您需要通过将 #![feature(collections)] 放在 crate 的根源文件顶部来明确选择加入.使用夜间编译器仅允许您使用不稳定的功能，它不会自动启用它们.

You need to explicitly opt-in by placing #![feature(collections)] at the top of your crate's root source file. Using a nightly compiler merely permits you to use unstable features, it doesn't automatically enable them.

另见这个相关的SO问题.