build-rs-libnix

In cargo projects often uses build.rs files to probe for libraries like openssl using pkg-config.

Often one sees text like:

cargo:rustc-cfg=libc_const_extern_fn
cargo:rustc-cfg=freebsd11
cargo:rustc-check-cfg=cfg(espidf_time32)
cargo:rustc-check-cfg=cfg(target_arch,values("mips64r6"))
cargo:rustc-env=VAR=VALUE
cargo:rustc-env=VAR2=
cargo:warning=This is a custom build warning from build.rs!

This command-line utility build-rs-libnix consumes this output and builds arguments like --cfg and --check-cfg which are used from the nix code to build the application.

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

Distributed as a Nix Flake

The motivation to keep this tool seperate from cargo is that when it changes I don't have to rebuild cargo all the time and building this is really fast! However, when the interface stabilizes it could be integated into cargo where it actually belongs - either as a standalone binary or as a part of cargo.

The original implementation is in cargo src/cargo/core/compiler/custom_build.rs at fn parse_metadata<'a>(

🔧 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
...

Writing these files

cargo  run  -- --script-output build-rs-libnix/test/output1 --out-dir nix/

Output

warning: This is a custom build warning from build.rs!
build.rs related nix files written to 'nix/'

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.

build_rs_libnix_0_1_11 = pkgs.callPackage ./default.nix {};

🧪 Testing

To verify functionality, run:

cargo test

The tests are written directly in main.rs.

📄 License

Cargo is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0). So is this project.

See LICENSE-APACHE and LICENSE-MIT for details.