/** Only works with thin binaries. */
  private CxxPlatform getCxxPlatformForBuildTarget(BuildTarget target) {
    CxxPlatform cxxPlatform;
    try {
      cxxPlatform = cxxPlatformFlavorDomain.getValue(target.getFlavors()).or(defaultCxxPlatform);
    } catch (FlavorDomainException e) {
      throw new HumanReadableException(e, "%s: %s", target, e.getMessage());
    }

    return cxxPlatform;
  }
  @Override
  public <A extends Arg> BuildRule createBuildRule(
      TargetGraph targetGraph, BuildRuleParams params, BuildRuleResolver ruleResolver, A args) {

    // See if we're building a particular "type" of this library, and if so, extract
    // it as an enum.
    Optional<Map.Entry<Flavor, Type>> type;
    Optional<Map.Entry<Flavor, CxxPlatform>> platform;
    try {
      type =
          LIBRARY_TYPE.getFlavorAndValue(ImmutableSet.copyOf(params.getBuildTarget().getFlavors()));
      platform =
          cxxPlatforms.getFlavorAndValue(ImmutableSet.copyOf(params.getBuildTarget().getFlavors()));
    } catch (FlavorDomainException e) {
      throw new HumanReadableException("%s: %s", params.getBuildTarget(), e.getMessage());
    }

    // If we *are* building a specific type of this lib, call into the type specific
    // rule builder methods.  Currently, we only support building a shared lib from the
    // pre-existing static lib, which we do here.
    if (type.isPresent()) {
      Preconditions.checkState(type.get().getValue() == Type.EXTENSION);
      Preconditions.checkState(platform.isPresent());
      return createExtensionBuildRule(
          targetGraph, params, ruleResolver, platform.get().getValue(), args);
    }

    // Otherwise, we return the generic placeholder of this library, that dependents can use
    // get the real build rules via querying the action graph.
    SourcePathResolver pathResolver = new SourcePathResolver(ruleResolver);
    Path baseModule = PythonUtil.getBasePath(params.getBuildTarget(), args.baseModule);
    return new CxxPythonExtension(
        params,
        ruleResolver,
        pathResolver,
        baseModule.resolve(getExtensionName(params.getBuildTarget())));
  }