add feature toggle for controlling gitoxide capabilities.

They are made to control which portions of typical git operations
should be handled by gitoxide and try to predict what'about
to be implemented one day.

That way testers on nightly can more easily A/B test different
capabilities as well.

src/cargo/core/features.rs

@@ -675,6 +675,7 @@ unstable_cli_options!(
     doctest_xcompile: bool = ("Compile and run doctests for non-host target using runner config"),
     dual_proc_macros: bool = ("Build proc-macros for both the host and the target"),
     features: Option<Vec<String>>  = (HIDDEN),
+    gitoxide: Option<GitoxideFeatures> = ("Use gitoxide for the given git interactions, or all of them if no argument is given"),
     jobserver_per_rustc: bool = (HIDDEN),
     minimal_versions: bool = ("Resolve minimal dependency versions instead of maximum"),
     mtime_on_use: bool = ("Configure Cargo to update the mtime of used files"),
@@ -778,6 +779,50 @@ where
     parse_check_cfg(crates.into_iter()).map_err(D::Error::custom)
 }
 
+#[derive(Debug, Copy, Clone, Default, Serialize, Deserialize)]
+pub struct GitoxideFeatures {
+    /// All featches are done with gitoxide, which includes git dependencies as well as the crates index.
+    pub fetch: bool,
+    /// When cloning the index, perform a shallow clone. Maintain shallowness upon subsequent fetches.
+    pub shallow_index: bool,
+    /// Checkout git dependencies using `gitoxide` (submodules are still handled by git2 ATM, and filters
+    /// like linefeed conversions are unsupported).
+    pub checkout: bool,
+}
+
+impl GitoxideFeatures {
+    fn all() -> Self {
+        GitoxideFeatures {
+            fetch: true,
+            shallow_index: true,
+            checkout: true,
+        }
+    }
+}
+
+fn parse_gitoxide(
+    it: impl Iterator<Item = impl AsRef<str>>,
+) -> CargoResult<Option<GitoxideFeatures>> {
+    let mut out = GitoxideFeatures::default();
+    let GitoxideFeatures {
+        fetch,
+        shallow_index,
+        checkout,
+    } = &mut out;
+
+    for e in it {
+        match e.as_ref() {
+            "fetch" => *fetch = true,
+            "shallow_index" => *shallow_index = true,
+            "checkout" => *checkout = true,
+            _ => {
+                bail!("unstable 'gitoxide' only takes `fetch`, 'shallow_index' and 'checkout' as valid inputs")
+            }
+        }
+    }
+    Ok(Some(out))
+}
+
 fn parse_check_cfg(
     it: impl Iterator<Item = impl AsRef<str>>,
 ) -> CargoResult<Option<(bool, bool, bool, bool)>> {
@@ -928,6 +973,12 @@ impl CliUnstable {
             "doctest-in-workspace" => self.doctest_in_workspace = parse_empty(k, v)?,
             "panic-abort-tests" => self.panic_abort_tests = parse_empty(k, v)?,
             "jobserver-per-rustc" => self.jobserver_per_rustc = parse_empty(k, v)?,
+            "gitoxide" => {
+                self.gitoxide = v.map_or_else(
+                    || Ok(Some(GitoxideFeatures::all())),
+                    |v| parse_gitoxide(v.split(',')),
+                )?
+            }
             "host-config" => self.host_config = parse_empty(k, v)?,
             "target-applies-to-host" => self.target_applies_to_host = parse_empty(k, v)?,
             "publish-timeout" => self.publish_timeout = parse_empty(k, v)?,