feat: Implementation and tests for `multiple-build-scripts` #15704

namanlp · 2025-06-25T19:09:05Z

Hi Everyone!

This is PR for the implementation of the first milestone of GSoC Project : Build Script Delegation

This will provide actual implementation for #15630

What does this PR try to resolve?

Now, multiple build scripts are parsed, this PR aims to implement the functioning the feature. This PR will allow users to use multiple build scripts, and is backward compatible with single script as well as boolean values.

Motivation : This will help users to maintain separate smaller and cleaner build scripts instead of one large build script. This is also necessary for build script delegation.

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

cargo-features = ["multiple-build-scripts"]

Preferably on the top of the Cargo.toml and use nightly toolchain to use the feature

epage · 2025-06-26T14:53:21Z

This is PR for the manifest parsing of the first milestone of GSoC Project : Build Script Delegation

Bad copy/paste?

src/cargo/core/compiler/build_runner/mod.rs

src/cargo/core/compiler/compilation.rs

src/cargo/core/compiler/build_runner/mod.rs

tests/testsuite/build_scripts_multiple.rs

src/cargo/core/compiler/unit_dependencies.rs

namanlp · 2025-06-30T19:44:19Z

I am marking this PR Ready for Review. Please do let me know if I have missed anything

rustbot · 2025-06-30T19:44:29Z

r? @weihanglo

rustbot has assigned @weihanglo.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

namanlp · 2025-06-30T19:46:43Z

r? @epage

epage · 2025-06-30T20:25:26Z

src/cargo/core/compiler/build_runner/mod.rs

Now that we have multiple build scripts working, we need to update

cargo/tests/testsuite/build_scripts_multiple.rs

Line 413 in 4a137fa

fn rerun_untracks_other_files() {

epage · 2025-06-30T20:31:21Z

src/cargo/core/compiler/compilation.rs

+        if let Some(meta_vec) = script_metas {
+            for meta in meta_vec {
+                if let Some(env) = self.extra_env.get(meta) {
+                    for (k, v) in env {
+                        cmd.env(k, v);
+                    }
                }


What should happen when two build scripts output conflicting information?

Last one specified wins (current behavior, see also feat: Implementation and tests for multiple-build-scripts #15704 (comment)

First one specified wins

Error

Unspecified

Whichever route we go, we should have a test for it. In fact, environment variable tests are probably the best way to test multiple build scripts ran.

While performing experiments, I don't know why, last one specified doesn't win. Rather, the lexicographically smallest build script wins.

For example, I have 2 build scripts, build1.rs and build2.rs, building files a.cpp and b.cpp (using cc), such that both a.cpp and b.cpp has functions foo, but foo is different in these files.

So, if we run it, no matter the order in Cargo.toml, build1.rs always wins, and hence, a.cpp always wins. Also, changing a.cpp to d.cpp doesn't change anything, but if we change build1.rs to build3.rs, build2.rs wins.

And it is deterministic, and not random, I have tested it like 50 times on my system ( I made a little bash script that cargo clean and then cargo run ). I am not completely sure why though...

For some reason, it is exactly opposite in environment variables. The lexicographically largest script, ie, build3.rs always wins.

epage · 2025-07-01T14:10:15Z

Also, another case we need to handle is OUT_DIR. That could be a follow up PR.

rustbot added A-build-execution Area: anything dealing with executing the compiler A-build-scripts Area: build.rs scripts A-dep-info Area: dep-info, .d files Command-run Command-test labels Jun 25, 2025