cfg-feature

一个进程宏，用于确保一次只启用一组特性中的一个。

• 问题
• 解决方案
• 许可证
  • 贡献

通常，特性应该是累加的。然而，有时这不可能或不是所希望的。

对于此类情况，如果特性的数量很少，或者不经常改变/发展，一些冗长的 #[cfg] 属性可能足够。

问题

例如，想象一个虚构的crate，它只能与特性 feat1 或 feat2 之一一起构建。

可以创建一个如 build.rs 的构建脚本。

fn main() {
    #[cfg(all(feature = "feat1", feature = "feat2"))]
    compile_error!("Only one of the features can be enabled at a time");
}

现在想象，我们添加了 feat3。

我们的 build.rs 更改为

fn main() {
    #[cfg(any(
        all(feature = "feat1", any(feature = "feat2", feature = "feat3")),
        all(feature = "feat2", any(feature = "feat1", feature = "feat3")),
        all(feature = "feat3", any(feature = "feat1", feature = "feat2")),
    ))]
    compile_error!("Only one of the features can be enabled at a time");
}

添加第四个 feat4 会变成 12 种组合！

fn main() {
    #[cfg(any(
        all(feature = "feat1", any(feature = "feat2", feature = "feat3", feature = "feat4")),
        all(feature = "feat2", any(feature = "feat1", feature = "feat3", feature = "feat4")),
        all(feature = "feat3", any(feature = "feat1", feature = "feat2", feature = "feat4")),
        all(feature = "feat4", any(feature = "feat1", feature = "feat2", feature = "feat3")),
    ))]
    compile_error!("Only one of the features can be enabled at a time");
}

这很快就变得难以控制。

解决方案

从两个特性开始，可以使用 cfg-exclusive 进程宏来简化 build.rs 脚本。

cfg_exclusive::cfg_exclusive! {
    validate_feats,
    ["feat1", "feat2"],
    "Only one of the features can be enabled at a time"
}

fn main() {
    validate_feats();
}

如果特性变为三个

-    ["feat1", "feat2"],
+    ["feat1", "feat2", "feat3"],

或者四个

-    ["feat1", "feat2", "feat3"],
+    ["feat1", "feat2", "feat3", "feat4"],

许可证

此crate可以按照您选择的许可证之一进行许可

• Apache许可证，版本2.0
• MIT许可证

任选其一。

贡献

除非您明确说明，否则您有意提交给工作以供包括在内的任何贡献，根据Apache-2.0许可证定义，应作为上述双许可证提供，不附加任何额外条款或条件。