Refactor how things are migrated

src/util/migrate.rs

@@ -0,0 +1,75 @@
+use serde::{Deserialize, Serialize};
+
+pub trait Migrate: Serialize + for<'de> Deserialize<'de> + 'static {
+	/// A sequence of bytes to add at the beginning of the serialized
+	/// string, to identify that the data is of this version.
+	const MARKER: &'static [u8] = b"";
+
+	/// The previous version of this data type, from which items of this version
+	/// can be migrated. Set `type Previous = NoPrevious` to indicate that this datatype
+	/// is the initial schema and cannot be migrated.
+	type Previous: Migrate;
+
+	/// This function must be filled in by implementors to migrate from a previons iteration
+	/// of the data format.
+	fn migrate(previous: Self::Previous) -> Self;
+
+	fn decode(bytes: &[u8]) -> Option<Self> {
+		if bytes.len() >= Self::MARKER.len() && &bytes[..Self::MARKER.len()] == Self::MARKER {
+			if let Ok(value) =
+				rmp_serde::decode::from_read_ref::<_, Self>(&bytes[Self::MARKER.len()..])
+			{
+				return Some(value);
+			}
+		}
+
+		Self::Previous::decode(bytes).map(Self::migrate)
+	}
+
+	fn encode(&self) -> Result<Vec<u8>, rmp_serde::encode::Error> {
+		let mut wr = Vec::with_capacity(128);
+		wr.extend_from_slice(Self::MARKER);
+		let mut se = rmp_serde::Serializer::new(&mut wr)
+			.with_struct_map()
+			.with_string_variants();
+		self.serialize(&mut se)?;
+		Ok(wr)
+	}
+}
+
+pub trait InitialFormat: Serialize + for<'de> Deserialize<'de> + 'static {
+	/// A sequence of bytes to add at the beginning of the serialized
+	/// string, to identify that the data is of this version.
+	const MARKER: &'static [u8] = b"";
+}
+
+// ----
+
+impl<T: InitialFormat> Migrate for T {
+	const MARKER: &'static [u8] = <T as InitialFormat>::MARKER;
+
+	type Previous = NoPrevious;
+
+	fn migrate(_previous: Self::Previous) -> Self {
+		unreachable!();
+	}
+}
+
+#[derive(Serialize, Deserialize)]
+pub struct NoPrevious;
+
+impl Migrate for NoPrevious {
+	type Previous = NoPrevious;
+
+	fn migrate(_previous: Self::Previous) -> Self {
+		unreachable!();
+	}
+
+	fn decode(_bytes: &[u8]) -> Option<Self> {
+		None
+	}
+
+	fn encode(&self) -> Result<Vec<u8>, rmp_serde::encode::Error> {
+		unreachable!()
+	}
+}