rust_module_confusion.md

From Mastadon post.

Working my way through the #rust book. Little confused by something in the modules section. Can't quite fit the whole thing in one toot, so here's a gist: <here>

Short version, useing nested modules. Does this allow access to ancestor modules or not?

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In the modules section, the author refers back to the "guessing game" section and shows this:

use rand::Rng;

fn main() {
    let secret_number = rand::thread_rng().gen_range(1..=100);
}

From this, I figure "oh ok. We've included Rng from the rand module. Then we use the thread_rng function which is in the rand module. I guess because we've useed rand::Rng we now have access to public things in the rand module?

But then, testing this out, I do this:

// src/main.rs
use module_test::foo::bar;

fn main() {
    println!("{}", foo::get_3());
    println!("{}", bar::get_5());
}

// src/lib.rs
pub mod foo {
    pub fn get_3() -> u32 { 3 }

    pub mod bar {
        pub fn get_5() -> u32 { 5 }
    }
}

This fails to compile with error: use of undeclared crate or module foo

So what gives? If I use foo::bar::baz, does that allow me to do things off of foo itself or not?

Seems like not. But if not, then how does the use rand::Rng allow us to then access rand::other_non_Rng_things?

fn main() {
    println!("{:?}", rand::thread_rng());
}

as an addition to that, this works perfectly fine with no use statements because we aren't relying on any of the implementations of the Rng trait. in short, if you want anything from a trait, you have to explicitly opt-in by use'ing the trait.