ch2: unused result warning differs

[nathany]

When running cargo build with rust 1.21.0 (rustc 1.18.0), the warning appears as:

warning: unused result which must be used

rather than

unused `std::result::Result` which must be used

[nathany]

I'm seeing other differences in compiler errors later on. Not sure if I'm using an old rustc release compared to the book or if the book is out-of-date.

variables/main.rs

re-assignment of immutable variable `x`

rather than

cannot assign twice to immutable variable `x`

[steveklabnik]

The book is out of date; for your second example, that error actually changed due to the book!

I believe @carols10cents and I were going to go over all of the error messages at the very end and make sure they're up to date, but I also don't see the harm in merging this right now. Carol, whatcha think?

[carols10cents]

rust 1.21.0 (rustc 1.18.0)

I'm confused about this part? What do you mean by this? The version of Rust that you're using should be exactly what rustc --version prints out....

[carols10cents]

Aside from that, the book's error messages are out of date and I'm slowly updating them to be the ones generated by rustc 1.21.0. I thought I did chapter 2 recently, checking...

[carols10cents]

Ok yeah I updated this recently with rustc 1.21.0.

When I run this code:

use std::io;

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

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

    let mut guess = String::new();

    io::stdin().read_line(&mut guess);

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

I get this output:

$ cargo run
   Compiling guessing_game v0.1.0 (file:///Users/carolnichols/rust/guessing_game)
warning: unused `std::result::Result` which must be used
  --> src/main.rs:10:5
   |
10 |     io::stdin().read_line(&mut guess);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_must_use)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.36 secs
     Running `target/debug/guessing_game`
Guess the number!
Please input your guess.

and that's what we're going for in the book.

We're going to have a notice in Chapter 1 (we're revising that chapter last) that we've used Rust 1.21.0 for all the examples in the book and that the exact output you see might be slightly different, but everything that compiles in the book should continue to compile with future versions (and since we've been working on this book for a while, with past versions even though that's not a guarantee of Rust's per se).

Please let us know if you need any help getting the right Rust version, I'm still confused about what you mean by "rust 1.21.0 (rustc 1.18.0)" but if there's an upgrading problem we can help fix we'd be happy to help!

[nathany]

thanks @carols10cents and @steveklabnik

I'm confused by the version mismatch too. If it's not an expected situation, then there's probably something wrong with the setup on my Mac.

❯ brew list | grep rust 
rust

❯ brew info rust
rust: stable 1.21.0 (bottled), HEAD

❯ which rustc
/Users/nathany/.cargo/bin/rustc
                                                                                
❯ rustc --version
rustc 1.18.0 (03fc9d622 2017-06-06)

Maybe I should try uninstalling Rust entirely and reinstalling from scratch.

[carols10cents]

Yeah I highly recommend using rustup rather than homebrew for managing Rust versions. I know, I know, it's another tool, but rustup Just Works for me and manages a lot of other aspects of your Rust dev setup.

[steveklabnik]

It sounds to me like you have both brew and rustup trying to manage a rust, and they're getting crossed.

[nathany]

@steveklabnik Yup, that appears to be the case, with rustup taking precedence over Homebrew, so I was using an older version than I thought.

@carols10cents Thanks for pointing out the issue and encouraging me to switch to just rustup. Appreciate it! Also thanks for caring about the small details, like the error messages shown in the book.