Implement Display for Version in build.rs

Previously, we implemented the std::string::ToString trait for the
Version enum in build.rs for converting the libnitrokey version number
to a string.  This patch replaces this implementation with an
implementation of the std::fmt::Display trait, which automatically
generates a ToString implementation and is more idiomatic.

2 files changed, 9 insertions, 6 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a7af9fb..6480c5f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 # Unreleased
 - Derive `Debug` for all structs generated by `bindgen`.
+- Implement `std::fmt::Display` instead of `std::string::ToString` for the
+  `Version` enum in `build.rs`.
 
 # v3.5.0 (2019-07-04)
 - Mark deprecated functions using the `deprecated` attribute.
diff --git a/build.rs b/build.rs
index a9d0778..24fdf0a 100644
--- a/build.rs
+++ b/build.rs
@@ -1,9 +1,9 @@
 use std::env;
+use std::fmt;
 use std::fs;
 use std::io;
 use std::io::{Read, Write};
 use std::path;
-use std::string;
 
 use cc;
 
@@ -14,12 +14,13 @@ struct Version {
     patch: Option<u32>,
 }
 
-impl string::ToString for Version {
-    fn to_string(&self) -> String {
-        match self.patch {
-            Some(patch) => format!("v{}.{}.{}", self.major, self.minor, patch),
-            None => format!("v{}.{}", self.major, self.minor),
+impl fmt::Display for Version {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "v{}.{}", self.major, self.minor)?;
+        if let Some(patch) = self.patch {
+            write!(f, ".{}", patch)?;
         }
+        Ok(())
     }
 }