rust - 为什么我不能通过 `std::assert` 导入 `use` 而它适用于来自 std 的其他宏？

Question

对于 Rust 2018，此代码有效(Playground):

use std::panic;
use std::format;
use std::assert_eq;

但是这个:

use std::assert;

导致此错误:

error[E0432]: unresolved import `std::assert`
 --> src/lib.rs:4:5
  |
4 | use std::assert;
  |     ^^^^^^^^^^^ no `assert` in the root

我读了the edition guide about this topic它说 use 应该与 macro_rules! 宏和过程宏一起使用。因此，我很困惑。

Answer 1

use should work with macro_rules! macros and procedural macros

除了 assert 是 neither of those :

/// Built-in macros to the compiler itself.
///
/// These macros do not have any corresponding definition with a `macro_rules!`
/// macro, but are documented here. Their implementations can be found hardcoded
/// into libsyntax itself.

这是一个compiler built-in :

#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_doc_only_macro]
macro_rules! assert {
    ($cond:expr) => ({ /* compiler built-in */ });
    ($cond:expr,) => ({ /* compiler built-in */ });
    ($cond:expr, $($arg:tt)+) => ({ /* compiler built-in */ });
}

其他伪宏包括:

• 编译错误
• format_args
• 环境
• option_env
• concat_idents
• 连接
• 行
• 列
• 文件
• 字符串化
• include_str
• include_bytes
• 模块路径
• 配置
• 包含

assert 的实际定义隐藏在 libsyntax_ext/assert.rs 中

---

Stabilize uniform paths on Rust 2018 (#56417)确实顺便提到了这些:

Built-in macros, for example use env. Currently an error due to some (fixable) implementation details of built-in macros. No known issues to resolve before stabilization (after the error is removed).