Add test that {:p} prints the right address

    thread 'test_pointer' panicked at tests/test_display.rs:265:5:
    assertion `left == right` failed
      left: "0x7fbbd7df10"
     right: "0x7fac000e30"

tests/test_display.rs

@@ -251,6 +251,20 @@ fn test_nested_tuple_field() {
     assert("0", Error(Inner(0)));
 }
 
+#[test]
+fn test_pointer() {
+    #[derive(Error, Debug)]
+    #[error("{field:p}")]
+    pub struct Struct {
+        field: Box<i32>,
+    }
+
+    let s = Struct {
+        field: Box::new(-1),
+    };
+    assert_eq!(s.to_string(), format!("{:p}", s.field));
+}
+
 #[test]
 fn test_macro_rules() {
     // Regression test for https://github.com/dtolnay/thiserror/issues/86