Expose env vars pkg-config depends on

[kornelski]

No description provided.

[alexcrichton]

Thanks! I wonder, this may need to be opt-in for now? Consumer libraries may not currently be prepared for this if they're using pkg-config otherwise with a different fallback. I'd imagine for a 1.0 release we'd do this by default but for now with the same 0.1 release I'm thinking we may want to stick to being conservative?

[kornelski]

How can a consumer be not prepared for this?

I assume it has no effect in older versions of Cargo, so won't cause back compat problems.

And I assume it only causes rebuild when Cargo sees the env var change, so it's a bug fix for builds that weren't rebuilt, and the worst thing that can happen is that Cargo will rebuild the project too often.

Note that it marks env vars as used only when they're actually used, so if false {pkg_config::probe();} won't have any effect.

[alexcrichton]

Ah so the problem here is that if you use pkg-config to locate a library and then use cc the crate to compile some support code, that build script won't get rerun when the C code changes. By default the cc crate doesn't print out the rerun-if-used annotations, so this could technically be a breaking change for that specific situation or situations like that

[kornelski]

Oh, so the rerun-if-env-changed annotation replaces all default file-based checks, instead of just adding one more check?

[alexcrichton]

Right yeah, Cargo's got a default "when do I rerun the build script" heuristic which is quite conservative by default, and printing anything as a dependency will replace this heuristic