Add a way to set arbitrary rpaths.

[vext01]

Hi,

Consider this minimal example project:
https://github.com/vext01/rust_link_test

The project has a dependency written in C which is built by the build.rs.

For the examples or tests to work, the shared object for the C code needs to be in the linker path:

     Running target/debug/deps/link_test-8d72230c7bcf790a
/home/vext01/research/link_test/target/debug/deps/link_test-8d72230c7bcf790a: error while loading shared libraries: libcstuff.so: cannot open shared object file: No such file or directory

If I set LD_LIBRARY_PATH to c_lib then everything works. The standard way to work around having to set the LD_LIBRARY_PATH is to encode an rpath into the consuming binary (in this case the example or test binary).

Whilst the example be fixed using, e.g.:

#![link_args="-Wl,-rpath /home/vext01/research/link_test/c_lib"]

At the top of the file, the same doesn't appear to be true of tests.

Another thing that bothers me about using link_args is that it assumes that the path is known, but typically that's a detail that only the build script will know.

Cargo build scripts already have cargo:rustc-link-lib which allows this kind of thing to add linkage to a given library:

println!("cargo:rustc-link-lib=cstuff");

Why not have a more general interface whereby arbitrary linker flags can be passed?

Hope this make sense.

[alexcrichton]

Does RUSTFLAGS or cargo rustc help for this use case?

[vext01]

Hi Alex.

I'm looking at the --help messages for these commands wondering which flags you mean. I can't see one that would allow my to pass a linker flag.

Even if there were, it would be useful if we could control this stuff in build.rs, rather than having the user have to specify in their env.

[vext01]

Any thoughts on this Alex?

[alexcrichton]

Ah sorry, but RUSTFLAGS is an environment variable that Cargo uses to pass to rustc invocations, and cargo rustc is a way to pass custom flags to the final invocation. Currently build scripts can't embed rpaths because they can't pass along arbitrary flags to rustc.

Currently rustc itself basically doesn't have great support for this unfortunately, the story around passing linker flags and such hasn't been fully fleshed out there.

[vext01]

Thanks Alex.

I'm aware of RUSTFLAGS but I don't think it helps in this scenario.

Indeed, linkage options are a little limited and somewhat inconsistent in Rust.

I notice that #[link_args] is always deemed an "unused attribute" also. I tend to think that #[link_args] would be unnecessary if build.rs were able to set arbitrary linker flags, and this would also solve this issue. I suppose this would require a flag to be added to rustc also.

[vext01]

Just to elaborate on this, I think software authors should be able to specify their own link flags in build.rs without having to ask their downstream users to set environment variables.

[vext01]

I've been thinking about this again. I think the best thing we could do is allow -C codegen arguments to appear in the Cargo rustc-flags=... directive.

This should be inheritable, like rustc-link-search is, so that consumers of a Rust library which builds (e.g.) C shared objects in a non-standard location need do nothing extra. The Rust library providing the extra libs should be able to have:

println!("rustc-flags='-C link-arg=-Wl,-rpath=...'");

or similar in build.rs.

[mdsteele]

Being able to set a custom rpath is also important when bundling a binary as a Mac OS X app with embedded frameworks -- in this case, you need to encode an rpath of e.g. @executable_path/../Frameworks into the binary. It would be really nice to have a way of doing that from a build.rs script.

[emoon]

I'm having the exact same issue. In order to get this to work I have to do this in a .cargo/config

[target.x86_64-apple-darwin]
rustflags = ["-C", "link-arg=-Wl,-rpath,/some/path/here"]

Problem is that I either want:

1. /some/path to be an environment variable or
2. Be able to pass it from build.rs

But none of these works for me. In my case I would be fine with .cargo/config supporting to expand environment variables but of course passing it directly from build.rs would be ideal.

edit:

Now that I think about it. Maybe #![link_args="-Wl,-rpath /home/vext01/research/link_test/c_lib"] is usable in my case. I could have build.rs generate a file that includes that line where I have expanded the variable myself. This would be a work-around for sure but hopefully it will be helpful in my case but some better support would be nice without this of course :)