¶Build script links

2023-11-15

You have an “upstream” crate that wraps a C library and provides Rust bindings for it. You also have a “downstream” crate that wraps a different C library. The downstream C library depends on the upstream C library — for example, to compile the downstream C code, you need access to the header files of the upstream C code.

The problem: How do you tell the compiler where to find the upstream crate's header files when compiling the downstream library?

The answer: the package.links field and “build script metadata”. The upstream library needs to set package.links in its Cargo.toml. You can choose any value (which I'll call LINK_NAME), but convention is that it's the name of the library that the Rust crate is providing bindings for. In the upstream library's build.rs, you output additional cargo metadata:

cargo:VAR_NAME=VALUE

Then, in your downstream library, this custom metadata will be available in an environment variable that you can read: ‘DEP_${LINK_NAME}_${VAR_NAME}’.

For this compilation example, you output a carefully named include metadata variable, whose value is the actual location of the upstream header files. In your downstream build.rs, you read the ‘DEP_${LINK_NAME}_INCLUDE’ env var, and add that to the compiler's include search path.

The ‘links’ manifest key [The Cargo book]