Add test of deserializing a &RawValue in map key position

Currently fails with:

    ---- test_raw_value_in_map_key stdout ----
    thread 'test_borrowed_raw_value' panicked at 'called `Result::unwrap()`
    on an `Err` value: Error("invalid type: newtype struct, expected any
    valid JSON value", line: 1, column: 2)', tests/test.rs:2230:52

Author: dtolnay

Cargo.toml

@@ -20,6 +20,7 @@ ryu = "1.0"
 
 [dev-dependencies]
 automod = "1.0"
+ref-cast = "1.0"
 rustversion = "1.0"
 serde_bytes = "0.11"
 serde_derive = "1.0"

tests/test.rs

@@ -2193,6 +2193,46 @@ fn test_borrowed_raw_value() {
     assert_eq!(r#"["a",42,{"foo": "bar"},null]"#, array_to_string);
 }
 
+#[cfg(feature = "raw_value")]
+#[test]
+fn test_raw_value_in_map_key() {
+    use ref_cast::RefCast;
+
+    #[derive(RefCast)]
+    #[repr(transparent)]
+    struct RawMapKey(RawValue);
+
+    impl<'de> Deserialize<'de> for &'de RawMapKey {
+        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+        where
+            D: serde::Deserializer<'de>,
+        {
+            let raw_value = <&RawValue>::deserialize(deserializer)?;
+            Ok(RawMapKey::ref_cast(raw_value))
+        }
+    }
+
+    impl PartialEq for RawMapKey {
+        fn eq(&self, other: &Self) -> bool {
+            self.0.get() == other.0.get()
+        }
+    }
+
+    impl Eq for RawMapKey {}
+
+    impl Hash for RawMapKey {
+        fn hash<H: Hasher>(&self, hasher: &mut H) {
+            self.0.get().hash(hasher);
+        }
+    }
+
+    let map_from_str: std::collections::HashMap<&RawMapKey, &RawValue> =
+        serde_json::from_str(r#" {"\\k":"\\v"} "#).unwrap();
+    let (map_k, map_v) = map_from_str.into_iter().next().unwrap();
+    assert_eq!("\"\\\\k\"", map_k.0.get());
+    assert_eq!("\"\\\\v\"", map_v.get());
+}
+
 #[cfg(feature = "raw_value")]
 #[test]
 fn test_boxed_raw_value() {