cargo-build_script_build-parser

A command-line utility that extracts --cfg and --check-cfg flags from cargo: lines generated by a Rust build.rs script.

This is useful when integrating crates like libc, which emit configuration flags during their build process.

https://github.com/nixcloud/cargo-build_script_build-parser

https://doc.rust-lang.org/cargo/reference/build-scripts.html

• Outputs all flags as a single command-line string
• Simple command-line interface
• Tests included
• Distributed as a Nix Flake

🔧 Supported Features

cargo:cargo-cfg
cargo:cargo-check-cfg
cargo:rustc-env
cargo:warning
cargo:error
cargo:rustc-link-search
cargo:rustc-link-lib
cargo:version_number
cargo:include
cargo:root
cargo:conf
cargo:version_number
cargo:static

🔧 Experimental Features

🔧 Intentionally Ignored Features

cargo:rerun-if-changed
cargo:rerun-if-env-changed
cargo:rerun-if-changed-bin
cargo:rerun-if-changed-glob
cargo:rerun-if-changed-dir
cargo:rerun-if-changed-recursive
cargo:rerun-if-changed-env
cargo:lib_dir

🔧 Missing Features

cargo:rustc-flags
cargo:rustc-cdylib-link-arg
cargo:rustc-bin-link-arg
cargo:rustc-link-arg-bin

Extract rustc-arguments

nix run .#default -- test/output1 rustc-arguments

Output:

--cfg 'freebsd11' --cfg 'libc_const_extern_fn' --check-cfg 'cfg(espidf_time32)' --check-cfg 'cfg(target_arch,values("mips64r6"))'

Extract environment-variables

nix run .#default -- test/output1 environment-variables

Output:

VAR=VALUE
VAR2=

Extract rustc-propagated-arguments

nix run .#default -- test/output3 rustc-propagated-arguments

Output:

warning: In file included from /nix/store/x4cz3spvw0bwwz5sjsdn2qm4f89rcryn-glibc-2.40-66-dev/include/bits/libc-header-start.h:33,
warning: from /nix/store/x4cz3spvw0bwwz5sjsdn2qm4f89rcryn-glibc-2.40-66-dev/include/stdio.h:28,
warning: from sqlite3/sqlite3.c:14884:
warning: /nix/store/x4cz3spvw0bwwz5sjsdn2qm4f89rcryn-glibc-2.40-66-dev/include/features.h:422:4: warning: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Wcpp]
warning: 422 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
warning: |    ^~~~~~~
-L 'native=${rust-embed-8_6_0-50d2bdadc507cf36}'

Writing these files

nix run .#default -- test/output1 --out-path out/ write-results

Output

warning: This is a custom build warning from build.rs!
Successfully created files for nix to process from build.rs output in 'out/'

Hacking

When traditional cargo executes build.rs scripts it stores the output of each in this folder structure:

du -a target/ | grep output | grep -v '.finger'
4       target/debug/build/openssl-sys-bf6c2c38618f44c9/root-output
8       target/debug/build/openssl-sys-bf6c2c38618f44c9/output
4       target/debug/build/parking_lot_core-7bae69a33df63e20/root-output
4       target/debug/build/parking_lot_core-7bae69a33df63e20/output
4       target/debug/build/generic-array-39173d8ec8bd3f99/root-output
4       target/debug/build/generic-array-39173d8ec8bd3f99/output
4       target/debug/build/im-rc-723454c6ae50b5e6/root-output
4       target/debug/build/im-rc-723454c6ae50b5e6/output
4       target/debug/build/pulldown-cmark-1b80d253c6640a7e/root-output
...

So look at the *./output files and filter them with:

cat target/debug/build/openssl-sys-bf6c2c38618f44c9/output | grep '^cargo:'

🚀 Installation

This project is distributed as a Nix Flake.

{
  cargo-build_script_build-parser.url = "github:nixcloud/cargo-build_script_build-parser";

  outputs = { self, cargo-build_script_build-parser, ... }: {
    packages.x86_64-linux.cargo-build_script_build-parser = cargo-build_script_build-parser.packages.x86_64-linux.default;
  };
}

🧪 Testing

To verify functionality, run:

cargo test

The tests are written directly in main.rs.

📄 License

MIT © 2025 nixcloud

🙏 Acknowledgements

This utility was inspired by how libc and similar crates communicate configuration to cargo and downstream consumers. It simplifies extracting those configurations in reproducible or cross-compiled environments.