Optional dependencies

[mahkoh]

Consider the case of a music player that supports multiple codecs. To compile such a program, the build system has to find out which libraries are currently available on the system and has to compile only those parts that use the available libraries. For rust this includes passing --cfg arguments to the compiler.

This is true not only for binaries but also for libraries that might use bindgen to create bindings from the header files.

Since checking for dependencies might take some time, this step should not be executed with cargo build every time.

[alexcrichton]

This is certainly planned! We're going to support this through the planned feature of profiles and features.

[huonw]

cc #385

[alexcrichton]

I believe that this will be entirely covered by the features mentioned in #385, so I'm going to close this in favor of that issue for now. Once #385 is implemented, if it doesn't solve this, then we can definitely reopen!

[mahkoh]

Not sure how this addresses the problem at all.

To compile such a program, the build system has to find out which libraries are currently available on the system

What I read in #385 sounds like I'll have to write a classical configure file that creates a make file that calls cargo with the correct flags.

[alexcrichton]

Due to cargo's nature of encouraging repeatable builds, this will likely manifest itself with features in that you explicitly opt in to building a feature and then the build will go awry if a system dependency was not discovered. Otherwise I suspect that syntax extensions will serve the purpose of determining whether a dependency is available locally or not.