add display

single_file_tests/display.rs

@@ -0,0 +1,16 @@
+use std::fmt;
+
+struct X {
+    i: i32,
+}
+
+impl fmt::Display for X {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Hello: {}", self.i)
+    }
+}
+
+fn main() {
+    let x = X { i: 111, };
+    println!("{}", x);
+}