Enable serde::expose_secret to deserialize Option<Secret<T>> (#24)

* Experiment with more flexiable `expose_secret` that supports `Option`

* `SerializableSecret`

* Add test

Cargo.toml

@@ -19,12 +19,14 @@ std = []
 fake = ["dep:fake", "dep:rand"]
 
 [dependencies]
-serde = { version = "1.0", optional = true, default-features = false }
 fake = { version = "2.5", optional = true, default-features = false }
 rand = { version = "0.8", optional = true, default-features = false }
+serde = { version = "1.0", optional = true, default-features = false }
 
 [dev-dependencies]
+fake = { version = "2.5", features = ["derive"] }
 serde = { version = "1.0", features = ["derive"] }
+serde_json = "1.0"
 
 [package.metadata.docs.rs]
 all-features = true

src/serde.rs

@@ -10,6 +10,36 @@ impl<'de, T: Deserialize<'de>> Deserialize<'de> for Secret<T> {
     }
 }
 
+/// A serializable type that contains a secret.
+///
+/// This abstraction enables [expose_secret] to be used to serialize both `Secret<T>` and
+/// `Option<Secret<T>`.
+pub trait SerializableSecret<T> {
+    type Exposed<'a>: Serialize
+    where
+        Self: 'a;
+    /// To reduce the number of functions that are able to expose secrets we require
+    /// that the [`Secret::expose_secret`] function is passed in here.
+    fn expose_via<'a>(&'a self, expose: impl Fn(&Secret<T>) -> &T) -> Self::Exposed<'a>;
+}
+
+impl<T: Serialize> SerializableSecret<T> for Secret<T> {
+    type Exposed<'a> = &'a T where T: 'a;
+
+    fn expose_via<'a>(&'a self, expose: impl Fn(&Secret<T>) -> &T) -> Self::Exposed<'a> {
+        expose(self)
+    }
+}
+
+impl<T: Serialize> SerializableSecret<T> for Option<Secret<T>> {
+    type Exposed<'a> = Option<&'a T> where T: 'a;
+
+    fn expose_via<'a>(&'a self, expose: impl Fn(&Secret<T>) -> &T) -> Self::Exposed<'a> {
+        self.as_ref().map(expose)
+    }
+}
+
+#[cfg(feature = "serde")]
 /// Exposes a [Secret] for serialization.
 ///
 /// For general-purpose secret exposing see [Secret::expose_secret].
@@ -19,8 +49,36 @@ impl<'de, T: Deserialize<'de>> Deserialize<'de> for Secret<T> {
 /// *This API requires the following crate features to be activated: `serde`*
 #[inline]
 pub fn expose_secret<S: Serializer, T: Serialize>(
-    secret: &Secret<T>,
+    secret: &impl SerializableSecret<T>,
     serializer: S,
 ) -> Result<S::Ok, S::Error> {
-    secret.expose_secret().serialize(serializer)
+    secret
+        .expose_via(Secret::expose_secret)
+        .serialize(serializer)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use fake::{Fake, Faker};
+    use serde::{Deserialize, Serialize};
+
+    #[test]
+    fn deserialize_the_serialized() {
+        #[derive(Serialize, Deserialize, fake::Dummy, PartialEq, Debug)]
+        struct Test {
+            #[serde(serialize_with = "expose_secret")]
+            one: Secret<String>,
+            #[serde(serialize_with = "expose_secret")]
+            two: Option<Secret<String>>,
+        }
+
+        let to_serialize: Test = Faker.fake();
+
+        let serialized = serde_json::to_string(&to_serialize).unwrap();
+
+        let deserialized = serde_json::from_str(&serialized).unwrap();
+
+        assert_eq!(to_serialize, deserialized)
+    }
 }