## Appendix D - Useful Development Tools

In this appendix, we talk about some useful development tools that the Rust project provides. We’ll look at automatic formatting, quick ways to apply warning fixes, a linter, and integrating with IDEs.

### Automatic Formatting with `rustfmt`

The `rustfmt` tool reformats your code according to the community code style. Many collaborative projects use `rustfmt` to prevent arguments about which style to use when writing Rust: everyone formats their code using the tool.

To install `rustfmt`, enter the following:

    $ rustup component add rustfmt

This command gives you `rustfmt` and `cargo-fmt`, similar to how Rust gives you both `rustc` and `cargo`. To format any Cargo project, enter the following:

    $ cargo fmt

Running this command reformats all the Rust code in the current crate. This should only change the code style, not the code semantics. For more information on `rustfmt`, see its documentation.

### Fix Your Code with `rustfix`

The rustfix tool is included with Rust installations and can automatically fix compiler warnings that have a clear way to correct the problem that’s likely what you want. It’s likely you’ve seen compiler warnings before. For example, consider this code:

Filename: src/main.rs

    fn do_something() {}

    fn main() {
        for i in 0..100 {
            do_something();
        }
    }

Here, we’re calling the `do_something` function 100 times, but we never use the variable `i` in the body of the `for` loop. Rust warns us about that:

    $ cargo build
       Compiling myprogram v0.1.0 (file:///projects/myprogram)
    warning: unused variable: `i`
     --> src/main.rs:4:9
      |
    4 |     for i in 0..100 {
      |         ^ help: consider using `_i` instead
      |
      = note: #[warn(unused_variables)] on by default

        Finished dev [unoptimized + debuginfo] target(s) in 0.50s

The warning suggests that we use `_i` as a name instead: the underscore indicates that we intend for this variable to be unused. We can automatically apply that suggestion using the `rustfix` tool by running the command `cargo fix`:

    $ cargo fix
        Checking myprogram v0.1.0 (file:///projects/myprogram)
          Fixing src/main.rs (1 fix)
        Finished dev [unoptimized + debuginfo] target(s) in 0.59s

When we look at src/main.rs again, we’ll see that `cargo fix` has changed the code:

Filename: src/main.rs

    fn do_something() {}

    fn main() {
        for _i in 0..100 {
            do_something();
        }
    }

The `for` loop variable is now named `_i`, and the warning no longer appears.

You can also use the `cargo fix` command to transition your code between different Rust editions. Editions are covered in Appendix E.

### More Lints with Clippy

The Clippy tool is a collection of lints to analyze your code so you can catch common mistakes and improve your Rust code.

To install Clippy, enter the following:

    $ rustup component add clippy

To run Clippy’s lints on any Cargo project, enter the following:

    $ cargo clippy

For example, say you write a program that uses an approximation of a mathematical constant, such as pi, as this program does:

Filename: src/main.rs

    fn main() {
        let x = 3.1415;
        let r = 8.0;
        println!("the area of the circle is {}", x * r * r);
    }

Running `cargo clippy` on this project results in this error:

    error: approximate value of `f{32, 64}::consts::PI` found
     --> src/main.rs:2:13
      |
    2 |     let x = 3.1415;
      |             ^^^^^^
      |
      = note: `#[deny(clippy::approx_constant)]` on by default
      = help: consider using the constant directly
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#approx_constant

This error lets you know that Rust already has a more precise `PI` constant defined, and that your program would be more correct if you used the constant instead. You would then change your code to use the `PI` constant. The following code doesn’t result in any errors or warnings from Clippy:

Filename: src/main.rs

    fn main() {
        let x = std::f64::consts::PI;
        let r = 8.0;
        println!("the area of the circle is {}", x * r * r);
    }

For more information on Clippy, see its documentation.

### IDE Integration Using `rust-analyzer`

To help IDE integration, the Rust community recommends using `rust-analyzer`. This tool is a set of compiler-centric utilities that speaks the Language Server Protocol, which is a specification for IDEs and programming languages to communicate with each other. Different clients can use `rust-analyzer`, such as the Rust analyzer plug-in for Visual Studio Code.

Visit the `rust-analyzer` project’s home page for installation instructions, then install the language server support in your particular IDE. Your IDE will gain abilities such as autocompletion, jump to definition, and inline errors.