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flip1995 committed Aug 16, 2023
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The first step in the journey of a new lint is the definition
and registration of the lint in Clippy's codebase.
We can use the Clippy dev tools to handle this step since setting up the
We can use the Clippy dev tools to handle this step since setting up the
lint involves some boilerplate code.

#### Lint types

A lint type is the category of items and expressions in which your lint focuses on.

As of the writing of this documentation update, there are 12 groups (a.k.a. _types_)
of lints besides the numerous standalone lints living under `clippy_lints/src/`:
As of the writing of this documentation update, there are 12 _types_ of lints
besides the numerous standalone lints living under `clippy_lints/src/`:

- `cargo`
- `casts`
Expand All @@ -25,47 +25,52 @@ of lints besides the numerous standalone lints living under `clippy_lints/src/`:
- `unit_types`
- `utils / internal` (Clippy internal lints)

These types group together lints that share some common behaviors.
For instance, `functions` groups together lints
that deal with some aspects of function calls in Rust.
These types group together lints that share some common behaviors. For instance,
`functions` groups together lints that deal with some aspects of functions in
Rust, like definitions, signatures and attributes.

For more information, feel free to compare the lint files under any category
with [All Clippy lints][all_lints] or
ask one of the maintainers.
with [All Clippy lints][all_lints] or ask one of the maintainers.

## Lint name

A good lint name is important, make sure to check the [lint naming guidelines][lint_naming]. Don't worry, if
the lint name doesn't fit, a Clippy team member will alert you in the PR process.
A good lint name is important, make sure to check the [lint naming
guidelines][lint_naming]. Don't worry, if the lint name doesn't fit, a Clippy
team member will alert you in the PR process.

---

We'll name our example lint that detects functions named "foo" `foo_functions`. Check the
[lint naming guidelines][lint_naming] to see why this name makes sense.
We'll name our example lint that detects functions named "foo" `foo_functions`.
Check the [lint naming guidelines][lint_naming] to see why this name makes
sense.

## Add and Register the Lint

Now that a name is chosen, we shall register `foo_functions` as a lint to the codebase.
There are two ways to register a lint.
Now that a name is chosen, we shall register `foo_functions` as a lint to the
codebase. There are two ways to register a lint.

### Standalone

If you believe that this new lint is a standalone lint (that doesn't belong to any specific [type](#lint-types) like `functions` or `loops`), you can run the following
command in your Clippy project:
If you believe that this new lint is a standalone lint (that doesn't belong to
any specific [type](#lint-types) like `functions` or `loops`), you can run the
following command in your Clippy project:

```sh
$ cargo dev new_lint --name=lint_name --pass=late --category=pedantic
```

There are two things to note here:

1. We set `--pass=late` in this command to do a late lint pass. The alternative
is an `early` lint pass. We will discuss this difference in a later chapter.
<!-- FIXME: Link that "later chapter" when lint_passes.md is merged -->
2. If not provided, the `category` of this new lint will default to `nursery`.
1. `--pass`: We set `--pass=late` in this command to do a late lint pass. The
alternative is an `early` lint pass. We will discuss this difference in a
later chapter.
<!-- FIXME: Link that "later chapter" when lint_passes.md is merged -->
2. `--category`: If not provided, the `category` of this new lint will default
to `nursery`.

The `cargo dev new_lint` command will create a new file: `clippy_lints/src/foo_functions.rs`
as well as [register the lint](#lint-registration).
The `cargo dev new_lint` command will create a new file:
`clippy_lints/src/foo_functions.rs` as well as [register the
lint](#lint-registration).

Overall, you should notice that the following files are modified or created:

Expand Down Expand Up @@ -115,7 +120,6 @@ call your lint from within the type's lint pass, found in `clippy_lints/src/{typ
A _type_ is just the name of a directory in `clippy_lints/src`, like `functions` in
the example command. Clippy groups together some lints that share common behaviors,
so if your lint falls into one, it would be best to add it to that type.
Read more about [lint types](#lint-types) below.

Overall, you should notice that the following files are modified or created:

Expand All @@ -138,24 +142,25 @@ Untracked files:

## The `define_clippy_lints` macro

After `cargo dev new_lint`, you should see a macro with the name `define_clippy_lints`. It will be in the same file
if you defined a standalone lint, and it will be in `mod.rs` if you defined a type-specific lint.
After `cargo dev new_lint`, you should see a macro with the name
`define_clippy_lints`. It will be in the same file if you defined a standalone
lint, and it will be in `mod.rs` if you defined a type-specific lint.

The macro looks something like this:

```rust
declare_clippy_lint! {
/// ### What it does
/// ### What it does
///
/// // Describe here what does the lint do.
///
/// Triggers when detects...
///
/// // Describe here what does the lint do.
///
/// Triggers when detects...
///
/// ### Why is this bad?
///
/// // Describe why this pattern would be bad
///
/// It can lead to...
///
/// // Describe why this pattern would be bad
///
/// It can lead to...
///
/// ### Example
/// ```rust
Expand All @@ -174,25 +179,26 @@ declare_clippy_lint! {

## Lint registration

If we run the `cargo dev new_lint` command for a new lint,
the lint will be automatically registered and there is nothing more to do.
If we run the `cargo dev new_lint` command for a new lint, the lint will be
automatically registered and there is nothing more to do.

However, sometimes we might want to declare a new lint by hand.
In this case, we'd use `cargo dev update_lints` command afterwards.
However, sometimes we might want to declare a new lint by hand. In this case,
we'd use `cargo dev update_lints` command afterwards.

When a lint is manually declared, we might need to register the lint pass
manually in the `register_plugins` function in `clippy_lints/src/lib.rs`:

```rust
store.register_late_pass(|| Box::new(foo_functions::FooFunctions));
store.register_late_pass(|_| Box::new(foo_functions::FooFunctions));
```

As you might have guessed, where there's something late, there is something early:
in Clippy there is a `register_early_pass` method as well.
More on early vs. late passes in a later chapter.
As you might have guessed, where there's something late, there is something
early: in Clippy there is a `register_early_pass` method as well. More on early
vs. late passes in a later chapter.
<!-- FIXME: Link that "later chapter" when lint_passes.md is merged -->

Without a call to one of `register_early_pass` or `register_late_pass`,
the lint pass in question will not be run.
Without a call to one of `register_early_pass` or `register_late_pass`, the lint
pass in question will not be run.


[all_lints]: https://rust-lang.github.io/rust-clippy/master/
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