refactor: make clear which resolve versions are stable

src/cargo/core/resolver/resolve.rs

@@ -88,6 +88,17 @@ pub enum ResolveVersion {
     V4,
 }
 
+impl ResolveVersion {
+    /// The maximum version of lockfile made into the stable channel.
+    ///
+    /// Any version larger than this needs `-Znext-lockfile-bump` to enable.
+    ///
+    /// Update this when you're going to stabilize a new lockfile version
+    pub fn max_stable() -> ResolveVersion {
+        ResolveVersion::V3
+    }
+}
+
 impl Resolve {
     pub fn new(
         graph: Graph<PackageId, HashSet<Dependency>>,

src/cargo/ops/lockfile.rs

@@ -64,17 +64,16 @@ pub fn write_pkg_lockfile(ws: &Workspace<'_>, resolve: &mut Resolve) -> CargoRes
     // out lock file updates as they're otherwise already updated, and changes
     // which don't touch dependencies won't seemingly spuriously update the lock
     // file.
-    if resolve.version() < ResolveVersion::default() {
-        resolve.set_version(ResolveVersion::default());
+    let default_version = ResolveVersion::default();
+    let current_version = resolve.version();
+    let next_lockfile_bump = ws.config().cli_unstable().next_lockfile_bump;
+
+    if current_version < default_version {
+        resolve.set_version(default_version);
         out = serialize_resolve(resolve, orig.as_deref());
-    } else if resolve.version() > ResolveVersion::default()
-        && !ws.config().cli_unstable().next_lockfile_bump
-    {
+    } else if current_version > ResolveVersion::max_stable() && !next_lockfile_bump {
         // The next version hasn't yet stabilized.
-        anyhow::bail!(
-            "lock file version `{:?}` requires `-Znext-lockfile-bump`",
-            resolve.version()
-        )
+        anyhow::bail!("lock file version `{current_version:?}` requires `-Znext-lockfile-bump`")
     }
 
     // Ok, if that didn't work just write it out