implement Format instead of just ToString

src/exit.rs

@@ -2,6 +2,7 @@ use crate::Position;
 use std::str::FromStr;
 use std::error::Error;
 use std::fmt;
+use std::fmt::{Display, Formatter};
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum Transition {
@@ -32,9 +33,9 @@ impl From<&str> for Transition {
     }
 }
 
-impl ToString for Transition {
-    fn to_string(&self) -> String {
-        match &self {
+impl Display for Transition {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", match &self {
             Transition::FadeToWhite => " FX fade_w",
             Transition::FadeToBlack => " FX fade_b",
             Transition::Wave        => " FX wave",
@@ -44,8 +45,7 @@ impl ToString for Transition {
             Transition::SlideLeft   => " FX slide_l",
             Transition::SlideRight  => " FX slide_r",
             Transition::None        => "",
-        }
-        .to_string()
+        })
     }
 }