Macro std::is_mips_feature_detected

macro_rules! is_mips_feature_detected {
    ($t:tt) => { ... };
}

🔬 This is a nightly-only experimental API. (stdsimd #27731)

如果在 MIPS 目标以外的其他地方使用 is_mips_feature_detected，则阻止编译。