Accessing each build script's OUT_DIR and in the correct order (#15776)

Hi Everyone!

This PR is aimed to have some improvements over #15704

### What does this PR try to resolve?

Now, multiple build scripts are built correctly. But there are some
underlying issues, that this PR is targeting.

- Preserving the order of the build scripts: Earlier the build scripts
were sorted by default, but now, the order will be preserved.

### How to test and review this PR?

There is a feature gate `multiple-build-scripts` that can be passed via
`cargo-features` in `Cargo.toml`. So, you have to add
```toml
cargo-features = ["multiple-build-scripts"]
```
Preferably on the top of the `Cargo.toml` and use nightly toolchain to
use the feature

src/cargo/core/compiler/unit_dependencies.rs

@@ -30,7 +30,9 @@ use crate::core::dependency::{Artifact, ArtifactKind, ArtifactTarget, DepKind};
 use crate::core::profiles::{Profile, Profiles, UnitFor};
 use crate::core::resolver::Resolve;
 use crate::core::resolver::features::{FeaturesFor, ResolvedFeatures};
-use crate::core::{Dependency, Package, PackageId, PackageSet, Target, TargetKind, Workspace};
+use crate::core::{
+    Dependency, Feature, Package, PackageId, PackageSet, Target, TargetKind, Workspace,
+};
 use crate::ops::resolve_all_features;
 use crate::util::GlobalContext;
 use crate::util::interning::InternedString;
@@ -142,8 +144,27 @@ pub fn build_unit_dependencies<'a, 'gctx>(
     // which affect the determinism of the build itself. As a result be sure
     // that dependency lists are always sorted to ensure we've always got a
     // deterministic output.
-    for list in state.unit_dependencies.values_mut() {
-        list.sort();
+    for (unit, list) in &mut state.unit_dependencies {
+        let is_multiple_build_scripts_enabled = unit
+            .pkg
+            .manifest()
+            .unstable_features()
+            .require(Feature::multiple_build_scripts())
+            .is_ok();
+
+        if is_multiple_build_scripts_enabled {
+            list.sort_by_key(|unit_dep| {
+                if unit_dep.unit.target.is_custom_build() {
+                    // We do not sort build scripts to preserve the user-defined order.
+                    // In terms of determinism, we are assuming nothing interferes with order from when the user set it in `Cargo.toml` to here
+                    (0, None)
+                } else {
+                    (1, Some(unit_dep.clone()))
+                }
+            });
+        } else {
+            list.sort();
+        }
     }
     trace!("ALL UNIT DEPENDENCIES {:#?}", state.unit_dependencies);
 

tests/testsuite/build_scripts_multiple.rs

@@ -608,6 +608,85 @@ Hello, from Build Script 2!
         .run();
 }
 
+#[cargo_test]
+fn build_script_with_conflicts_reverse_sorted() {
+    // In this, multiple scripts create file with same name in their respective OUT_DIR.
+    // It is different from above because `package.build` is not sorted in this.
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                cargo-features = ["multiple-build-scripts"]
+
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2024"
+
+                build = ["build2.rs", "build1.rs"]
+            "#,
+        )
+        // OUT_DIR is set to the lexicographically largest build script's OUT_DIR by default
+        .file(
+            "src/main.rs",
+            r#"
+                include!(concat!(env!("OUT_DIR"), "/foo.rs"));
+                fn main() {
+                    println!("{}", message());
+                }
+            "#,
+        )
+        .file(
+            "build1.rs",
+            r#"
+            use std::env;
+            use std::fs;
+            use std::path::Path;
+
+            fn main() {
+                let out_dir = env::var_os("OUT_DIR").unwrap();
+                let dest_path = Path::new(&out_dir).join("foo.rs");
+                fs::write(
+                    &dest_path,
+                    "pub fn message() -> &'static str {
+                        \"Hello, from Build Script 1!\"
+                    }
+                    "
+                ).unwrap();
+             }"#,
+        )
+        .file(
+            "build2.rs",
+            r#"
+            use std::env;
+            use std::fs;
+            use std::path::Path;
+
+            fn main() {
+                let out_dir = env::var_os("OUT_DIR").unwrap();
+                let dest_path = Path::new(&out_dir).join("foo.rs");
+                fs::write(
+                    &dest_path,
+                    "pub fn message() -> &'static str {
+                        \"Hello, from Build Script 2!\"
+                    }
+                    "
+                ).unwrap();
+             }"#,
+        )
+        .build();
+
+    p.cargo("run -v")
+        .masquerade_as_nightly_cargo(&["multiple-build-scripts"])
+        .with_status(0)
+        .with_stdout_data(str![[r#"
+Hello, from Build Script 1!
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn rerun_untracks_other_files() {
     let p = project()