Rust unused import: `rand::Rng` and no method named `gen_range` found for struct `ThreadRng`

I am new to rust. I am trying to learn how modular system works in rust and I tried to import rand and io at the top line of the file like I would have in nodejs or python. But when I tried to import the rand and io on top, I was getting the error as:
unused import: rand::Rng and no method named gen_range found for struct ThreadRng errors.

Help

Can anyone explain me, how can I work the use keyword as above like in the bugy code and what is the issue really about?

Guessed it

As I have experience in JS and python, I guessed it was related about something with the scoping of variables. But the nature of scoping is completely different. So can anyone explain to me what was the reason of the error.

Here are two codes.

Buggy Code:

use rand::Rng;
use std::io;

pub mod guess_game{
    
    pub fn start(){
        
        let random_generated_number = rand::thread_rng().gen_range(1, 10);  
        // Error was here      ---------        ^ no method named `gen_range` found for struct `ThreadRng` in the current scope
items from traits can only be used if the trait is in scope

        let mut user_input = String::new();
        
        println!("Enter a number: ");
    
        io::stdin()
//      ^ a builtin type with a similar name exists: `i8`rustc E0433
            .read_line(&mut user_input)
            .expect("Error ayo hai ta");
    
        let user_input_number:i32 = user_input.trim().parse().expect("Invalid integer provided");
    
        println!("I see you entered {}", user_input);
    
        if user_input_number == random_generated_number {
            println!("RIGHT, You guessed it, {}", random_generated_number);
        }else{
            println!("WRONG!, You will have to guess {}", random_generated_number);
        };
    }
}

Working code

pub mod guess_game{

    use rand::Rng;
    use std::io;

    pub fn start(){
        
        let random_generated_number = rand::thread_rng().gen_range(1, 10);  
    
        let mut user_input = String::new();
        
        println!("Enter a number: ");
    
        io::stdin()
            .read_line(&mut user_input)
            .expect("Error ayo hai ta");
    
        let user_input_number:i32 = user_input.trim().parse().expect("Invalid integer provided");
    
        println!("I see you entered {}", user_input);
    
        if user_input_number == random_generated_number {
            println!("RIGHT, You guessed it, {}", random_generated_number);
        }else{
            println!("WRONG!, You will have to guess {}", random_generated_number);
        };
    }
}

> I am new to rust. I am trying to learn how modular system works in rust and I tried to import rand and io at the top line of the file like I would have in nodejs or python [...] Can anyone explain me, how can I work the use keyword as above like in the bugy code and what is the issue really about?

Modules do not have lexical visibility.

While Rust lets you create nested modules without creating new files the semantics don't change. That is, by design

mod foo {
    // content here
}

and

mod foo;

// foo.rs
// content here

have exactly the same behaviour, intentionally: it makes the implementation simpler, and lets you break up files by moving contents from nested modules into external files, and nothing more.

That means exactly as in Python, for one module to see the contents of an other, you need some sort of cross-module import.

So your original code is exactly the same as:

// mod.rs 

use rand::Rng;
use std::io;

pub mod guess_game;

// guess_game.rs
pub fn start(){
    ...
}

which in Python translates to:

# __init__.py
import random
import io

from . import guess_game

# guess_game.py
def start():
    # uses random and io

Do you expect that to work? No? There's your answer.