如何在Rust中导入和引用枚举类型？

Question

How do you import and reference enum types from the Rust std lib?

I'm trying to use the Ordering enum from the std::sync::atomics module. My attempts so far have all ended in failure:

use std::sync::atomics::AtomicBool;
use std::sync::atomics::Ordering;

// error unresolved import: there is no `Relaxed` in `std::sync::atomics::Ordering`
// use std::sync::atomics::Ordering::Relaxed;  

fn main() {
    let mut ab = AtomicBool::new(false);
    let val1 = ab.load(Ordering::Relaxed); // error: unresolved import:
                                           // there is no `Relaxed` in `std::sync::atomics::Ordering`
    println!("{:?}", val1);

    ab.store(true, Ordering.Relaxed);      // error: unresolved name `Ordering`
    let val2 = ab.load(Ordering(Relaxed)); // error: unresolved name `Relaxed`
    println!("{:?}", val2);    
}

I'm currently using Rust v. 0.9.

Answer 1

As of Rust 1.0, enum variants are scoped inside of their enum type. They can be directly use d:

pub use self::Foo::{A, B};

pub enum Foo {
    A,
    B,
}

fn main() {
    let a = A;
}

or you can use the type-qualified name:

pub enum Foo {
    A,
    B,
}

fn main() {
    let a = Foo::A;
}

Answer 2

Editor's note: This answer predates Rust 1.0 and is not applicable for Rust 1.0.

Enum variants are not scoped inside the enum; they are thus std::sync::atomics::Relaxed , &c.

Scoped enum variants is the subject of issue 10090 .