Add test case for mem::discriminant inside a macro

Author: HMPerson1

tests/ui/mem_discriminant.rs

@@ -34,6 +34,11 @@ fn main() {
     mem::discriminant(rro);
     mem::discriminant(&rro);
 
+    macro_rules! mem_discriminant_but_in_a_macro {
+        ($param:expr) => (mem::discriminant($param))
+    }
+
+    mem_discriminant_but_in_a_macro!(&rro);
 
     // ok
     mem::discriminant(&Some(2));

tests/ui/mem_discriminant.stderr

@@ -72,5 +72,17 @@ error: calling `mem::discriminant` on non-enum type `&&std::option::Option<i32>`
    |                       |
    |                       help: try dereferencing: `*rro`
 
-error: aborting due to 9 previous errors
+error: calling `mem::discriminant` on non-enum type `&&std::option::Option<i32>`
+  --> $DIR/mem_discriminant.rs:38:27
+   |
+38 |         ($param:expr) => (mem::discriminant($param))
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+41 |     mem_discriminant_but_in_a_macro!(&rro);
+   |     ---------------------------------------
+   |     |                                |
+   |     |                                help: try dereferencing: `*rro`
+   |     in this macro invocation
+
+error: aborting due to 10 previous errors