refactor(nix-compat): Move serialization machinery into wire.

This groups most `wire` feature gated logic into a single module.
The nix_daemon module will be gated by a feature that adds
nix-compat-derive as a dependency.

All of this is a way to break the crate2nix dependency cycle between
nix-compat and nix-compat-derive(which depends on nix-compat for its
doctests).

Change-Id: I95938a6f280c11967371ff21f8b5a19e6d3d3805
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12761
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/nix-compat/src/wire/protocol_version.rs

@@ -0,0 +1,139 @@
+/// The latest version that is currently supported by nix-compat.
+static DEFAULT_PROTOCOL_VERSION: ProtocolVersion = ProtocolVersion::from_parts(1, 37);
+
+/// Protocol versions are represented as a u16.
+/// The upper 8 bits are the major version, the lower bits the minor.
+/// This is not aware of any endianness, use [crate::wire::read_u64] to get an
+/// u64 first, and the try_from() impl from here if you're receiving over the
+/// Nix Worker protocol.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct ProtocolVersion(u16);
+
+impl ProtocolVersion {
+    pub const fn from_parts(major: u8, minor: u8) -> Self {
+        Self(((major as u16) << 8) | minor as u16)
+    }
+
+    pub fn major(&self) -> u8 {
+        ((self.0 & 0xff00) >> 8) as u8
+    }
+
+    pub fn minor(&self) -> u8 {
+        (self.0 & 0x00ff) as u8
+    }
+}
+
+impl Default for ProtocolVersion {
+    fn default() -> Self {
+        DEFAULT_PROTOCOL_VERSION
+    }
+}
+
+impl PartialOrd for ProtocolVersion {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for ProtocolVersion {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        match self.major().cmp(&other.major()) {
+            std::cmp::Ordering::Less => std::cmp::Ordering::Less,
+            std::cmp::Ordering::Greater => std::cmp::Ordering::Greater,
+            std::cmp::Ordering::Equal => {
+                // same major, compare minor
+                self.minor().cmp(&other.minor())
+            }
+        }
+    }
+}
+
+impl From<u16> for ProtocolVersion {
+    fn from(value: u16) -> Self {
+        Self::from_parts(((value & 0xff00) >> 8) as u8, (value & 0x00ff) as u8)
+    }
+}
+
+#[cfg(any(test, feature = "test"))]
+impl From<(u8, u8)> for ProtocolVersion {
+    fn from((major, minor): (u8, u8)) -> Self {
+        Self::from_parts(major, minor)
+    }
+}
+
+impl TryFrom<u64> for ProtocolVersion {
+    type Error = &'static str;
+
+    fn try_from(value: u64) -> Result<Self, Self::Error> {
+        if value & !0xffff != 0 {
+            return Err("only two least significant bits might be populated");
+        }
+
+        Ok((value as u16).into())
+    }
+}
+
+impl From<ProtocolVersion> for u16 {
+    fn from(value: ProtocolVersion) -> Self {
+        value.0
+    }
+}
+
+impl From<ProtocolVersion> for u64 {
+    fn from(value: ProtocolVersion) -> Self {
+        value.0 as u64
+    }
+}
+
+impl std::fmt::Display for ProtocolVersion {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}.{}", self.major(), self.minor())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::ProtocolVersion;
+
+    #[test]
+    fn from_parts() {
+        let version = ProtocolVersion::from_parts(1, 37);
+        assert_eq!(version.major(), 1, "correct major");
+        assert_eq!(version.minor(), 37, "correct minor");
+        assert_eq!("1.37", &version.to_string(), "to_string");
+
+        assert_eq!(0x0125, Into::<u16>::into(version));
+        assert_eq!(0x0125, Into::<u64>::into(version));
+    }
+
+    #[test]
+    fn from_u16() {
+        let version = ProtocolVersion::from(0x0125_u16);
+        assert_eq!("1.37", &version.to_string());
+    }
+
+    #[test]
+    fn from_u64() {
+        let version = ProtocolVersion::try_from(0x0125_u64).expect("must succeed");
+        assert_eq!("1.37", &version.to_string());
+    }
+
+    /// This contains data in higher bits, which should fail.
+    #[test]
+    fn from_u64_fail() {
+        ProtocolVersion::try_from(0xaa0125_u64).expect_err("must fail");
+    }
+
+    #[test]
+    fn ord() {
+        let v0_37 = ProtocolVersion::from_parts(0, 37);
+        let v1_37 = ProtocolVersion::from_parts(1, 37);
+        let v1_40 = ProtocolVersion::from_parts(1, 40);
+
+        assert!(v0_37 < v1_37);
+        assert!(v1_37 > v0_37);
+        assert!(v1_37 < v1_40);
+        assert!(v1_40 > v1_37);
+        assert!(v1_40 <= v1_40);
+    }
+}