# Lint levels

In `rustc`, lints are divided into four *levels*:

1. allow
2. warn
3. deny
4. forbid

Each lint has a default level (explained in the lint listing later in this
chapter), and the compiler has a default warning level. First, let's explain
what these levels mean, and then we'll talk about configuration.

## allow

These lints exist, but by default, do nothing. For example, consider this
source:

```rust
pub fn foo() {}
```

Compiling this file produces no warnings:

```bash
$ rustc lib.rs --crate-type=lib
$
```

But this code violates the `missing_docs` lint.

These lints exist mostly to be manually turned on via configuration, as we'll
talk about later in this section.

## warn

The 'warn' lint level will produce a warning if you violate the lint. For example,
this code runs afoul of the `unused_variables` lint:

```rust
pub fn foo() {
    let x = 5;
}
```

This will produce this warning:

```bash
$ rustc lib.rs --crate-type=lib
warning: unused variable: `x`
 --> lib.rs:2:9
  |
2 |     let x = 5;
  |         ^
  |
  = note: `#[warn(unused_variables)]` on by default
  = note: to avoid this warning, consider using `_x` instead
```

## deny

A 'deny' lint produces an error if you violate it. For example, this code
runs into the `exceeding_bitshifts` lint.

```rust,ignore
fn main() {
    100u8 << 10;
}
```

```bash
$ rustc main.rs
error: bitshift exceeds the type's number of bits
 --> main.rs:2:13
  |
2 |     100u8 << 10;
  |     ^^^^^^^^^^^
  |
  = note: `#[deny(exceeding_bitshifts)]` on by default
```

What's the difference between an error from a lint and a regular old error?
Lints are configurable via levels, so in a similar way to 'allow' lints,
warnings that are 'deny' by default let you allow them. Similarly, you may
wish to set up a lint that is `warn` by default to produce an error instead.
This lint level gives you that.

## forbid

'forbid' is a special lint level that's stronger than 'deny'. It's the same
as 'deny' in that a lint at this level will produce an error, but unlike the
'deny' level, the 'forbid' level can not be overridden to be anything lower
than an error.  However, lint levels may still be capped with `--cap-lints`
(see below) so `rustc --cap-lints warn` will make lints set to 'forbid' just
warn.

## Configuring warning levels

Remember our `missing_docs` example from the 'allow' lint level?

```bash
$ cat lib.rs
pub fn foo() {}
$ rustc lib.rs --crate-type=lib
$
```

We can configure this lint to operate at a higher level, both with
compiler flags, as well as with an attribute in the source code.

You can also "cap" lints so that the compiler can choose to ignore
certain lint levels. We'll talk about that last.

### Via compiler flag

The `-A`, `-W`, `-D`, and `-F` flags let you turn one or more lints
into allowed, warning, deny, or forbid levels, like this:

```bash
$ rustc lib.rs --crate-type=lib -W missing-docs
warning: missing documentation for crate
 --> lib.rs:1:1
  |
1 | pub fn foo() {}
  | ^^^^^^^^^^^^
  |
  = note: requested on the command line with `-W missing-docs`

warning: missing documentation for a function
 --> lib.rs:1:1
  |
1 | pub fn foo() {}
  | ^^^^^^^^^^^^
```

```bash
$ rustc lib.rs --crate-type=lib -D missing-docs
error: missing documentation for crate
 --> lib.rs:1:1
  |
1 | pub fn foo() {}
  | ^^^^^^^^^^^^
  |
  = note: requested on the command line with `-D missing-docs`

error: missing documentation for a function
 --> lib.rs:1:1
  |
1 | pub fn foo() {}
  | ^^^^^^^^^^^^

error: aborting due to 2 previous errors
```

You can also pass each flag more than once for changing multiple lints:

```bash
$ rustc lib.rs --crate-type=lib -D missing-docs -D unused-variables
```

And of course, you can mix these four flags together:

```bash
$ rustc lib.rs --crate-type=lib -D missing-docs -A unused-variables
```

### Via an attribute

You can also modify the lint level with a crate-wide attribute:

```bash
$ cat lib.rs
#![warn(missing_docs)]

pub fn foo() {}
$ rustc lib.rs --crate-type=lib
warning: missing documentation for crate
 --> lib.rs:1:1
  |
1 | / #![warn(missing_docs)]
2 | |
3 | | pub fn foo() {}
  | |_______________^
  |
note: lint level defined here
 --> lib.rs:1:9
  |
1 | #![warn(missing_docs)]
  |         ^^^^^^^^^^^^

warning: missing documentation for a function
 --> lib.rs:3:1
  |
3 | pub fn foo() {}
  | ^^^^^^^^^^^^
```

All four, `warn`, `allow`, `deny`, and `forbid` all work this way.

You can also pass in multiple lints per attribute:

```rust
#![warn(missing_docs, unused_variables)]

pub fn foo() {}
```

And use multiple attributes together:

```rust
#![warn(missing_docs)]
#![deny(unused_variables)]

pub fn foo() {}
```

### Capping lints

`rustc` supports a flag, `--cap-lints LEVEL` that sets the "lint cap level."
This is the maximum level for all lints. So for example, if we take our
code sample from the "deny" lint level above:

```rust,ignore
fn main() {
    100u8 << 10;
}
```

And we compile it, capping lints to warn:

```bash
$ rustc lib.rs --cap-lints warn
warning: bitshift exceeds the type's number of bits
 --> lib.rs:2:5
  |
2 |     100u8 << 10;
  |     ^^^^^^^^^^^
  |
  = note: `#[warn(exceeding_bitshifts)]` on by default

warning: this expression will panic at run-time
 --> lib.rs:2:5
  |
2 |     100u8 << 10;
  |     ^^^^^^^^^^^ attempt to shift left with overflow
```

It now only warns, rather than errors. We can go further and allow all lints:

```bash
$ rustc lib.rs --cap-lints allow
$
```

This feature is used heavily by Cargo; it will pass `--cap-lints allow` when
compiling your dependencies, so that if they have any warnings, they do not
pollute the output of your build.
