
+                    }
+                }
+            };
+
+            if self.lookup_op_method(fn_sig.output(),
+                                    &[other_ty],
+                                    Op::Binary(op, is_assign))
+                    .is_ok() {
+                let variable_snippet = if fn_sig.inputs().len() > 0 {
+                    format!("{}( /* arguments */ )", source_map.span_to_snippet(span).unwrap())
+                } else {
+                    format!("{}()", source_map.span_to_snippet(span).unwrap())
+                };
+
+                err.span_suggestion(
+                    span,
+                    "did you forget",
+                    variable_snippet,
+                    Applicability::MachineApplicable,
+                );
+            }
         }
     }
 

src/test/ui/fn/fn-compare-mismatch.stderr

@@ -2,12 +2,9 @@ error[E0369]: binary operation `==` cannot be applied to type `fn() {main::f}`
   --> $DIR/fn-compare-mismatch.rs:4:15
    |
 LL |     let x = f == g;
-   |             - ^^ -
-   |             |    |
-   |             |    fn() {main::g}
-   |             |    did you forget `()`?
+   |             - ^^ - fn() {main::g}
+   |             |
    |             fn() {main::f}
-   |             did you forget `()`?
    |
    = note: an implementation of `std::cmp::PartialEq` might be missing for `fn() {main::f}`
 

src/test/ui/issues/issue-59488.rs

@@ -1,9 +1,19 @@
+// ignore-tidy-linelength
+
 fn foo() -> i32 {
     42
 }
 
+fn bar(a: i64) -> i64 {
+    43
+}
+
 fn main() {
     foo > 12;
-    //~^ ERROR 6:9: 6:10: binary operation `>` cannot be applied to type `fn() -> i32 {foo}` [E0369]
-    //~| ERROR 6:11: 6:13: mismatched types [E0308]
+    //~^ ERROR 12:9: 12:10: binary operation `>` cannot be applied to type `fn() -> i32 {foo}` [E0369]
+    //~| ERROR 12:11: 12:13: mismatched types [E0308]
+
+    bar > 13;
+    //~^ ERROR 16:9: 16:10: binary operation `>` cannot be applied to type `fn(i64) -> i64 {bar}` [E0369]
+    //~| ERROR 16:11: 16:13: mismatched types [E0308]
 }

src/test/ui/issues/issue-59488.stderr

@@ -1,24 +1,44 @@
 error[E0369]: binary operation `>` cannot be applied to type `fn() -> i32 {foo}`
-  --> $DIR/issue-59488.rs:6:9
+  --> $DIR/issue-59488.rs:12:9
    |
 LL |     foo > 12;
    |     --- ^ -- {integer}
    |     |
    |     fn() -> i32 {foo}
-   |     did you forget `()`?
+   |     help: did you forget: `foo()`
    |
    = note: an implementation of `std::cmp::PartialOrd` might be missing for `fn() -> i32 {foo}`
 
 error[E0308]: mismatched types
-  --> $DIR/issue-59488.rs:6:11
+  --> $DIR/issue-59488.rs:12:11
    |
 LL |     foo > 12;
    |           ^^ expected fn item, found integer
    |
    = note: expected type `fn() -> i32 {foo}`
-              found type `{integer}`
+              found type `i32`
 
-error: aborting due to 2 previous errors
+error[E0369]: binary operation `>` cannot be applied to type `fn(i64) -> i64 {bar}`
+  --> $DIR/issue-59488.rs:16:9
+   |
+LL |     bar > 13;
+   |     --- ^ -- {integer}
+   |     |
+   |     fn(i64) -> i64 {bar}
+   |     help: did you forget: `bar( /* arguments */ )`
+   |
+   = note: an implementation of `std::cmp::PartialOrd` might be missing for `fn(i64) -> i64 {bar}`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-59488.rs:16:11
+   |
+LL |     bar > 13;
+   |           ^^ expected fn item, found integer
+   |
+   = note: expected type `fn(i64) -> i64 {bar}`
+              found type `i64`
+
+error: aborting due to 4 previous errors
 
 Some errors occurred: E0308, E0369.
 For more information about an error, try `rustc --explain E0308`.

src/test/ui/parser/require-parens-for-chained-comparison.stderr

@@ -44,7 +44,6 @@ LL |     f<X>();
    |     -^- X
    |     |
    |     fn() {f::<_>}
-   |     did you forget `()`?
    |
    = note: an implementation of `std::cmp::PartialOrd` might be missing for `fn() {f::<_>}`