Added build.build_dir configuration option

This commit adds a `build_dir` option to the `build` table in
`config.toml` and adds the equivalent field to `Workspace` and `GlobalContext`.

src/cargo/core/workspace.rs

@@ -66,6 +66,10 @@ pub struct Workspace<'gctx> {
     /// `None` if the default path of `root/target` should be used.
     target_dir: Option<Filesystem>,
 
+    /// Shared build directory for intermediate build artifacts.
+    /// This directory may be shared between multiple workspaces.
+    build_dir: Option<Filesystem>,
+
     /// List of members in this workspace with a listing of all their manifest
     /// paths. The packages themselves can be looked up through the `packages`
     /// set above.
@@ -207,6 +211,7 @@ impl<'gctx> Workspace<'gctx> {
     pub fn new(manifest_path: &Path, gctx: &'gctx GlobalContext) -> CargoResult<Workspace<'gctx>> {
         let mut ws = Workspace::new_default(manifest_path.to_path_buf(), gctx);
         ws.target_dir = gctx.target_dir()?;
+        ws.build_dir = gctx.build_dir()?;
 
         if manifest_path.is_relative() {
             bail!(
@@ -236,6 +241,7 @@ impl<'gctx> Workspace<'gctx> {
             },
             root_manifest: None,
             target_dir: None,
+            build_dir: None,
             members: Vec::new(),
             member_ids: HashSet::new(),
             default_members: Vec::new(),
@@ -279,6 +285,7 @@ impl<'gctx> Workspace<'gctx> {
         } else {
             ws.gctx.target_dir()?
         };
+        ws.build_dir = ws.target_dir.clone();
         ws.members.push(ws.current_manifest.clone());
         ws.member_ids.insert(id);
         ws.default_members.push(ws.current_manifest.clone());
@@ -408,6 +415,15 @@ impl<'gctx> Workspace<'gctx> {
             .unwrap_or_else(|| self.default_target_dir())
     }
 
+    pub fn build_dir(&self) -> Filesystem {
+        if !self.gctx().cli_unstable().build_dir {
+            return self.target_dir();
+        }
+        self.build_dir
+            .clone()
+            .unwrap_or_else(|| self.default_target_dir())
+    }
+
     fn default_target_dir(&self) -> Filesystem {
         if self.root_maybe().is_embedded() {
             let hash = crate::util::hex::short_hash(&self.root_manifest().to_string_lossy());

src/cargo/util/context/mod.rs

@@ -603,7 +603,7 @@ impl GlobalContext {
     ///
     /// Returns `None` if the user has not chosen an explicit directory.
     ///
-    /// Callers should prefer `Workspace::target_dir` instead.
+    /// Callers should prefer [`Workspace::target_dir`] instead.
     pub fn target_dir(&self) -> CargoResult<Option<Filesystem>> {
         if let Some(dir) = &self.target_dir {
             Ok(Some(dir.clone()))
@@ -634,6 +634,35 @@ impl GlobalContext {
         }
     }
 
+    /// The directory to use for intermediate build artifacts.
+    ///
+    /// Fallsback to the target directory if not specified.
+    ///
+    /// Callers should prefer [`Workspace::build_dir`] instead.
+    pub fn build_dir(&self) -> CargoResult<Option<Filesystem>> {
+        if !self.cli_unstable().build_dir {
+            return self.target_dir();
+        }
+
+        if let Some(val) = &self.build_config()?.build_dir {
+            let path = val.resolve_path(self);
+
+            // Check if the target directory is set to an empty string in the config.toml file.
+            if val.raw_value().is_empty() {
+                bail!(
+                    "the build directory is set to an empty string in {}",
+                    val.value().definition
+                )
+            }
+
+            Ok(Some(Filesystem::new(path)))
+        } else {
+            // For now, fallback to the previous implementation.
+            // This will change in the future.
+            return self.target_dir();
+        }
+    }
+
     /// Get a configuration value by key.
     ///
     /// This does NOT look at environment variables. See `get_cv_with_env` for
@@ -2653,6 +2682,7 @@ pub struct CargoBuildConfig {
     pub pipelining: Option<bool>,
     pub dep_info_basedir: Option<ConfigRelativePath>,
     pub target_dir: Option<ConfigRelativePath>,
+    pub build_dir: Option<ConfigRelativePath>,
     pub incremental: Option<bool>,
     pub target: Option<BuildTargetConfig>,
     pub jobs: Option<JobsConfig>,

src/doc/src/reference/unstable.md

@@ -77,6 +77,7 @@ Each new feature described below should explain how to use it.
     * [update-breaking](#update-breaking) --- Allows upgrading to breaking versions with `update --breaking`
 * Output behavior
     * [artifact-dir](#artifact-dir) --- Adds a directory where artifacts are copied to.
+    * [build-dir](#build-dir) --- Adds a directory where intermediate build artifacts are stored.
     * [Different binary name](#different-binary-name) --- Assign a name to the built binary that is separate from the crate name.
     * [root-dir](#root-dir) --- Controls the root directory relative to which paths are printed
 * Compile behavior
@@ -237,6 +238,22 @@ This can also be specified in `.cargo/config.toml` files.
 artifact-dir = "out"
 ```
 
+## build-dir
+* Original Issue: [#14125](https://github.com/rust-lang/cargo/issues/14125)
+* Tracking Issue: [#14125](https://github.com/rust-lang/cargo/issues/14125)
+
+This feature allows you to specify the directory where intermediate build artifacts will be stored.
+The final artifacts will also be contained in this directory but will be hardlinked into the artifact directory (usually `target`)
+
+This can be specified in `.cargo/config.toml` files using the `build.build-dir`.
+
+Example:
+
+```toml
+[build]
+build-dir = "out"
+```
+
 ## root-dir
 * Original Issue: [#9887](https://github.com/rust-lang/cargo/issues/9887)
 * Tracking Issue: None (not currently slated for stabilization)