Add missing deny unknown fields (#371)

* Remove unused struct

No clue why clippy didn't catch this.

* Add missing deny_unknown_fields

* Add changelog

buildpacks/ruby/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Layer metadata deserialization to Rust structs is now using `#[serde(deny_unknown_fields)]` this prevents the accidental scenario where metadata containing a superset of fields could accidentally be deserialized to the wrong struct. It's unlikely this is currently happening with the current buildpack, but it's a possibly-observable difference so it's being listed ([#371](https://github.com/heroku/buildpacks-ruby/pull/371))
+
 ## [4.0.1] - 2024-12-11
 
 ### Fixed

buildpacks/ruby/src/layers/bundle_install_layer.rs

@@ -145,6 +145,7 @@ pub(crate) struct MetadataV2 {
 }
 
 #[derive(Deserialize, Serialize, Debug, Clone, Eq, PartialEq, CacheDiff)]
+#[serde(deny_unknown_fields)]
 pub(crate) struct MetadataV3 {
     #[cache_diff(rename = "OS Distribution")]
     pub(crate) os_distribution: OsDistribution,
@@ -316,14 +317,6 @@ fn display_name(cmd: &mut Command, env: &Env) -> String {
     )
 }
 
-#[derive(Deserialize, Serialize, Debug, Clone, Eq, PartialEq, Default)]
-#[serde(deny_unknown_fields)]
-pub(crate) struct BundleDigest {
-    env: String,
-    gemfile: String,
-    lockfile: String,
-}
-
 #[cfg(test)]
 mod test {
     use crate::layers::shared::strip_ansi;

buildpacks/ruby/src/layers/ruby_install_layer.rs

@@ -92,6 +92,7 @@ pub(crate) struct MetadataV2 {
 }
 
 #[derive(Deserialize, Serialize, Debug, Clone, Eq, PartialEq, CacheDiff)]
+#[serde(deny_unknown_fields)]
 pub(crate) struct MetadataV3 {
     #[cache_diff(rename = "OS Distribution")]
     pub(crate) os_distribution: OsDistribution,