Proper way to use rerun-if-env-changed?

[eberkund]

What the proper way of using rerun-if-env-changed? I am using a library where I am trying to pass in an env var from my application build.rs using cargo:rustc-env but the problem is the library which depends on the env var fails before it can be rerun. If set the env var prior or if I change the code around in the library then it can compile fine. Just not sure what the intended use case is.

[Eh2406]

I am sorry, I don't quite understand the problem you are hitting. Can you post a minimal reproducible example? Then maybe we can figure out if rerun-if-env-changed is relevant, and how to improve its documentation.

[eberkund]

Okay, well this is the 3rd party package which I was talking about: https://github.com/pedrosland/mmal-sys/blob/master/build.rs

The libs are not located in the default path it panics which means it never gets to be rerun since the build stops on a panic.

So I was wondering what is the proper way to communicate this error (not being able to find the libs) while also maintaining compatibility with overriding and rerunning the build script?

[ishitatsuyuki]

See also: rust-lang/rfcs#2391

[Eh2406]

So let me see if I understand. Your code depends on mmal-sys. mmal-sys uses env:MMAL_INCLUDE_DIR and panics if not set correctly. So you want to set env:MMAL_INCLUDE_DIR to the correct value in your build.rs and then have mmal-sys see the value you set. However this is not working, as your build.rs only gets run after your dependencies are built, and mmal-sys panics leading to your build.rs never being run to fix the env:MMAL_INCLUDE_DIR. Do I understand correctly?

[eberkund]

@Eh2406 That is exactly correct.

[Eh2406]

I don't think a build.rs can affect the environment for its dependencies. But I am not an expert on this part of the code. @alexcrichton, @jsgf, @matklad Am I correct about this?

If you are building a final artifact than you may need to have a build system around cargo to set the env before the build starts.
If you are building a library you may want to document that the env needs to be set up for your dependencies.
Alternatively if you think it is widely usable you could make a PR to mmal-sys to make it better at finding the correct value.

[alexcrichton]

@Eh2406 you are indeed correct! This is something that'll need to be set by an outer build system rather than a crate

[eberkund]

Okay, thank you for the info everyone.