为什么Option的Some和None变体不需要合格？

Question

According to the docs for Option , Option is an enum with variants Some<T> and None .

Why is it possible to refer to Some and None without qualifying them?

For example, this works fine:

let x = Option::Some(5);
match x {
    Some(a) => println!("Got {}", a),
    None => println!("Got None"),
}

But this fails to compile:

enum Foo<T> {
    Bar(T),
    Baz,
}
let x = Foo::Bar(5);
match x {
    Bar(a) => println!("Got {}", a),
    Baz => println!("Got Baz"),
}

The error from the compiler is unresolved enum variant, struct or const `Bar`

Answer 1

The Rust prelude , which is automatically inserted into every source file, contains this line:

pub use option::Option::{self, Some, None};

Which brings Option and both its variants in scope.