Add a test for fn pointer calls in consts

Author: mbartlett21

src/test/ui/consts/const-fn-ptr.rs

@@ -0,0 +1,16 @@
+const fn make_fn_ptr() -> fn() {
+    || {}
+}
+
+static STAT: () = make_fn_ptr()();
+//~^ ERROR function pointer
+
+const CONST: () = make_fn_ptr()();
+//~^ ERROR function pointer
+
+const fn call_ptr() {
+    make_fn_ptr()();
+    //~^ ERROR function pointer
+}
+
+fn main() {}

src/test/ui/consts/const-fn-ptr.stderr

@@ -0,0 +1,20 @@
+error: function pointer calls are not allowed in statics
+  --> $DIR/const-fn-ptr.rs:5:19
+   |
+LL | static STAT: () = make_fn_ptr()();
+   |                   ^^^^^^^^^^^^^^^
+
+error: function pointer calls are not allowed in constants
+  --> $DIR/const-fn-ptr.rs:8:19
+   |
+LL | const CONST: () = make_fn_ptr()();
+   |                   ^^^^^^^^^^^^^^^
+
+error: function pointer calls are not allowed in constant functions
+  --> $DIR/const-fn-ptr.rs:12:5
+   |
+LL |     make_fn_ptr()();
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+