Fix JSON serialization (#426)

We recently removed the custom serialization for InputSource.
This causes the JSON formatter to fail
with "key must be a string".
Add it back and add a comment on
why this is needed.

lychee-lib/src/types/input.rs

@@ -43,7 +43,7 @@ impl InputContent {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize)]
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
 #[non_exhaustive]
 /// Input types which lychee supports
 pub enum InputSource {
@@ -64,6 +64,18 @@ pub enum InputSource {
     String(String),
 }
 
+// Custom serialization for enum is needed
+// Otherwise we get "key must be a string" when using the JSON writer
+// Related: https://github.com/serde-rs/json/issues/45
+impl Serialize for InputSource {
+    fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.collect_str(self)
+    }
+}
+
 impl Display for InputSource {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         f.write_str(match self {