feat(unstable): Added -Zfine-grain-locking unstable feature

Author: ranger-ross

src/cargo/core/features.rs

@@ -861,6 +861,7 @@ unstable_cli_options!(
     dual_proc_macros: bool = ("Build proc-macros for both the host and the target"),
     feature_unification: bool = ("Enable new feature unification modes in workspaces"),
     features: Option<Vec<String>>,
+    fine_grain_locking: bool = ("Use fine grain locking instead of locking the entire build cache"),
     fix_edition: Option<FixEdition> = ("Permanently unstable edition migration helper"),
     gc: bool = ("Track cache usage and \"garbage collect\" unused files"),
     #[serde(deserialize_with = "deserialize_git_features")]
@@ -1241,6 +1242,9 @@ impl CliUnstable {
         if self.gitoxide.is_none() && cargo_use_gitoxide_instead_of_git2() {
             self.gitoxide = GitoxideFeatures::safe().into();
         }
+
+        self.fail_if_incompatible_opts()?;
+
         Ok(warnings)
     }
 
@@ -1379,6 +1383,7 @@ impl CliUnstable {
             "direct-minimal-versions" => self.direct_minimal_versions = parse_empty(k, v)?,
             "dual-proc-macros" => self.dual_proc_macros = parse_empty(k, v)?,
             "feature-unification" => self.feature_unification = parse_empty(k, v)?,
+            "fine-grain-locking" => self.fine_grain_locking = parse_empty(k, v)?,
             "fix-edition" => {
                 let fe = v
                     .ok_or_else(|| anyhow::anyhow!("-Zfix-edition expected a value"))?
@@ -1510,6 +1515,14 @@ impl CliUnstable {
             );
         }
     }
+
+    fn fail_if_incompatible_opts(&self) -> CargoResult<()> {
+        if self.fine_grain_locking && !self.build_dir_new_layout {
+            bail!("-Z fine-grain-locking requires -Z build-dir-new-layout");
+        }
+
+        Ok(())
+    }
 }
 
 /// Returns the current release channel ("stable", "beta", "nightly", "dev").

src/doc/src/reference/unstable.md

@@ -96,6 +96,7 @@ Each new feature described below should explain how to use it.
     * [gc](#gc) --- Global cache garbage collection.
     * [open-namespaces](#open-namespaces) --- Allow multiple packages to participate in the same API namespace
     * [panic-immediate-abort](#panic-immediate-abort) --- Passes `-Cpanic=immediate-abort` to the compiler.
+    * [fine-grain-locking](#fine-grain-locking) --- Use fine grain locking instead of locking the entire build cache
 * rustdoc
     * [rustdoc-map](#rustdoc-map) --- Provides mappings for documentation to link to external sites like [docs.rs](https://docs.rs/).
     * [scrape-examples](#scrape-examples) --- Shows examples within documentation.
@@ -1709,6 +1710,14 @@ panic-immediate-abort = true
 panic = "immediate-abort"
 ```
 
+## fine-grain-locking
+
+* Tracking Issue: [#4282](https://github.com/rust-lang/cargo/issues/4282)
+
+Use fine grain locking instead of locking the entire build cache.
+
+Note: Fine grain locking also requires enabling the [build-dir-new-layout](#build-dir-new-layout) as fine grain locking builds on that directory reoganization.
+
 ## `[lints.cargo]`
 
 * Tracking Issue: [#12235](https://github.com/rust-lang/cargo/issues/12235)

tests/testsuite/cargo/z_help/stdout.term.svg

@@ -2368,3 +2368,17 @@ fn invalid_feature_name_slash_error() {
 "#]])
         .run();
 }
+
+#[cargo_test]
+fn fine_grain_locking_should_require_new_build_dir_layout() {
+    let p = project().file("src/main.rs", "").build();
+
+    p.cargo("-Zfine-grain-locking check")
+        .masquerade_as_nightly_cargo(&["fine grain locking flag"])
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[ERROR] -Z fine-grain-locking requires -Z build-dir-new-layout
+
+"#]])
+        .run();
+}