Guessing Game rand example doesn't compile

[zvory]

Versions

Book:

• https://doc.rust-lang.org/book/ch02-00-guessing-game-tutorial.html
• https://doc.rust-lang.org/nightly/book/ch02-00-guessing-game-tutorial.html

rustc: rustc 1.31.0 (abe02cefd 2018-12-04)

cargo: cargo 1.31.0 (339d9f9c8 2018-11-16)

What is the issue:

When going through the Guessing Game tutorial:

Listing 2-3 contains some code that should use rand:

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

fn main() {
    println!("Guess the number!");

    let secret_number = rand::thread_rng().gen_range(1, 101);

    println!("The secret number is: {}", secret_number);

    println!("Please input your guess.");

    let mut guess = String::new();

    io::stdin().read_line(&mut guess)
        .expect("Failed to read line");

    println!("You guessed: {}", guess);
}

This code does not compile, with the following errors:

azvorygi@bit-shifter:~/Code/rst$ cargo run
   Compiling rst v0.1.0 (/users/azvorygi/Code/rst)                                                                                                                                            
error[E0432]: unresolved import `rand`                                                                                                                                                        
 --> src/main.rs:2:5                                                                                                                                                                          
  |                                                                                                                                                                                           
2 | use rand::Rng;                                                                                                                                                                            
  |     ^^^^ maybe a missing `extern crate rand;`?                                                                                                                                            
                                                                                                                                                                                              
warning: unused import: `rand::Rng`                                                                                                                                                           
 --> src/main.rs:2:5                                                                                                                                                                          
  |                                                                                                                                                                                           
2 | use rand::Rng;                                                                                                                                                                            
  |     ^^^^^^^^^                                                                                                                                                                             
  |                                                                                                                                                                                           
  = note: #[warn(unused_imports)] on by default                                                                                                                                               
                                                                                                                                                                                              
error[E0599]: no method named `gen_range` found for type `rand::ThreadRng` in the current scope                                                                                               
 --> src/main.rs:7:44                                                                                                                                                                         
  |                                                                                                                                                                                           
7 |     let secret_number = rand::thread_rng().gen_range(1, 101);                                                                                                                             
  |                                            ^^^^^^^^^                                                                                                                                      
  |                                                                                                                                                                                           
  = help: items from traits can only be used if the trait is in scope                                                                                                                         
help: the following trait is implemented but not in scope, perhaps add a `use` for it:                                                                                                        
  |                                                                                                                                                                                           
1 | use rand::Rng;                                                                                                                                                                            
  |                                                                                                                                                                                           
                                                                                                                                                                                              
error: aborting due to 2 previous errors                                                                                                                                                      
                                                                                                                                                                                              
Some errors occurred: E0432, E0599.                                                                                                                                                           
For more information about an error, try `rustc --explain E0432`.                                                                                                                             
error: Could not compile `rst`.                                                                                                                                                               

This is of course after adding the following to my Cargo.toml:

[dependencies]
rand = "0.3.14"

Here's a github repo with my repo when it is breaking: https://github.com/zvory/guessing-game-breaking

[carols10cents]

Hi, your Cargo.toml also needs to have edition="2018" in it. If you ran cargo new with Rust 1.31.0, your Cargo.toml should have this in it automatically. If you created the project with a version of Rust earlier than 1.31, you need to opt in to the edition by adding edition="2018" to your Cargo.toml manually.

When I build this code with edition="2018" in Cargo.toml, it completes successfully.

I see that the Cargo.toml shown at the beginning of Chapter 2 doesn't have the edition metadata, so perhaps that caused you to remove it? That's the only bug I can see here; I'm going to fix that.

Please let me know if anything else regarding the edition key was confusing.

[carols10cents]

BTW thank you for providing a repo, that made it very easy to check what the problem was!

[zvory]

😊 the "thanks for providing a repo" thing was very validating and nice, thanks! also thanks for taking care of this issue, <3 !

[adamkpickering]

I found a related inconsistency. Right below the example code that @zvory quoted (listing 2-3), the first paragraph says:

First, we add a line that lets Rust know we’ll be using the rand crate as an external dependency. This also does the equivalent of calling use rand, so now we can call anything in the rand crate by placing rand:: before it.

It appears that this is referring to a extern crate rand; line at the top of the file. However, this line is not in the example code!

As a sidenote, it would make sense to add a note in section 1.1 that explains that the book is assuming that the reader is using 1.31.0 or later. In my case, I did not use the rustup tool, and instead used sudo apt install rustc, which on Ubuntu 16.04 installed 1.30.0.

Thanks for your work on the book!

[carols10cents]

I found a related inconsistency. Right below the example code that @zvory quoted (listing 2-3), the first paragraph says:

First, we add a line that lets Rust know we’ll be using the rand crate as an external dependency. This also does the equivalent of calling use rand, so now we can call anything in the rand crate by placing rand:: before it.

It appears that this is referring to a extern crate rand; line at the top of the file. However, this line is not in the example code!

Yep! That has been fixed in master, but we haven't pulled the fix into rust-lang/rust to update doc.rust-lang.org/book yet. It'll be there soon!

As a sidenote, it would make sense to add a note in section 1.1 that explains that the book is assuming that the reader is using 1.31.0 or later. In my case, I did not use the rustup tool, and instead used sudo apt install rustc, which on Ubuntu 16.04 installed 1.30.0.

Yup! We plan to add that; we're just deciding exactly where it should go in both online and print. It's tracked as an item in the "Smaller changes" section of this issue.

Thanks for your work on the book!

You're welcome, thank you for your comments! ❤️

[apghero]

Hi @carols10cents -- just wanted to bump this, as I came here to open an issue for the extern crate rand stuff adam mentioned. There's a legitimate reason that it hasn't been pushed to the web yet, but it's been more than 2 months since it was fixed, and the old text is still confusing to readers.

[steveklabnik]

There's nothing to do; it will make its way to stable like everything else. It can take up to 12 weeks, and since I think we may have missed one update, 18 weeks, for things to percolate up. That's just the way things work, unfortunately.

[apghero]

Thanks, @steveklabnik, for the explanation.

That's just the way things work, unfortunately.

I can think of legitimate reasons that it works this way (I'm new here, btw), but if it's "unfortunate" that it works this way, is it worth reconsidering? Perhaps this isn't the change to die on a hill for, though...

[steveklabnik]

Well, there's pros and cons, like everything else. The pros far outweigh the cons. This might make for a good thread on users.rust-lang.org though!