为什么 `CMSG_SPACE` 不是 Rust 中的常量函数？

Question

根据the documentation：

pub const unsafe extern "C" fn CMSG_SPACE(length: c_uint) -> c_uint

但是，如果我编译

fn main() {
    let _ = [0u8; libc::CMSG_SPACE(1) as usize];
}

我收到以下错误：

error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
 --> src/bin/libc-const.rs:2:27
  |
2 |     let _ = [0u8; unsafe {libc::CMSG_SPACE(1) as usize}];
  |                           ^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0015`.

Answer 1

const-extern-fn feature for libc is required 激活此功能 const。 docs.rs 上记录的版本启用了此功能，这就是它（不幸地）默认显示为 const 的原因。此功能需要每晚 rustc。