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[WIP] Accessing each build script's OUT_DIR #15891

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6c8bc08
Add test for multiple `OUT_DIR`
namanlp Aug 28, 2025
1a9ce6f
Add environment variables for accessing the `OUT_DIR`
namanlp Sep 7, 2025
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19 changes: 15 additions & 4 deletions src/cargo/core/compiler/build_runner/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -210,7 +210,7 @@ impl<'a, 'gctx> BuildRunner<'a, 'gctx> {
plan.output_plan(self.bcx.gctx);
}

// Add `OUT_DIR` to env vars if unit has a build script.
// Add `OUT_DIR` to runtime env vars if unit has a build script.
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Saying runtime here might be confusing as we are adding it for use at compile time of the package and not for cargo run or cargo test running binaries.

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Wait, this is us setting the specialized OUT_DIR on the build script itself? We probably shouldn't be doing that. Depending on how we do build script delegation, the name will be dependent on how the caller sees the build script which will make it undiscoverable for build scripts to use.

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So, should I remove it?

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Yes. While this could make it easier to match up build script to rust source, it runs into the problem mentioned.

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Not the earlier code, right?

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Not sure what you are asking about. We should only set the new environment variable on the libs/bins/etc and not on the build script execution.

let units_with_build_script = &self
.bcx
.roots
Expand All @@ -227,11 +227,22 @@ impl<'a, 'gctx> BuildRunner<'a, 'gctx> {
.display()
.to_string();
let script_meta = self.get_run_build_script_metadata(&dep.unit);
self.compilation

// It creates environment variable named build-script-<name>_OUT_DIR
let mut out_dir_name = dep.unit.target.name().to_owned();
out_dir_name = out_dir_name
.strip_prefix("build-script-")
.unwrap()
.to_string();
out_dir_name.push_str("_OUT_DIR");

let env = self
.compilation
.extra_env
.entry(script_meta)
.or_insert_with(Vec::new)
.push(("OUT_DIR".to_string(), out_dir));
.or_insert_with(Vec::new);
env.push(("OUT_DIR".to_string(), out_dir.clone()));
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Can we make OUT_DIR an explicit choice for when we write it, rather than relying on it by incident? That'll make it less likely for people to break this in the future.

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I'll get back to this

env.push((out_dir_name, out_dir));
Comment on lines +239 to +245
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We'll need to update https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-crates

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In a separate commit maybe? Or should I update the latest commit itself?

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Unless the commit is ginormous (e.g. adding all of cargo add into cargo), I would generally update docs as part of commit that added the feature, like tests.

}
}
}
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13 changes: 9 additions & 4 deletions src/cargo/core/compiler/mod.rs
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Expand Up @@ -1706,10 +1706,15 @@ fn build_deps_args(

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),
);
let out_dir = &build_runner.files().build_script_out_dir(&dep.unit);
let mut out_dir_name = dep.unit.target.name().to_owned();
out_dir_name = out_dir_name
.strip_prefix("build-script-")
.unwrap()
.to_string();
out_dir_name.push_str("_OUT_DIR");
cmd.env("OUT_DIR", &out_dir);
cmd.env(&out_dir_name, &out_dir);
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don't forget to update the test from 6c8bc08

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Done. I added an extra test for duplicated/conflicting declarations of same function

}
}

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179 changes: 177 additions & 2 deletions tests/testsuite/build_scripts_multiple.rs
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Expand Up @@ -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 last build script in the array
.file(
"src/main.rs",
r#"
Expand Down Expand Up @@ -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 last build script in the array
.file(
"src/main.rs",
r#"
Expand Down Expand Up @@ -764,3 +764,178 @@ fn bar() {
"#]])
.run();
}

#[cargo_test]
fn multiple_out_dirs() {
// Test to verify access to the `OUT_DIR` of the respective build scripts.

let p = project()
.file(
"Cargo.toml",
r#"
cargo-features = ["multiple-build-scripts"]

[package]
name = "foo"
version = "0.1.0"
edition = "2024"
build = ["build1.rs", "build2.rs"]
"#,
)
.file(
"src/main.rs",
r#"
include!(concat!(env!("build1_OUT_DIR"), "/foo.rs"));
include!(concat!(env!("build2_OUT_DIR"), "/foo.rs"));
fn main() {
println!("{}", message1());
println!("{}", message2());
}
"#,
)
.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 message1() -> &'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 message2() -> &'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!
Hello, from Build Script 2!

"#]])
.run();
}

#[cargo_test]
fn build_script_with_fn_conflicts() {
// In this, multiple scripts have duplicated/conflicting declarations of same function

let p = project()
.file(
"Cargo.toml",
r#"
cargo-features = ["multiple-build-scripts"]

[package]
name = "foo"
version = "0.1.0"
edition = "2024"

build = ["build1.rs", "build2.rs"]
"#,
)
.file(
"src/main.rs",
r#"
include!(concat!(env!("build1_OUT_DIR"), "/foo.rs"));
include!(concat!(env!("build2_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("check -v")
.masquerade_as_nightly_cargo(&["multiple-build-scripts"])
.with_status(101)
.with_stderr_data(str![[r#"
[COMPILING] foo v0.1.0 ([ROOT]/foo)
...
--> [ROOT]/foo/target/debug/build/foo-[HASH]/out/foo.rs:1:1
|
1 | pub fn message() -> &'static str {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `message` redefined here
|
::: [ROOT]/foo/target/debug/build/foo-[HASH]/out/foo.rs:1:1
|
1 | pub fn message() -> &'static str {
| -------------------------------- previous definition of the value `message` here
|
= [NOTE] `message` must be defined only once in the value namespace of this module

For more information about this error, try `rustc --explain E0428`.
[ERROR] could not compile `foo` (bin "foo") due to 1 previous error

Caused by:
process didn't exit successfully: `rustc --crate-name foo --edition=2024 src/main.rs [..] ([EXIT_STATUS]: 1)

"#]])
.run();
}
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