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Match guessing game output to newest language version #37483

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Nov 9, 2016
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Match guessing game output to newest language version #37483

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66 changes: 43 additions & 23 deletions src/doc/book/guessing-game.md
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,7 @@ has a command that does that for us. Let’s give it a shot:
```bash
$ cd ~/projects
$ cargo new guessing_game --bin
Created binary (application) `guessing_game` project
$ cd guessing_game
```

Expand Down Expand Up @@ -51,6 +52,7 @@ Let’s try compiling what Cargo gave us:
```{bash}
$ cargo build
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 0.53 secs
```

Excellent! Open up your `src/main.rs` again. We’ll be writing all of
Expand All @@ -61,6 +63,7 @@ Remember the `run` command from last chapter? Try it out again here:
```bash
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target/debug/guessing_game`
Hello, world!
```
Expand Down Expand Up @@ -282,10 +285,13 @@ we’ll get a warning:
```bash
$ cargo build
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
src/main.rs:10:5: 10:39 warning: unused result which must be used,
#[warn(unused_must_use)] on by default
src/main.rs:10 io::stdin().read_line(&mut guess);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
warning: unused result which must be used, #[warn(unused_must_use)] on by default
--> src/main.rs:10:5
|
10 | io::stdin().read_line(&mut guess);
| ^

Finished debug [unoptimized + debuginfo] target(s) in 0.42 secs
```

Rust warns us that we haven’t used the `Result` value. This warning comes from
Expand Down Expand Up @@ -321,6 +327,7 @@ Anyway, that’s the tour. We can run what we have with `cargo run`:
```bash
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 0.44 secs
Running `target/debug/guessing_game`
Guess the number!
Please input your guess.
Expand Down Expand Up @@ -374,11 +381,12 @@ Now, without changing any of our code, let’s build our project:
```bash
$ cargo build
Updating registry `https://github.com/rust-lang/crates.io-index`
Downloading rand v0.3.8
Downloading libc v0.1.6
Compiling libc v0.1.6
Compiling rand v0.3.8
Downloading rand v0.3.14
Downloading libc v0.2.17
Compiling libc v0.2.17
Compiling rand v0.3.14
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 5.88 secs
```

(You may see different versions, of course.)
Expand All @@ -400,22 +408,24 @@ If we run `cargo build` again, we’ll get different output:

```bash
$ cargo build
Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
```

That’s right, no output! Cargo knows that our project has been built, and that
That’s right, nothing was done! Cargo knows that our project has been built, and that
all of its dependencies are built, and so there’s no reason to do all that
stuff. With nothing to do, it simply exits. If we open up `src/main.rs` again,
make a trivial change, and then save it again, we’ll only see one line:
make a trivial change, and then save it again, we’ll only see two lines:

```bash
$ cargo build
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 0.45 secs
```

So, we told Cargo we wanted any `0.3.x` version of `rand`, and so it fetched the latest
version at the time this was written, `v0.3.8`. But what happens when next
week, version `v0.3.9` comes out, with an important bugfix? While getting
bugfixes is important, what if `0.3.9` contains a regression that breaks our
version at the time this was written, `v0.3.14`. But what happens when next
week, version `v0.3.15` comes out, with an important bugfix? While getting
bugfixes is important, what if `0.3.15` contains a regression that breaks our
code?

The answer to this problem is the `Cargo.lock` file you’ll now find in your
Expand All @@ -424,11 +434,11 @@ figures out all of the versions that fit your criteria, and then writes them
to the `Cargo.lock` file. When you build your project in the future, Cargo
will see that the `Cargo.lock` file exists, and then use that specific version
rather than do all the work of figuring out versions again. This lets you
have a repeatable build automatically. In other words, we’ll stay at `0.3.8`
have a repeatable build automatically. In other words, we’ll stay at `0.3.14`
until we explicitly upgrade, and so will anyone who we share our code with,
thanks to the lock file.

What about when we _do_ want to use `v0.3.9`? Cargo has another command,
What about when we _do_ want to use `v0.3.15`? Cargo has another command,
`update`, which says ‘ignore the lock, figure out all the latest versions that
fit what we’ve specified. If that works, write those versions out to the lock
file’. But, by default, Cargo will only look for versions larger than `0.3.0`
Expand Down Expand Up @@ -511,13 +521,15 @@ Try running our new program a few times:
```bash
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 0.55 secs
Running `target/debug/guessing_game`
Guess the number!
The secret number is: 7
Please input your guess.
4
You guessed: 4
$ cargo run
Finished debug [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target/debug/guessing_game`
Guess the number!
The secret number is: 83
Expand Down Expand Up @@ -619,15 +631,20 @@ I did mention that this won’t quite compile yet, though. Let’s try it:
```bash
$ cargo build
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
src/main.rs:28:21: 28:35 error: mismatched types:
expected `&collections::string::String`,
found `&_`
(expected struct `collections::string::String`,
found integral variable) [E0308]
src/main.rs:28 match guess.cmp(&secret_number) {
^~~~~~~~~~~~~~
error[E0308]: mismatched types
--> src/main.rs:23:21
|
23 | match guess.cmp(&secret_number) {
| ^^^^^^^^^^^^^^ expected struct `std::string::String`, found integral variable
|
= note: expected type `&std::string::String`
= note: found type `&{integer}`

error: aborting due to previous error
Could not compile `guessing_game`.

error: Could not compile `guessing_game`.

To learn more, run the command again with --verbose.
```

Whew! This is a big error. The core of it is that we have ‘mismatched types’.
Expand Down Expand Up @@ -723,6 +740,7 @@ Let’s try our program out!
```bash
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 0.57 secs
Running `target/guessing_game`
Guess the number!
The secret number is: 58
Expand Down Expand Up @@ -786,6 +804,7 @@ and quit. Observe:
```bash
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 0.58 secs
Running `target/guessing_game`
Guess the number!
The secret number is: 59
Expand Down Expand Up @@ -920,6 +939,7 @@ Now we should be good! Let’s try:
```bash
$ cargo run
Compiling guessing_game v0.1.0 (file:///home/you/projects/guessing_game)
Finished debug [unoptimized + debuginfo] target(s) in 0.57 secs
Running `target/guessing_game`
Guess the number!
The secret number is: 61
Expand Down