Make Platform a newtype

Summary:
Just a bit of extra type safety. The previous diff caused a bit of a regression
by making things stringly-typed.

Reviewed By: lblasa

Differential Revision: D35902107

fbshipit-source-id: a599106f899ec3d205663b5791420aee29f3eeae

packer/src/main.rs

@@ -178,7 +178,7 @@ fn pack_platform_glob(
         let full_path = path::Path::new(&base_dir).join(&path);
         if !full_path.exists() {
             bail!(error::Error::MissingPackFile(
-                platform.to_string(),
+                platform.clone(),
                 pack_type,
                 full_path,
             ));
@@ -204,7 +204,7 @@ fn pack_platform_exact(
         let full_path = path::Path::new(&base_dir).join(f);
         if !full_path.exists() {
             bail!(error::Error::MissingPackFile(
-                platform.to_string(),
+                platform.clone(),
                 pack_type,
                 full_path,
             ));

packer/src/types.rs

@@ -8,8 +8,22 @@
 use std::fmt::{self, Display};
 use std::str;
 
-// TODO: Make this a newtype.
-pub type Platform = String;
+#[derive(Eq, PartialEq, Debug, PartialOrd, Ord, Clone, serde::Deserialize)]
+pub struct Platform(pub String);
+
+impl str::FromStr for Platform {
+    type Err = &'static str;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(Platform(s.to_string()))
+    }
+}
+
+impl Display for Platform {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
 
 #[derive(
     Debug,