Improvement for comparision against fn

src/librustc_typeck/check/op.rs

@@ -3,7 +3,7 @@
 use super::{FnCtxt, Needs};
 use super::method::MethodCallee;
 use rustc::ty::{self, Ty, TypeFoldable};
-use rustc::ty::TyKind::{Ref, Adt, Str, Uint, Never, Tuple, Char, Array};
+use rustc::ty::TyKind::{Ref, Adt, FnDef, Str, Uint, Never, Tuple, Char, Array};
 use rustc::ty::adjustment::{Adjustment, Adjust, AllowTwoPhase, AutoBorrow, AutoBorrowMutability};
 use rustc::infer::type_variable::TypeVariableOrigin;
 use errors::{self,Applicability};
@@ -333,8 +333,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                 lhs_ty);
 
                             if !lhs_expr.span.eq(&rhs_expr.span) {
-                                err.span_label(lhs_expr.span, lhs_ty.to_string());
-                                err.span_label(rhs_expr.span, rhs_ty.to_string());
+                                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);
                             }
 
                             let mut suggested_deref = false;
@@ -415,6 +415,18 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         (lhs_ty, rhs_ty, return_ty)
     }
 
+    fn add_type_neq_err_label(
+        &self,
+        err: &mut errors::DiagnosticBuilder<'_>,
+        span: Span,
+        ty: Ty<'tcx>,
+    ) {
+        err.span_label(span, ty.to_string());
+        if let FnDef(..) = ty.sty {
+            err.span_label(span, "did you forget `()`?");
+        }
+    }
+
     fn check_str_addition(
         &self,
         expr: &'gcx hir::Expr,

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

@@ -2,9 +2,12 @@ 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}
-   |             |
+   |             - ^^ -
+   |             |    |
+   |             |    fn() {main::g}
+   |             |    did you forget `()`?
    |             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

@@ -0,0 +1,9 @@
+fn foo() -> i32 {
+    42
+}
+
+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]
+}

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

@@ -0,0 +1,24 @@
+error[E0369]: binary operation `>` cannot be applied to type `fn() -> i32 {foo}`
+  --> $DIR/issue-59488.rs:6:9
+   |
+LL |     foo > 12;
+   |     --- ^ -- {integer}
+   |     |
+   |     fn() -> i32 {foo}
+   |     did you forget `()`?
+   |
+   = note: an implementation of `std::cmp::PartialOrd` might be missing for `fn() -> i32 {foo}`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-59488.rs:6:11
+   |
+LL |     foo > 12;
+   |           ^^ expected fn item, found integer
+   |
+   = note: expected type `fn() -> i32 {foo}`
+              found type `{integer}`
+
+error: aborting due to 2 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

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