Auto merge of #59798 - rchaser53:issue-59488, r=estebank

Improvement for comparision against fn

I try to add error message.
related: https://github.com/rust-lang/rust/issues/59488

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

@@ -7,6 +7,14 @@ LL |     let x = f == g;
    |             fn() {main::f}
    |
    = note: an implementation of `std::cmp::PartialEq` might be missing for `fn() {main::f}`
+help: you might have forgotten to call this function
+   |
+LL |     let x = f() == g;
+   |             ^^^
+help: you might have forgotten to call this function
+   |
+LL |     let x = f == g();
+   |                  ^^^
 
 error[E0308]: mismatched types
   --> $DIR/fn-compare-mismatch.rs:4:18

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

@@ -0,0 +1,26 @@
+// ignore-tidy-linelength
+
+fn foo() -> i32 {
+    42
+}
+
+fn bar(a: i64) -> i64 {
+    43
+}
+
+fn main() {
+    foo > 12;
+    //~^ 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]
+
+    foo > foo;
+    //~^ ERROR 20:9: 20:10: binary operation `>` cannot be applied to type `fn() -> i32 {foo}` [E0369]
+
+    foo > bar;
+    //~^ ERROR 23:9: 23:10: binary operation `>` cannot be applied to type `fn() -> i32 {foo}` [E0369]
+    //~| ERROR 23:11: 23:14: mismatched types [E0308]
+}

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

@@ -0,0 +1,81 @@
+error[E0369]: binary operation `>` cannot be applied to type `fn() -> i32 {foo}`
+  --> $DIR/issue-59488.rs:12:9
+   |
+LL |     foo > 12;
+   |     --- ^ -- {integer}
+   |     |
+   |     fn() -> i32 {foo}
+   |     help: you might have forgotten to call this function: `foo()`
+   |
+   = note: an implementation of `std::cmp::PartialOrd` might be missing for `fn() -> i32 {foo}`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-59488.rs:12:11
+   |
+LL |     foo > 12;
+   |           ^^ expected fn item, found integer
+   |
+   = note: expected type `fn() -> i32 {foo}`
+              found type `i32`
+
+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: you might have forgotten to call this function: `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[E0369]: binary operation `>` cannot be applied to type `fn() -> i32 {foo}`
+  --> $DIR/issue-59488.rs:20:9
+   |
+LL |     foo > foo;
+   |     --- ^ --- fn() -> i32 {foo}
+   |     |
+   |     fn() -> i32 {foo}
+   |
+   = note: an implementation of `std::cmp::PartialOrd` might be missing for `fn() -> i32 {foo}`
+help: you might have forgotten to call this function
+   |
+LL |     foo() > foo;
+   |     ^^^^^
+help: you might have forgotten to call this function
+   |
+LL |     foo > foo();
+   |           ^^^^^
+
+error[E0369]: binary operation `>` cannot be applied to type `fn() -> i32 {foo}`
+  --> $DIR/issue-59488.rs:23:9
+   |
+LL |     foo > bar;
+   |     --- ^ --- fn(i64) -> i64 {bar}
+   |     |
+   |     fn() -> i32 {foo}
+   |
+   = note: an implementation of `std::cmp::PartialOrd` might be missing for `fn() -> i32 {foo}`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-59488.rs:23:11
+   |
+LL |     foo > bar;
+   |           ^^^ expected fn item, found a different fn item
+   |
+   = note: expected type `fn() -> i32 {foo}`
+              found type `fn(i64) -> i64 {bar}`
+
+error: aborting due to 7 previous errors
+
+Some errors occurred: E0308, E0369.
+For more information about an error, try `rustc --explain E0308`.