自定义条件

有部分条件如 target_os 是由 rustc 隐式地提供的,但是自定义条件必须使用 --cfg 标记来传给 rustc

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

试试不使用自定义的 cfg 标记会发生什么:

$ rustc custom.rs && ./custom No such file or directory (os error 2)

使用自定义的 cfg 标记:

$ rustc --cfg some_condition custom.rs && ./custom condition met!