Mutually exclusive features

In Windows API there is the notion of a family and partitions. Your choice of family is basically which platform you are targeting and there is a choice between WINAPI_FAMILY_DESKTOP_APP WINAPI_FAMILY_PC_APP WINAPI_FAMILY_PHONE_APP WINAPI_FAMILY_SYSTEM and WINAPI_FAMILY_SERVER. Your choice of family in turn enables certain partitions which enable access to certain functionality. At the moment I cannot use cargo features for this because they are not mutually exclusive, so two downstream dependencies could specify different families which is completely wrong. Even if I use a build script to ensure only one such feature is selected, it would be a nightmare to ensure that every single downstream dependency chooses the same family, threading the choice of family feature through long chains of transitive dependencies that shouldn't have to care about that choice.

What I think would work well here is some sort of feature selection that has to choose exactly one option from a closed set, and that choice is controlled by the root crate. All dependencies that are affected by that choice will then need some way to be informed of the choice so they can adjust which functionality to use depending on that choice.

At the moment the only workaround I see is for the user to pass an environment variable while building the project so winapi's build script can read that to choose the family followed by emitting key=value stuff which downstream dependencies can read via build scripts through the DEP_WINAPI_key=value environment variables to emit cfgs to control their own code.

Also, this can also apply to other choices, such as which version of Windows you want to target, newer versions have more functionality that dependencies might want to be able to take advantage of, but the user might also want them to target an older version instead so it can run on more systems.

Start by reviewing Cargo's feature resolution and build-script behavior described in the issue, focusing on how choices from the root crate could affect dependencies. Define how exactly one option would be selected and propagated through dependency chains, including Windows family and version choices; the work is done when the behavior and scope are specified clearly enough to implement and test.