Rust: the usage of "self" when bringing paths into scope

user_3741571 0
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mod front_of_house {
    pub mod hosting {
        pub fn add_to_waitlist() {}
    }
}

use self::front_of_house::hosting;

pub fn eat_at_restaurant() {
    hosting::add_to_waitlist();
    hosting::add_to_waitlist();
    hosting::add_to_waitlist();
}

The code is from Rust Book. In this case, if using the relative path, why bother to use self? I find that use front_of_house::hosting works well here.

So, any necessary reason to introduce self here?

rust
1 Answers

The outcome of use front_of_house::hosting depends on the context. If the crate root contains extern crate front_of_house or front_of_house is a dependency in Cargo.toml, this will refer to hosting in that crate. In your case, however, there is the local module front_of_house, which takes priority over external crates. Conveniently, it's located just before the code that uses it, so it's obvious that it's the actual module being used. In larger files, however, there is a chance that the source of a module is ambiguous, i.e. if there is a dependency named the same as a local module. In that case, the extra self adds clarity to the code, helping you differentiate between dependencies and local modules. If, in that case, you wanted to use the dependency, you'd use ::front_of_house::hosting, which unconditionally refers to the dependency and fails if there isn't one named like that.

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