more clever version number handling

src/lib.rs

@@ -47,9 +47,12 @@ pub struct JsonDb<T: Schema> {
 /// other fields of the corresponding schema version; earlier versions
 /// will be migrated to the current version automatically.
 pub trait Schema: DeserializeOwned + Serialize {
-    const VERSION: u32;
+    /// Previous schema that can be migrated into the new schema
     type Prev: Schema + Into<Self>;
 
+    /// Schema version number
+    const VERSION: u32 = Self::Prev::VERSION + 1;
+
     fn parse(s: &str) -> Result<Self, Error> {
         let Version { version } = serde_json::from_str(s)?;
         match version.cmp(&Self::VERSION) {
@@ -64,11 +67,25 @@ pub trait Schema: DeserializeOwned + Serialize {
 ///
 /// Implementing this will automatically implement [`Schema`], with
 /// version number `0` and `Self` as the previous version.
-pub trait SchemaV0: DeserializeOwned + Serialize {}
+pub trait SchemaV0: DeserializeOwned + Serialize {
+    /// Set this to false if your version 0 is a pre-`JsonDb` schema
+    /// that does not include a version number.
+    const EXPECT_VERSION_NUMBER: bool = true;
+}
 
 impl<T: SchemaV0> Schema for T {
-    const VERSION: u32 = 0;
     type Prev = Self;
+    const VERSION: u32 = 0;
+
+    fn parse(s: &str) -> Result<Self, Error> {
+        if Self::EXPECT_VERSION_NUMBER {
+            let Version { version } = serde_json::from_str(s)?;
+            if version != 0 {
+                return Err(Error::UnknownVersion(version));
+            }
+        }
+        Ok(serde_json::from_str(s)?)
+    }
 }
 
 #[derive(Deserialize)]
@@ -216,7 +233,6 @@ mod tests {
         last_updated: i64,
     }
     impl Schema for V1 {
-        const VERSION: u32 = 1;
         type Prev = V0;
     }
     impl From<V0> for V1 {
@@ -248,7 +264,6 @@ mod tests {
         }
     }
     impl Schema for V2 {
-        const VERSION: u32 = 2;
         type Prev = V1;
     }
     impl From<V1> for V2 {