feat: prepare for the next lockfile bump

src/cargo/core/features.rs

@@ -741,6 +741,7 @@ unstable_cli_options!(
     minimal_versions: bool = ("Resolve minimal dependency versions instead of maximum"),
     msrv_policy: bool = ("Enable rust-version aware policy within cargo"),
     mtime_on_use: bool = ("Configure Cargo to update the mtime of used files"),
+    next_lockfile_bump: bool = (HIDDEN),
     no_index_update: bool = ("Do not update the registry index even if the cache is outdated"),
     panic_abort_tests: bool = ("Enable support to run tests with -Cpanic=abort"),
     profile_rustflags: bool = ("Enable the `rustflags` option in profiles in .cargo/config.toml file"),
@@ -1110,6 +1111,7 @@ impl CliUnstable {
             "host-config" => self.host_config = parse_empty(k, v)?,
             "jobserver-per-rustc" => self.jobserver_per_rustc = parse_empty(k, v)?,
             "lints" => self.lints = parse_empty(k, v)?,
+            "next-lockfile-bump" => self.next_lockfile_bump = parse_empty(k, v)?,
             "minimal-versions" => self.minimal_versions = parse_empty(k, v)?,
             "msrv-policy" => self.msrv_policy = parse_empty(k, v)?,
             // can also be set in .cargo/config or with and ENV

src/cargo/core/resolver/encode.rs

@@ -154,10 +154,18 @@ impl EncodableResolve {
     /// primary uses is to be used with `resolve_with_previous` to guide the
     /// resolver to create a complete Resolve.
     pub fn into_resolve(self, original: &str, ws: &Workspace<'_>) -> CargoResult<Resolve> {
+        let unstable_lockfile_version_allowed = ws.config().cli_unstable().next_lockfile_bump;
         let path_deps = build_path_deps(ws)?;
         let mut checksums = HashMap::new();
 
         let mut version = match self.version {
+            Some(4) => {
+                if unstable_lockfile_version_allowed {
+                    ResolveVersion::V4
+                } else {
+                    anyhow::bail!("lock file version 4 requires `-Znext-lockfile-bump`",)
+                }
+            }
             Some(3) => ResolveVersion::V3,
             Some(n) => bail!(
                 "lock file version `{}` was found, but this version of Cargo \
@@ -612,6 +620,7 @@ impl ser::Serialize for Resolve {
             metadata,
             patch,
             version: match self.version() {
+                ResolveVersion::V4 => Some(4),
                 ResolveVersion::V3 => Some(3),
                 ResolveVersion::V2 | ResolveVersion::V1 => None,
             },

src/cargo/core/resolver/resolve.rs

@@ -80,6 +80,10 @@ pub enum ResolveVersion {
     /// V3 by default staring in 1.53.
     #[default]
     V3,
+    /// Unstable. Will collect a certain amount of changes and then go.
+    ///
+    /// Changes made:
+    V4,
 }
 
 impl Resolve {

src/cargo/ops/lockfile.rs

@@ -69,6 +69,14 @@ pub fn write_pkg_lockfile(ws: &Workspace<'_>, resolve: &mut Resolve) -> CargoRes
     if resolve.version() < ResolveVersion::default() {
         resolve.set_version(ResolveVersion::default());
         out = serialize_resolve(resolve, orig.as_deref());
+    } else if resolve.version() > ResolveVersion::default()
+        && !ws.config().cli_unstable().next_lockfile_bump
+    {
+        // The next version hasn't yet stabilized.
+        anyhow::bail!(
+            "lock file version `{:?}` requires `-Znext-lockfile-bump`",
+            resolve.version()
+        )
     }
 
     // Ok, if that didn't work just write it out

tests/testsuite/lockfile_compat.rs

@@ -888,3 +888,66 @@ perhaps a crate was updated and forgotten to be re-vendored?
         )
         .run();
 }
+
+#[cargo_test]
+fn v4_is_unstable() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [package]
+                    name = "foo"
+                    version = "0.0.1"
+                "#,
+            ),
+        )
+        .file("src/lib.rs", "")
+        .file("Cargo.lock", "version = 4")
+        .build();
+
+    p.cargo("fetch")
+        .with_status(101)
+        .with_stderr(
+            "\
+error: failed to parse lock file at: [CWD]/Cargo.lock
+
+Caused by:
+  lock file version 4 requires `-Znext-lockfile-bump`
+",
+        )
+        .run();
+}
+
+#[cargo_test]
+fn v4_cannot_be_created_from_scratch() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [package]
+                    name = "foo"
+                    version = "0.0.1"
+                "#,
+            ),
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("fetch -Znext-lockfile-bump")
+        .masquerade_as_nightly_cargo(&["-Znext-lockfile-bump"])
+        .run();
+
+    let lockfile = r#"# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "foo"
+version = "0.0.1"
+"#;
+
+    let lock = p.read_lockfile();
+    assert_match_exact(lockfile, &lock);
+}