add more complete CowStr impl

proto/src/serializers/cow_str.rs

@@ -1,11 +1,71 @@
-use std::borrow::Cow;
+//! Wrapper `Cow<'_, str>` for deserializing without allocation.
+//!
+//! This is a workaround for [serde's issue 1852](https://github.com/serde-rs/serde/issues/1852).
+
+use serde::{de, Deserialize, Deserializer, Serialize, Serializer};
+use std::borrow::{Cow, ToOwned};
 use std::fmt::{self, Debug, Display, Formatter};
 use std::ops::Deref;
 
-use serde::Deserialize;
+/// Wrapper `Cow<'_, str>` for deserializing without allocation.
+#[derive(Default)]
+pub struct CowStr<'a>(Cow<'a, str>);
+
+impl<'a> CowStr<'a> {
+    /// Convert into `Cow<'a, str>`.
+    pub fn into_inner(self) -> Cow<'a, str> {
+        self.0
+    }
+}
+
+impl<'de> Deserialize<'de> for CowStr<'de> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct Visitor;
+
+        impl<'de> serde::de::Visitor<'de> for Visitor {
+            type Value = CowStr<'de>;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("a string")
+            }
+
+            fn visit_borrowed_str<E>(self, value: &'de str) -> Result<Self::Value, E>
+            where
+                E: de::Error,
+            {
+                Ok(CowStr(Cow::Borrowed(value)))
+            }
+
+            fn visit_str<E>(self, value: &str) -> Result<Self::Value, E>
+            where
+                E: de::Error,
+            {
+                Ok(CowStr(Cow::Owned(value.to_owned())))
+            }
 
-#[derive(Default, Deserialize)]
-pub struct CowStr<'a>(#[serde(borrow)] Cow<'a, str>);
+            fn visit_string<E>(self, value: String) -> Result<Self::Value, E>
+            where
+                E: de::Error,
+            {
+                Ok(CowStr(Cow::Owned(value)))
+            }
+        }
+
+        deserializer.deserialize_str(Visitor)
+    }
+}
+
+impl<'a> Serialize for CowStr<'a> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_str(&self.0)
+    }
+}
 
 impl<'a> Debug for CowStr<'a> {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
@@ -38,3 +98,75 @@ impl<'a> AsRef<[u8]> for CowStr<'a> {
         self.0.as_bytes()
     }
 }
+
+impl<'a> From<CowStr<'a>> for Cow<'a, str> {
+    fn from(value: CowStr<'a>) -> Self {
+        value.0
+    }
+}
+
+impl<'a> From<Cow<'a, str>> for CowStr<'a> {
+    fn from(value: Cow<'a, str>) -> Self {
+        CowStr(value)
+    }
+}
+
+/// Serialize `Cow<'_, str>`.
+pub fn serialize<'a, S>(value: &Cow<'a, str>, serializer: S) -> Result<S::Ok, S::Error>
+where
+    S: Serializer,
+{
+    serializer.serialize_str(value)
+}
+
+/// Deserialize `Cow<'_, str>`.
+pub fn deserialize<'de, D>(deserializer: D) -> Result<Cow<'de, str>, D::Error>
+where
+    D: Deserializer<'de>,
+{
+    CowStr::deserialize(deserializer).map(|value| value.into_inner())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn borrowed() {
+        struct Test(u32);
+
+        impl<'de> Deserialize<'de> for Test {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                let s = CowStr::deserialize(deserializer)?;
+                assert!(matches!(s.0, Cow::Borrowed(_)));
+                Ok(Test(s.parse().unwrap()))
+            }
+        }
+
+        let v = serde_json::from_str::<Test>("\"2\"").unwrap();
+        assert_eq!(v.0, 2);
+    }
+
+    #[test]
+    fn owned() {
+        struct Test(u32);
+
+        impl<'de> Deserialize<'de> for Test {
+            fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+            where
+                D: Deserializer<'de>,
+            {
+                let s = CowStr::deserialize(deserializer)?;
+                assert!(matches!(s.0, Cow::Owned(_)));
+                Ok(Test(s.parse().unwrap()))
+            }
+        }
+
+        let json_value = serde_json::from_str::<serde_json::Value>("\"2\"").unwrap();
+        let v = serde_json::from_value::<Test>(json_value).unwrap();
+        assert_eq!(v.0, 2);
+    }
+}