Add regression test

Author: oli-obk

tests/ui/consts/dangling_raw_ptr.rs

@@ -3,6 +3,16 @@ const FOO: *const u32 = { //~ ERROR it is undefined behavior
     &x
 };
 
+union Union {
+    ptr: *const u32
+}
+
+const BAR: Union = { //~ ERROR encountered dangling pointer in final value
+    let x = 42;
+    Union { ptr: &x }
+};
+
 fn main() {
     let x = FOO;
+    let x = BAR;
 }

tests/ui/consts/dangling_raw_ptr.stderr

@@ -9,6 +9,12 @@ LL | const FOO: *const u32 = {
                ╾ALLOC0<imm>╼                         │ ╾──────╼
            }
 
-error: aborting due to 1 previous error
+error: encountered dangling pointer in final value of constant
+  --> $DIR/dangling_raw_ptr.rs:10:1
+   |
+LL | const BAR: Union = {
+   | ^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0080`.