Add a way to define relative paths in rustc-link-search

[oblique]

Describe the problem you are trying to solve

I'm writing a project that uses zmq and zmq-sys which need to be cross-compiled for arm-unknown-linux-gnueabi.

zmq-sys doesn't compile libzmq on its own but we can specify the lib and include directories by using LIBZMQ_LIB_DIR and LIBZMQ_INCLUDE_DIR. However zmq's build.rs requires zmq-sys, which means I also need a compiled binary for x86_64-unknown-linux-gnu target too. So I can not just use the environment variables of zmq-sys since LIBZMQ_LIB_DIR will point to a wrong directory.

I did the following workaround:

[target.arm-unknown-linux-gnueabi]
linker = "arm-none-linux-gnueabi-gcc"

[target.arm-unknown-linux-gnueabi.zmq]
rustc-link-search = ["native=/home/user/zmq-example/vendor/libzmq/lib/arm"]
rustc-link-lib = ["static=zmq", "dylib=stdc++"]

[target.x86_64-unknown-linux-gnu.zmq]
rustc-link-search = ["native=/home/user/zmq-example/vendor/libzmq/lib/x86_64"]
rustc-link-lib = ["static=zmq", "dylib=stdc++"]

The problem with the above workaround is that all developers need to have the same absolute path to the compiled binaries.

Describe the solution you'd like

I would like to have a way to define a relative path in rustc-link-search.

Another solution could be a way to define a environment variables per target.

[ehuss]

I'm a little confused about the request for relative paths. The paths should support being relative to the package root. Is that not working for you?

[oblique]

You are correct, so let me rephrase: Is there a way to provide paths that are relative to my project's root (/home/user/zmq-example) instead of zmq-sys package's root (/home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/zmq-sys-0.9.0).

I don't want to have absolute paths of my project's root in .cargo/config since it restricts other developers.

[nikonthethird]

So I have a workaround for this problem. I am cross-compiling DBus and have the same problem as you: I have to point the rustc-link-search to some place inside my repository, relative to the root of the repository, not relative to where Cargo extracted the package.

In short, you have to use a build.rs build script to set rustc-link-search.

This is my Cargo config file:

[build]
target = "armv7-unknown-linux-gnueabihf"

[target.armv7-unknown-linux-gnueabihf.dbus]
rustc-link-search = [
    # Provided by the build script.
]

rustc-link-lib = [
    "dbus-1",
    "gcrypt",
    "gpg-error",
    "lz4",
    "lzma",
    "pcre",
    "selinux",
    "systemd",
]

And this is my build.rs build script:

use std::env::var;

fn main() {
    let manifest_dir = var("CARGO_MANIFEST_DIR").unwrap();
    println!("cargo:rustc-link-search={}/libraries/lib/arm-linux-gnueabihf", manifest_dir);
    println!("cargo:rustc-link-search={}/libraries/usr/lib/arm-linux-gnueabihf", manifest_dir);
}

The CARGO_MANIFEST_DIR environment variable points to the crate with the build script, and I placed the external libraries into a libraries folder there. After doing this the build works again, no more hard-coded absolute paths.

Furthermore in your case you would also have to read the TARGET environment variable and set the paths according to it.