Add environment variables for accessing the OUT_DIR

Author: namanlp

src/cargo/core/compiler/mod.rs

@@ -1704,15 +1704,34 @@ fn build_deps_args(
 
     let mut unstable_opts = false;
 
-    for dep in deps {
-        if dep.unit.mode.is_run_custom_build() {
-            cmd.env(
-                "OUT_DIR",
-                &build_runner.files().build_script_out_dir(&dep.unit),
-            );
+    // Add `OUT_DIR` environment variables for build scripts
+    let first_custom_build_dep = deps.iter().find(|dep| dep.unit.mode.is_run_custom_build());
+    if let Some(dep) = first_custom_build_dep {
+        let out_dir = &build_runner.files().build_script_out_dir(&dep.unit);
+        cmd.env("OUT_DIR", &out_dir);
+    }
+
+    // Adding output directory for each build script
+    let is_multiple_build_scripts_enabled = unit
+        .pkg
+        .manifest()
+        .unstable_features()
+        .require(Feature::multiple_build_scripts())
+        .is_ok();
+
+    if is_multiple_build_scripts_enabled {
+        for dep in deps {
+            if dep.unit.mode.is_run_custom_build() {
+                let out_dir = &build_runner.files().build_script_out_dir(&dep.unit);
+                let target_name = dep.unit.target.name();
+                let out_dir_prefix = target_name
+                    .strip_prefix("build-script-")
+                    .unwrap_or(target_name);
+                let out_dir_name = format!("{out_dir_prefix}_OUT_DIR");
+                cmd.env(&out_dir_name, &out_dir);
+            }
         }
     }
-
     for arg in extern_args(build_runner, unit, &mut unstable_opts)? {
         cmd.arg(arg);
     }

src/doc/src/reference/unstable.md

@@ -324,6 +324,10 @@ version = "0.0.1"
 build = ["foo.rs", "bar.rs"]
 ```
 
+**Accessing Output Directories**:  Output directory of each build script can be accessed by using `<script-name>_OUT_DIR` 
+  where the `<script-name>` is the file-stem of the build script, exactly as-is.
+  For example, `bar_OUT_DIR` for script at `foo/bar.rs`. (Only set during compilation, can be accessed via `env!` macro)
+
 ## public-dependency
 * Tracking Issue: [#44663](https://github.com/rust-lang/rust/issues/44663)
 

tests/testsuite/build_scripts_multiple.rs

@@ -549,7 +549,7 @@ fn build_script_with_conflicting_out_dirs() {
                 build = ["build1.rs", "build2.rs"]
             "#,
         )
-        // OUT_DIR is set to the lexicographically largest build script's OUT_DIR by default
+        // By default, OUT_DIR is set to that of the first build script in the array
         .file(
             "src/main.rs",
             r#"
@@ -603,7 +603,7 @@ fn build_script_with_conflicting_out_dirs() {
         .masquerade_as_nightly_cargo(&["multiple-build-scripts"])
         .with_status(0)
         .with_stdout_data(str![[r#"
-Hello, from Build Script 2!
+Hello, from Build Script 1!
 
 "#]])
         .run();
@@ -628,7 +628,7 @@ fn build_script_with_conflicts_reverse_sorted() {
                 build = ["build2.rs", "build1.rs"]
             "#,
         )
-        // OUT_DIR is set to the lexicographically largest build script's OUT_DIR by default
+        // By default, OUT_DIR is set to that of the first build script in the array
         .file(
             "src/main.rs",
             r#"
@@ -682,7 +682,7 @@ fn build_script_with_conflicts_reverse_sorted() {
         .masquerade_as_nightly_cargo(&["multiple-build-scripts"])
         .with_status(0)
         .with_stdout_data(str![[r#"
-Hello, from Build Script 1!
+Hello, from Build Script 2!
 
 "#]])
         .run();
@@ -785,9 +785,11 @@ fn multiple_out_dirs() {
         .file(
             "src/main.rs",
             r#"
-                include!(concat!(env!("OUT_DIR"), "/foo.rs"));
+                include!(concat!(env!("build1_OUT_DIR"), "/foo.rs"));
+                include!(concat!(env!("build2_OUT_DIR"), "/foo.rs"));
                 fn main() {
-                    println!("{}", message());
+                    println!("{}", message1());
+                    println!("{}", message2());
                 }
             "#,
         )
@@ -803,7 +805,7 @@ fn multiple_out_dirs() {
                 let dest_path = Path::new(&out_dir).join("foo.rs");
                 fs::write(
                     &dest_path,
-                    "pub fn message() -> &'static str {
+                    "pub fn message1() -> &'static str {
                         \"Hello, from Build Script 1!\"
                     }
                     "
@@ -822,7 +824,7 @@ fn multiple_out_dirs() {
                 let dest_path = Path::new(&out_dir).join("foo.rs");
                 fs::write(
                     &dest_path,
-                    "pub fn message() -> &'static str {
+                    "pub fn message2() -> &'static str {
                         \"Hello, from Build Script 2!\"
                     }
                     "
@@ -835,6 +837,7 @@ fn multiple_out_dirs() {
         .masquerade_as_nightly_cargo(&["multiple-build-scripts"])
         .with_status(0)
         .with_stdout_data(str![[r#"
+Hello, from Build Script 1!
 Hello, from Build Script 2!
 
 "#]])