book/src/usage.md

# Usage

This chapter describes how to use Clippy to get the most out of it. Clippy can
be used as a `cargo` subcommand or, like `rustc`, directly with the
`clippy-driver` binary.

> _Note:_ This chapter assumes that you have Clippy installed already. If you're
> not sure, take a look at the [Installation] chapter.

## Cargo subcommand

The easiest and most common way to run Clippy is through `cargo`. To do that,
just run

```bash
cargo clippy
```

### Lint configuration

The above command will run the default set of lints, which are included in the
lint group `clippy::all`. You might want to use even more lints, or you may not
agree with every Clippy lint, and for that there are ways to configure lint
levels.

> _Note:_ Clippy is meant to be used with a generous sprinkling of
> `#[allow(..)]`s through your code. So if you disagree with a lint, don't feel
> bad disabling them for parts of your code or the whole project.

#### Command line

You can configure lint levels on the command line by adding
`-A/W/D clippy::lint_name` like this:

```bash
cargo clippy -- -Aclippy::style -Wclippy::box_default -Dclippy::perf
```

For [CI] all warnings can be elevated to errors which will in turn fail
the build and cause Clippy to exit with a code other than `0`.

```
cargo clippy -- -Dwarnings
```

> _Note:_ Adding `-D warnings` will cause your build to fail if **any** warnings
> are found in your code. That includes warnings found by rustc (e.g.
> `dead_code`, etc.).

For more information on configuring lint levels, see the [rustc documentation].

[rustc documentation]: https://doc.rust-lang.org/rustc/lints/levels.html#configuring-warning-levels

#### Even more lints

Clippy has lint groups which are allow-by-default. This means, that you will
have to enable the lints in those groups manually.

For a full list of all lints with their description and examples, please refer
to [Clippy's lint list]. The two most important allow-by-default groups are
described below:

[Clippy's lint list]: https://rust-lang.github.io/rust-clippy/master/index.html

##### `clippy::pedantic`

The first group is the `pedantic` group. This group contains really opinionated
lints, that may have some intentional false positives in order to prevent false
negatives. So while this group is ready to be used in production, you can expect
to sprinkle multiple `#[allow(..)]`s in your code. If you find any false
positives, you're still welcome to report them to us for future improvements.

> FYI: Clippy uses the whole group to lint itself.

##### `clippy::restriction`

The second group is the `restriction` group. This group contains lints that
"restrict" the language in some way. For example the `clippy::unwrap` lint from
this group won't allow you to use `.unwrap()` in your code. You may want to look
through the lints in this group and enable the ones that fit your need.

> _Note:_ You shouldn't enable the whole lint group, but cherry-pick lints from
> this group. Some lints in this group will even contradict other Clippy lints!

#### Too many lints

The most opinionated warn-by-default group of Clippy is the `clippy::style`
group. Some people prefer to disable this group completely and then cherry-pick
some lints they like from this group. The same is of course possible with every
other of Clippy's lint groups.

> _Note:_ We try to keep the warn-by-default groups free from false positives
> (FP). If you find that a lint wrongly triggers, please report it in an issue
> (if there isn't an issue for that FP already)

#### Source Code

You can configure lint levels in source code the same way you can configure
`rustc` lints:

```rust,ignore
#![allow(clippy::style)]

#[warn(clippy::box_default)]
fn main() {
    let _ = Box::<String>::new(Default::default());
    // ^ warning: `Box::new(_)` of default value
}
```

### Automatically applying Clippy suggestions

Clippy can automatically apply some lint suggestions, just like the compiler. Note that `--fix` implies
`--all-targets`, so it can fix as much code as it can.

```terminal
cargo clippy --fix
```

### Workspaces

All the usual workspace options should work with Clippy. For example the
following command will run Clippy on the `example` crate in your workspace:

```terminal
cargo clippy -p example
```

As with `cargo check`, this includes dependencies that are members of the
workspace, like path dependencies. If you want to run Clippy **only** on the
given crate, use the `--no-deps` option like this:

```terminal
cargo clippy -p example -- --no-deps
```

## Using Clippy without `cargo`: `clippy-driver`

Clippy can also be used in projects that do not use cargo. To do so, run
`clippy-driver` with the same arguments you use for `rustc`. For example:

```terminal
clippy-driver --edition 2018 -Cpanic=abort foo.rs
```

> _Note:_ `clippy-driver` is designed for running Clippy and should not be used
> as a general replacement for `rustc`. `clippy-driver` may produce artifacts
> that are not optimized as expected, for example.

[Installation]: installation.md
[CI]: continuous_integration/index.md