何时在导入中使用 crate:: vs proj_name::？

Question

I understand that one can import functionality within the project in 3 different ways:

1. Using project name

use proj_name::mod1::mod2::fn_name;

2. Using crate

use crate::mod1::mod2::fn_name;

3. Using a relative path

use mod1::mod2::fn_name;

What I'm confused about is that it seems that sometimes the compiler wants me to use (1) and sometimes (2) / (3). I can't figure out the ruleset for which should be used when. Can someone help?

Answer 1

Only the crate itself is allowed to refer to itself as The crate . Every single dependant uses its name prefix. That includes binary targets within the same crate. They might be coupled together in the project, but they are NOT part of library crate.

Binary targets in fact are all separate crates for all purposes of the build system that work the same as libraries that have lib.rs at their root and writing crate in them refers to root of that specific binary target.

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Demo:

#[derive(Debug)]
pub struct A;

mod b {
    #[derive(Debug)]
    pub struct B(pub crate::A);
}

fn main() {
    println!("{:?}", b::B(A));
}

Playground